1.
Metal carbonyl
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Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation. In the Mond process, nickel carbonyl is used to produce pure nickel, in organometallic chemistry, metal carbonyls serve as precursors for the preparation of other organometalic complexes. Metal carbonyls are toxic by skin contact, inhalation or ingestion, in part because of their ability to carbonylate hemoglobin to give carboxyhemoglobin, which prevents the binding of O2. The nomenclature of the metal carbonyls depends on the charge of the complex, the number and type of atoms. They occur as neutral complexes, as positively charged metal cations or as negatively charged metal carbonylates. The carbon monoxide ligand may be bound terminally to a metal atom or bridging to two or more metal atoms. These complexes may be homoleptic, that is containing only CO ligands, such as nickel carbonyl, mononuclear metal carbonyls contain only one metal atom as the central atom. Except vanadium hexacarbonyl only metals with even number such as chromium, iron, nickel. Polynuclear metal carbonyls are formed from metals with odd order numbers, complexes with different metals, but only one type of ligand will be referred to as isoleptic. The number of carbon monoxide ligands in a metal carbonyl complex is described by a Greek numeral, Carbon monoxide has different binding modes in metal carbonyls. They differ in the hapticity and the bridging mode, the hapticity describes the number of carbon monoxide ligands, which are directly bonded to the central atom. The denomination shall be made by the letter ηn, which is prefixed to the name of the complex, the superscript n indicates the number of bounded atoms. In monohapto coordination, such as in terminally bonded carbon monoxide, If carbon monoxide is bound via the carbon atom and via the oxygen to the metal, it will be referred to as dihapto coordinated η2. The carbonyl ligand engages in a range of bonding modes in metal carbonyl dimers and clusters, in the most common bridging mode, the CO ligand bridges a pair of metals. This bonding mode is observed in the available metal carbonyls, Co28, Fe29, Fe312. In certain higher nuclearity clusters, CO bridges between three or even four metals and these ligands are denoted μ3-CO and μ4-CO. Less common are bonding modes in which both C and O bond to the metal, e. g. μ3-η2, Carbon monoxide bonds to transition metals using synergistic π* back-bonding
2.
Organic chemistry
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Study of structure includes many physical and chemical methods to determine the chemical composition and the chemical constitution of organic compounds and materials. In the modern era, the range extends further into the table, with main group elements, including, Group 1 and 2 organometallic compounds. They either form the basis of, or are important constituents of, many products including pharmaceuticals, petrochemicals and products made from them, plastics, fuels and explosives. Before the nineteenth century, chemists generally believed that compounds obtained from living organisms were endowed with a force that distinguished them from inorganic compounds. According to the concept of vitalism, organic matter was endowed with a vital force, during the first half of the nineteenth century, some of the first systematic studies of organic compounds were reported. Around 1816 Michel Chevreul started a study of soaps made from various fats and he separated the different acids that, in combination with the alkali, produced the soap. Since these were all compounds, he demonstrated that it was possible to make a chemical change in various fats, producing new compounds. In 1828 Friedrich Wöhler produced the chemical urea, a constituent of urine, from inorganic starting materials. The event is now accepted as indeed disproving the doctrine of vitalism. In 1856 William Henry Perkin, while trying to manufacture quinine accidentally produced the organic dye now known as Perkins mauve and his discovery, made widely known through its financial success, greatly increased interest in organic chemistry. A crucial breakthrough for organic chemistry was the concept of chemical structure, ehrlich popularized the concepts of magic bullet drugs and of systematically improving drug therapies. His laboratory made decisive contributions to developing antiserum for diphtheria and standardizing therapeutic serums, early examples of organic reactions and applications were often found because of a combination of luck and preparation for unexpected observations. The latter half of the 19th century however witnessed systematic studies of organic compounds, the development of synthetic indigo is illustrative. The production of indigo from plant sources dropped from 19,000 tons in 1897 to 1,000 tons by 1914 thanks to the methods developed by Adolf von Baeyer. In 2002,17,000 tons of indigo were produced from petrochemicals. In the early part of the 20th Century, polymers and enzymes were shown to be large organic molecules, the multiple-step synthesis of complex organic compounds is called total synthesis. Total synthesis of natural compounds increased in complexity to glucose. For example, cholesterol-related compounds have opened ways to synthesize complex human hormones, since the start of the 20th century, complexity of total syntheses has been increased to include molecules of high complexity such as lysergic acid and vitamin B12
3.
Functional group
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In organic chemistry, functional groups are specific groups of atoms or bonds within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of, however, its relative reactivity can be modified by other functional groups nearby. The atoms of functional groups are linked to other and to the rest of the molecule by covalent bonds. Any subgroup of atoms of a compound also may be called a radical, and if a covalent bond is broken homolytically, Functional groups can also be charged, e. g. in carboxylate salts, which turns the molecule into a polyatomic ion or a complex ion. Complexation and solvation is also caused by interactions of functional groups. In the common rule of thumb like dissolves like, it is the shared or mutually well-interacting functional groups give rise to solubility. For example, sugar dissolves in water because both share the functional group and hydroxyls interact strongly with each other. Combining the names of groups with the names of the parent alkanes generates what is termed a systematic nomenclature for naming organic compounds. In traditional nomenclature, the first carbon atom after the carbon that attaches to the group is called the alpha carbon, the second, beta carbon. IUPAC conventions call for numeric labeling of the position, e. g. 4-aminobutanoic acid, in traditional names various qualifiers are used to label isomers, for example isopropanol is an isomer is n-propanol. The following is a list of functional groups. In the formulas, the symbols R and R usually denote an attached hydrogen, or a side chain of any length. Functional groups, called hydrocarbyl, that only carbon and hydrogen. Each one differs in type of reactivity, there are also a large number of branched or ring alkanes that have specific names, e. g. tert-butyl, bornyl, cyclohexyl, etc. Hydrocarbons may form charged structures, positively charged carbocations or negative carbanions, examples are tropylium and triphenylmethyl cations and the cyclopentadienyl anion. Haloalkanes are a class of molecule that is defined by a carbon–halogen bond and this bond can be relatively weak or quite stable. In general, with the exception of fluorinated compounds, haloalkanes readily undergo nucleophilic substitution reactions or elimination reactions, the substitution on the carbon, the acidity of an adjacent proton, the solvent conditions, etc. all can influence the outcome of the reactivity. Compounds that contain nitrogen in this category may contain C-O bonds, compounds that contain sulfur exhibit unique chemistry due to their ability to form more bonds than oxygen, their lighter analogue on the periodic table
4.
Carbon
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Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds, three isotopes occur naturally, 12C and 13C being stable, while 14C is a radioactive isotope, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity, Carbon is the 15th most abundant element in the Earths crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is the second most abundant element in the body by mass after oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon, the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form, for example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper, while diamond is the hardest naturally occurring material known, graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials, all carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen, the most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil. For this reason, carbon has often referred to as the king of the elements. The allotropes of carbon graphite, one of the softest known substances, and diamond. It bonds readily with other small atoms including other carbon atoms, Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements, although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon is the element, with a ground-state electron configuration of 1s22s22p2. Its first four ionisation energies,1086.5,2352.6,4620.5 and 6222.7 kJ/mol, are higher than those of the heavier group 14 elements. Carbons covalent radii are normally taken as 77.2 pm,66.7 pm and 60.3 pm, although these may vary depending on coordination number, in general, covalent radius decreases with lower coordination number and higher bond order. Carbon compounds form the basis of all life on Earth
5.
Atom
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An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms, Atoms are very small, typical sizes are around 100 picometers. Atoms are small enough that attempting to predict their behavior using classical physics - as if they were billiard balls, through the development of physics, atomic models have incorporated quantum principles to better explain and predict the behavior. Every atom is composed of a nucleus and one or more bound to the nucleus. The nucleus is made of one or more protons and typically a number of neutrons. Protons and neutrons are called nucleons, more than 99. 94% of an atoms mass is in the nucleus. The protons have an electric charge, the electrons have a negative electric charge. If the number of protons and electrons are equal, that atom is electrically neutral, if an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion. The electrons of an atom are attracted to the protons in a nucleus by this electromagnetic force. The number of protons in the nucleus defines to what chemical element the atom belongs, for example, the number of neutrons defines the isotope of the element. The number of influences the magnetic properties of an atom. Atoms can attach to one or more other atoms by chemical bonds to form compounds such as molecules. The ability of atoms to associate and dissociate is responsible for most of the changes observed in nature. The idea that matter is made up of units is a very old idea, appearing in many ancient cultures such as Greece. The word atom was coined by ancient Greek philosophers, however, these ideas were founded in philosophical and theological reasoning rather than evidence and experimentation. As a result, their views on what look like. They also could not convince everybody, so atomism was but one of a number of competing theories on the nature of matter. It was not until the 19th century that the idea was embraced and refined by scientists, in the early 1800s, John Dalton used the concept of atoms to explain why elements always react in ratios of small whole numbers
6.
Double bond
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A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that is two carbon atoms, can be found in alkenes. Many types of double bonds exist between two different elements, for example, in a carbonyl group with a carbon atom and an oxygen atom. Other common double bonds are found in azo compounds, imines and sulfoxides, in skeletal formula the double bond is drawn as two parallel lines between the two connected atoms, typographically, the equals sign is used for this. Double bonds were first introduced in chemical notation by prominent Russian chemist Alexander Butlerov, double bonds involving carbon are stronger than single bonds and are also shorter. Double bonds are also electron-rich, which makes them reactive, the type of bonding can be explained in terms of orbital hybridization. In ethylene each carbon atom has three sp2 orbitals and one p-orbital, the three sp2 orbitals lie in a plane with ~120° angles. The p-orbital is perpendicular to this plane, when the carbon atoms approach each other, two of the sp2 orbitals overlap to form a sigma bond. At the same time, the two p-orbitals approach and together form a pi-bond. For maximum overlap, the p-orbitals have to parallel, and, therefore. This property gives rise to cis-trans isomerism, double bonds are shorter than single bonds because p-orbital overlap is maximized. With 133 pm, the ethylene C=C bond length is shorter than the C−C length in ethane with 154 pm. The double bond is stronger,636 kJ mol−1 versus 368 kJ mol−1. In an alternative representation, the double bond results from two overlapping sp3 orbitals as in a bent bond, in molecules, with alternating double bonds and single bonds, p-orbital overlap can exist over multiple atoms in a chain, giving rise to a conjugated system. Conjugation can be found in such as dienes and enones. In cyclic molecules, conjugation can lead to aromaticity, in cumulenes, two double bonds are adjacent. Double bonds are common for period 2 elements carbon, nitrogen, and oxygen, metals, too, can engage in multiple bonding in a metal ligand multiple bond. Double bonded compounds, alkene homologs, R2E=ER2 are now known for all of the heavier group 14 elements, unlike the alkenes these compounds are not planar but adopt twisted and/or trans bent structures
7.
Oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the group on the periodic table and is a highly reactive nonmetal. By mass, oxygen is the third-most abundant element in the universe, after hydrogen, at standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. This is an important part of the atmosphere and diatomic oxygen gas constitutes 20. 8% of the Earths atmosphere, additionally, as oxides the element makes up almost half of the Earths crust. Most of the mass of living organisms is oxygen as a component of water, conversely, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone, strongly absorbs ultraviolet UVB radiation, but ozone is a pollutant near the surface where it is a by-product of smog. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft, the name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle, Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philos work by observing that a portion of air is consumed during combustion and respiration, Oxygen was discovered by the Polish alchemist Sendivogius, who considered it the philosophers stone. In the late 17th century, Robert Boyle proved that air is necessary for combustion, English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. From this he surmised that nitroaereus is consumed in both respiration and combustion, Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract De respiratione. Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, one part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. The fact that a substance like wood gains overall weight in burning was hidden by the buoyancy of the combustion products
8.
Ligand
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In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the electron pairs. The nature of bonding can range from covalent to ionic. Furthermore, the bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic ligand, metals and metalloids are bound to ligands in virtually all circumstances, although gaseous naked metal ions can be generated in high vacuum. Ligands in a complex dictate the reactivity of the atom, including ligand substitution rates, the reactivity of the ligands themselves. Ligand selection is a consideration in many practical areas, including bioinorganic and medicinal chemistry, homogeneous catalysis. Ligands are classified in many ways, including, charge, size, the identity of the atom. The size of a ligand is indicated by its cone angle, the composition of coordination complexes have been known since the early 1800s, such as Prussian blue and copper vitriol. The key breakthrough occurred when Alfred Werner reconciled formulas and isomers and he showed, among other things, that the formulas of many cobalt and chromium compounds can be understood if the metal has six ligands in an octahedral geometry. The first to use the term ligand were Alfred Stock and Carl Somiesky, the theory allows one to understand the difference between coordinated and ionic chloride in the cobalt ammine chlorides and to explain many of the previously inexplicable isomers. He resolved the first coordination complex called hexol into optical isomers, in general, ligands are viewed as electron donors and the metals as electron acceptors. This is because the ligand and central metal are bonded to one another, bonding is often described using the formalisms of molecular orbital theory. The HOMO can be mainly of ligands or metal character, ligands and metal ions can be ordered in many ways, one ranking system focuses on ligand hardness. Metal ions preferentially bind certain ligands, in general, soft metal ions prefer weak field ligands, whereas hard metal ions prefer strong field ligands. According to the orbital theory, the HOMO of the ligand should have an energy that overlaps with the LUMO of the metal preferential. Metal ions bound to strong-field ligands follow the Aufbau principle, whereas complexes bound to weak-field ligands follow Hunds rule. Binding of the metal with the results in a set of molecular orbitals, where the metal can be identified with a new HOMO and LUMO
9.
Organometallic chemistry
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Moreover, some related compounds such as transition metal hydrides and metal phosphine complexes are often included in discussions of organometallic compounds. The field of organometallic chemistry combines aspects of inorganic and organic chemistry. Organometallic compounds are distinguished by the prefix organo- e. g. organopalladium compounds, examples of such organometallic compounds include all Gilman reagents, which contain lithium and copper. Tetracarbonyl nickel, and ferrocene are examples of compounds containing transition metals. The term metalorganics usually refers to metal-containing compounds lacking direct metal-carbon bonds, metal beta-diketonates, alkoxides, and dialkylamides are representative members of this class. Representative Organometallic Compounds Many complexes feature coordination bonds between a metal and organic ligands, the organic ligands often bind the metal through a heteroatom such as oxygen or nitrogen, in which case such compounds are considered coordination compounds. However, if any of the form a direct M-C bond, then complex is usually considered to be organometallic. Furthermore, many compounds such as metal acetylacetonates and metal alkoxides are called metalorganics. Many organic coordination compounds occur naturally, for example, hemoglobin and myoglobin contain an iron center coordinated to the nitrogen atoms of a porphyrin ring, magnesium is the center of a chlorin ring in chlorophyll. The field of inorganic compounds is known as bioinorganic chemistry. In contrast to these compounds, methylcobalamin, with a cobalt-methyl bond, is a true organometallic complex. This subset of complexes are often discussed within the subfield of bioorganometallic chemistry, the metal-carbon bond in organometallic compounds are generally highly covalent. For highly electropositive elements, such as lithium and sodium, the carbon ligand exhibits carbanionic character, but free carbon-based anions are extremely rare, as in other areas of chemistry, electron counting is useful for organizing organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of metal carbonyls, most organometallic compounds do not however follow the 18e rule. Chemical bonding and reactivity in organometallic compounds is often discussed from the perspective of the isolobal principle, as well as X-ray diffraction, NMR and infrared spectroscopy are common techniques used to determine structure. The dynamic properties of compounds is often probed with variable-temperature NMR. The abundant and diverse products from coal and petroleum led to Ziegler-Natta, Fischer-Tropsch, hydroformylation catalysis which employ CO, H2, recognition of organometallic chemistry as a distinct subfield culminated in the Nobel Prizes to Ernst Fischer and Geoffrey Wilkinson for work on metallocenes. In 2005, Yves Chauvin, Robert H. Grubbs and Richard R. Schrock shared the Nobel Prize for metal-catalyzed olefin metathesis,1900 Paul Sabatier works on hydrogenation organic compounds with metal catalysts
10.
Nickel tetracarbonyl
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Nickel carbonyl is the organonickel compound with the formula Ni4. This pale-yellow liquid is the carbonyl of nickel. It is an intermediate in the Mond process for the purification of nickel, Nickel carbonyl is one of the most toxic substances encountered in industrial processes. In nickel tetracarbonyl, the state for nickel is assigned as zero. The formula conforms to 18-electron rule, the molecule is tetrahedral, with four carbonyl ligands attached to nickel. The CO ligands, in which the C and the O are connected by bonds, are covalently bonded to the nickel atom via the carbon ends. Electron diffraction studies have been performed on this molecule, and the Ni–C, Ni4 was first synthesised in 1890 by Ludwig Mond by the direct reaction of nickel metal with CO. This pioneering work foreshadowed the existence of other metal carbonyl compounds, including those of V, Cr, Mn, Fe. It was also applied industrially to the purification of nickel by the end of the 19th century, at 323 K, carbon monoxide is passed over impure nickel. The optimal rate occurs at 130 °C, Ni4 is not readily available commercially. It is conveniently generated in the laboratory by carbonylation of commercially available bisnickel, on moderate heating, Ni4 decomposes to carbon monoxide and nickel metal. Combined with the formation from CO and even impure nickel. Thermal decomposition commences near 180 °C and increases at higher temperature, like other low-valent metal carbonyls, Ni4 is susceptible to attack by nucleophiles. Attack can occur at nickel center, resulting in displacement of CO ligands, or at CO. Thus, donor ligands such as triphenylphosphine react to give Ni3, bipyridine and related ligands behave similarly. The monosubstitution of nickel tetracarbonyl with other ligands can be used to determine the Tolman electronic parameter, treatment with hydroxides gives clusters such as 2− and 2−. These compounds can also be obtained by reduction of nickel carbonyl, Thus, treatment of Ni4 with carbon nucleophiles results in acyl derivatives such as −. Chlorine oxidizes nickel carbonyl into NiCl2, releasing CO gas and this reaction provides a convenient method for destroying unwanted portions of the toxic compound. Reactions of Ni4 with alkyl and aryl halides often result in carbonylated organic products, vinylic halides, such as PhCH=CHBr, are converted to the unsaturated esters upon treatment with Ni4 followed by sodium methoxide
11.
Aldehyde
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The group—without R—is the aldehyde group, also known as the formyl group. Aldehydes are common in organic chemistry, Aldehydes feature an sp2-hybridized, planar carbon center that is connected by a double bond to oxygen and a single bond to hydrogen. The C–H bond is not ordinarily acidic, because of resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde is far more acidic, with a pKa near 15, compared to the acidity of a typical alkane. This acidification is attributed to the quality of the formyl center and the fact that the conjugate base. Related to, the group is somewhat polar. Aldehydes can exist in either the keto or the enol tautomer, keto-enol tautomerism is catalyzed by either acid or base. Usually the enol is the minority tautomer, but it is more reactive, the common names for aldehydes do not strictly follow official guidelines, such as those recommended by IUPAC, but these rules are useful. IUPAC prescribes the following nomenclature for aldehydes, Acyclic aliphatic aldehydes are named as derivatives of the longest carbon chain containing the aldehyde group, thus, HCHO is named as a derivative of methane, and CH3CH2CH2CHO is named as a derivative of butane. The name is formed by changing the suffix -e of the parent alkane to -al, so that HCHO is named methanal, in other cases, such as when a -CHO group is attached to a ring, the suffix -carbaldehyde may be used. Thus, C6H11CHO is known as cyclohexanecarbaldehyde, if the presence of another functional group demands the use of a suffix, the aldehyde group is named with the prefix formyl-. This prefix is preferred to methanoyl-, the word aldehyde was coined by Justus von Liebig as a contraction of the Latin alcohol dehydrogenatus. In the past, aldehydes were sometimes named after the corresponding alcohols, for example, the term formyl group is derived from the Latin word formica ant. This word can be recognized in the simplest aldehyde, formaldehyde, Aldehydes have properties that are diverse and that depend on the remainder of the molecule. Smaller aldehydes are more soluble in water, formaldehyde and acetaldehyde completely so, the volatile aldehydes have pungent odors. Aldehydes degrade in air via the process of autoxidation, the two aldehydes of greatest importance in industry, formaldehyde and acetaldehyde, have complicated behavior because of their tendency to oligomerize or polymerize. They also tend to hydrate, forming the geminal diol, the oligomers/polymers and the hydrates exist in equilibrium with the parent aldehyde. Aldehydes are readily identified by spectroscopic methods, using IR spectroscopy, they display a strong νCO band near 1700 cm−1. In their 1H NMR spectra, the formyl hydrogen center absorbs near δH =9 and this signal shows the characteristic coupling to any protons on the alpha carbon
12.
Ketone
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In chemistry, a ketone /ˈkiːtoʊn/ is an organic compound with the structure RCR, where R and R can be a variety of carbon-containing substituents. Ketones and aldehydes are simple compounds that contain a carbonyl group and they are considered simple because they do not have reactive groups like −OH or −Cl attached directly to the carbon atom in the carbonyl group, as in carboxylic acids containing −COOH. Many ketones are known and many are of importance in industry. Examples include many sugars and the industrial solvent acetone, which is the smallest ketone, the word ketone is derived from Aketon, an old German word for acetone. According to the rules of IUPAC nomenclature, ketones are named by changing the suffix -ane of the parent alkane to -anone, the position of the carbonyl group is usually denoted by a number. For the most important ketones, however, traditional names are still generally used. The common names of ketones are obtained by writing separately the names of the two alkyl groups attached to the group, followed by ketone as a separate word. The names of the groups are written alphabetically. When the two groups are the same, the prefix di- is added before the name of alkyl group. The positions of other groups are indicated by Greek letters, the α-carbon being the adjacent to carbonyl group. If both alkyl groups in a ketone are the same then the ketone is said to be symmetrical, although used infrequently, oxo is the IUPAC nomenclature for a ketone functional group. Other prefixes, however, are also used, for some common chemicals, keto or oxo refer to the ketone functional group. The term oxo is used widely through chemistry, for example, it also refers to an oxygen atom bonded to a transition metal. The ketone carbon is often described as sp2 hybridized, a description that includes both their electronic and molecular structure, ketones are trigonal planar around the ketonic carbon, with C−C−O and C−C−C bond angles of approximately 120°. Ketones differ from aldehydes in that the group is bonded to two carbons within a carbon skeleton. In aldehydes, the carbonyl is bonded to one carbon and one hydrogen and are located at the ends of carbon chains, ketones are also distinct from other carbonyl-containing functional groups, such as carboxylic acids, esters and amides. The carbonyl group is polar because the electronegativity of the oxygen is greater than that for carbon, thus, ketones are nucleophilic at oxygen and electrophilic at carbon. Because the carbonyl group interacts with water by bonding, ketones are typically more soluble in water than the related methylene compounds
13.
Carboxylic acid
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A carboxylic acid /ˌkɑːrbɒkˈsɪlɪk/ is an organic compound that contains a carboxyl group. The general formula of an acid is R–COOH, with R referring to the rest of the molecule. Carboxylic acids occur widely and include the amino acids and acetic acid, salts and esters of carboxylic acids are called carboxylates. When a carboxyl group is deprotonated, its conjugate base forms a carboxylate anion, carboxylate ions are resonance-stabilized, and this increased stability makes carboxylic acids more acidic than alcohols. Carboxylic acids can be seen as reduced or alkylated forms of the Lewis acid carbon dioxide, carboxylic acids are commonly identified using their trivial names, and usually have the suffix -ic acid. IUPAC-recommended names also exist, in system, carboxylic acids have an -oic acid suffix. For example, butyric acid is butanoic acid by IUPAC guidelines, the -oic acid nomenclature detail is based on the name of the previously-known chemical benzoic acid. Alternately, it can be named as a carboxy or carboxylic acid substituent on another parent structure, for example, 2-carboxyfuran. The carboxylate anion of an acid is usually named with the suffix -ate, in keeping with the general pattern of -ic acid and -ate for a conjugate acid and its conjugate base. For example, the base of acetic acid is acetate. The radical •COOH has only a fleeting existence. The acid dissociation constant of •COOH has been measured using electron paramagnetic resonance spectroscopy, the carboxyl group tends to dimerise to form oxalic acid. Because they are both hydrogen-bond acceptors and hydrogen-bond donors, they participate in hydrogen bonding. Together the hydroxyl and carbonyl group forms the functional group carboxyl, carboxylic acids usually exist as dimeric pairs in nonpolar media due to their tendency to self-associate. Smaller carboxylic acids are soluble in water, whereas higher carboxylic acids are less due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be soluble in less-polar solvents such as ethers. Carboxylic acids tend to have higher boiling points than water, not only because of their surface area. Carboxylic acids tend to evaporate or boil as these dimers, for boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement must be vaporised, both of which increase the enthalpy of vaporization requirements significantly
14.
Ester
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In chemistry, esters are chemical compounds derived from an acid in which at least one –OH group is replaced by an –O–alkyl group. Usually, esters are derived from an acid and an alcohol. Glycerides, which are fatty acid esters of glycerol, are important esters in biology, being one of the classes of lipids. Esters with low weight are commonly used as fragrances and found in essential oils. Phosphoesters form the backbone of DNA molecules, nitrate esters, such as nitroglycerin, are known for their explosive properties, while polyesters are important plastics, with monomers linked by ester moieties. The word ester was coined in 1848 by German chemist Leopold Gmelin, probably as a contraction of the German Essigäther, ester names are derived from the parent alcohol and the parent acid, where the latter may be organic or inorganic. Esters derived from more complex carboxylic acids are, on the hand, more frequently named using the systematic IUPAC name. For example, the ester hexyl octanoate, also known under the trivial name hexyl caprylate, has the formula CH36CO25CH3, the chemical formulas of organic esters usually take the form RCO2R′, where R and R′ are the hydrocarbon parts of the carboxylic acid and the alcohol, respectively. For example, butyl acetate, derived from butanol and acetic acid would be written CH3CO2C4H9, alternative presentations are common including BuOAc and CH3COOC4H9. Cyclic esters are called lactones, regardless of whether they are derived from an organic or an inorganic acid, one example of a lactone is γ-valerolactone. An uncommon class of organic esters are the orthoesters, which have the formula RC3, triethylorthoformate is derived, in terms of its name from orthoformic acid and ethanol. Esters can also be derived from an acid and an alcohol. For example, triphenyl phosphate is the derived from phosphoric acid. Organic carbonates are derived from acid, for example, ethylene carbonate is derived from carbonic acid. So far an alcohol and inorganic acid are linked via oxygen atoms, in corollary, boron features borinic esters, boronic esters, and borates. As oxygen is a group 16 chemical element, sulfur atoms can replace some oxygen atoms in carbon–oxygen–central inorganic atom covalent bonds of an ester, esters contain a carbonyl center, which gives rise to 120 ° C–C–O and O–C–O angles. Unlike amides, esters are structurally flexible functional groups because rotation about the C–O–C bonds has a low barrier and their flexibility and low polarity is manifested in their physical properties, they tend to be less rigid and more volatile than the corresponding amides. The pKa of the alpha-hydrogens on esters is around 25, the preference for the Z conformation is influenced by the nature of the substituents and solvent, if present
15.
Amide
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An amide, also known as an acid amide, is a compound with the functional group RnExNR′2. Most common are carboxamides, but many other important types of amides are known, the term amide refers both to classes of compounds and to the functional group within those compounds. Amide can also refer to the base of ammonia or of an organic amine. For discussion of these anionic amides, see Alkali metal amides, the remainder of this article is about the carbonyl–nitrogen sense of amide. The simplest amides are derivatives of ammonia wherein one hydrogen atom has replaced by an acyl group. The ensemble is represented as RCNH2 and is described as a primary amide. Closely related and even more numerous are secondary amides which can be derived from amines and have the formula RCNHR′. Tertiary amides are commonly derived from amines and have the general structure RCNR′R″. Amides are usually regarded as derivatives of carboxylic acids in which the group has been replaced by an amine or ammonia. The lone pair of electrons on the nitrogen is delocalized into the carbonyl, consequently, the nitrogen in amides is not pyramidal. It is estimated that acetamide is described by resonance structure A for 62%, in the usual nomenclature, one adds the term amide to the stem of the parent acids name. For instance, the derived from acetic acid is named acetamide. IUPAC recommends ethanamide, but this and related names are rarely encountered. When the amide is derived from a primary or secondary amine, thus, the amide formed from dimethylamine and acetic acid is N, N-dimethylacetamide. Usually even this name is simplified to dimethylacetamide, cyclic amides are called lactams, they are necessarily secondary or tertiary amides. Functional groups consisting of –PNR2 and –SO2NR2 are phosphonamides and sulfonamides, some chemists make a pronunciation distinction between the two, saying /əˈmiːd/ for the carbonyl–nitrogen compound and /ˈeɪmaɪd/ for the anion. Others replace one of these with /ˈæmᵻd/, while others pronounce both /ˈæmᵻd/, making them homonyms. Compared to amines, amides are very weak bases, while the conjugate acid of an amine has a pKa of about 9.5, the conjugate acid of an amide has a pKa around −0.5
16.
Enone
–
An enone, also called an α, β-unsaturated carbonyl, is a type of organic compound consisting of an alkene conjugated to a ketone. The simplest enone is methyl vinyl ketone or CH2=CHCOCH3, an enal is the corresponding α, β-unsaturated aldehyde, an example being acrolein. Enones are typically produced using an Aldol condensation or Knoevenagel condensation, some commercially significant enones are produced by condensations of acetone, e. g. mesityl oxide and isophorone. In the Meyer–Schuster rearrangement the starting compound is a propargyl alcohol, cyclic enones can be prepared via the Pauson–Khand reaction. Enones undergo many kinds of reactions and they are electrophilic at both the carbonyl carbon as well as the β-carbon. Depending on conditions, either site is attacked by nucleophiles, addition to the alkene is called conjugate additions. Enones are often good dienophiles in Diels-Alder reactions and they are activated by Lewis acids, which bind to the carbonyl oxygen. They can undergo both reduction of the carbonyl and of the alkene, as well as both, enones undergo the Nazarov cyclization reaction and in the Rauhut–Currier reaction. Sterically unhindered enones such as methyl vinyl ketone are prone to polymerization, enones are good ligands for low-valent metal complexes, examples being Fe3 and trisdipalladium. Enone is not to be confused with ketene, an enamine is a cousin of an enone, with the carbonyl replaced by an amine group. Regiospecific formation is the controlled enolate formation by the specific deprotonation at one of the α-carbons of the starting molecule. This provides one of the best understood synthetic strategies to introduce chemical complexity in natural product, a prominent example of its use is in the total synthesis of progesterone illustrated in Figure Regiospecific enolate formation in the total synthesis of progesterone. When ketones are treated with base, enolates can be formed by deprotonation at either α-carbon, the selectivity is determined by both the steric and electronic effects on the α-carbons as well as the precise base used. Enolate formation will be thermodynamically favoured at the most acidic proton which depends on the stabilization of the resulting anion. However, the selectivity can be reversed by sterically hindering the thermodynamic product, traditional methods for regioselective enolate formation use either electronic activating groups or steric blocking groups. Enone can also serve as a precursor for regiospecific formation of enolate and this process is first described by Gilbert Stork who is best known for his contributions to the study of selective enolate formation methods in organic synthesis. The Stork method also uses enone as a “masked functionality” of enolate, reacting enone with lithium metal generates the enolate at the α-carbon of the enone. The enolate product can either be trapped or alkylated, by using “masked functionality”, it is possible to produce enolates that are not accessible by traditional methods
17.
Acyl halide
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An acyl halide is a chemical compound derived from an oxoacid by replacing a hydroxyl group with a halide group. If the acid is an acid, the compound contains a –COX functional group. The general formula for such an acyl halide can be written RCOX, where R may be, for example, a group, CO is the carbonyl group. Acyl chlorides are the most commonly encountered acyl halides, but acetyl iodide is the one produced on the largest scale, billions of kilograms are generated annually in the production of acetic acid. The hydroxyl group of a sulfonic acid may also be replaced by a halogen to produce the corresponding sulfonyl halide, in practical terms this is almost always chloride to give the sulfonyl chloride. Aromatic acyl halides can be prepared in similar reactions with similar reagents, however, for example, chloroformylation, a specific type of Friedel-Crafts acylation which uses formaldehyde as a reagent, or by the direct chlorination of benzaldehyde derivatives. Acyl halides are rather reactive compounds often synthesized to be used as intermediates in the synthesis of organic compounds. For example, an acyl halide can react with, water and this hydrolysis is the most heavily exploited reaction for acyl halides as it occurs in the industrial synthesis of acetic acid. An alcohol to form an ester an amine to form an amide an aromatic compound, using a Lewis acid catalyst such as AlCl3, carboxylic acids to form an organic acid anhydrides. In the above reactions, HX is also formed, for example, if the acyl halide is an acyl chloride, HCl is also formed. A molecule can have more than one acyl halide functional group, for example, adipoyl dichloride, usually simply called adipoyl chloride, has two acyl chloride functional groups, see the structure at right. It is the dichloride of the 6-carbon dicarboxylic acid adipic acid, an important use of adipoyl chloride is polymerization with an organic di-amino compound to form a polyamide called nylon or polymerization with certain other organic compounds to form polyesters. Phosgene is a toxic gas that is the dichloride of carbonic acid. Both chloride radicals in phosgene can undergo reactions analogous to the reactions of acyl halides. Phosgene is used a reactant in the production of polycarbonate polymers, volatile acyl halides are lachrymatory because they can react with water at the surface of the eye producing hydrohalic and organic acids irritating to the eye. Similar problems can result if one inhales acyl halide vapors, in general, acyl halides are irritants to the eyes, skin and mucous membranes
18.
Acid anhydride
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An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the oxygen atom. A common type of acid anhydride is a carboxylic anhydride, where the parent acid is a carboxylic acid. Symmetrical acid anhydrides of this type are named by replacing the word acid in the name of the parent carboxylic acid by the word anhydride, thus, 2O is called acetic anhydride. Mixed acid anhydrides, such as acetic anhydride, are known. One or both groups of an acid anhydride may also be derived from another type of organic acid. One of the groups of an acid anhydride can be derived from an inorganic acid such as phosphoric acid. The mixed anhydride 1, 3-bisphosphoglycerate is an intermediate in the formation of ATP via glycolysis, is the mixed anhydride between 3-phosphoglyceric acid and phosphoric acid, acidic oxides are often classified as acid anhydrides. Acid anhydrides are prepared in industry by diverse means, Acetic anhydride is mainly produced by the carbonylation of methyl acetate. Maleic anhydride is produced by the oxidation of benzene or butane, laboratory routes emphasize the dehydration of the corresponding acids. In reactions with protic substrates, the reactions afford equal amounts of the acylated product, acid anhydrides tend to be less electrophilic than acyl chlorides, and only one acyl group is transferred per molecule of acid anhydride, which leads to a lower atom efficiency. The low cost, however, of acetic anhydride makes it a choice for acetylation reactions. Illustrative acid anhydrides Acetic anhydride is an industrial chemical widely used for preparing acetate esters. Maleic anhydride is the precursor to various resins by copolymerization with styrene, maleic anhydride is a dienophile in the Diels-Alder reaction. Dianhydrides, molecules containing two acid anhydride functions, are used to synthesize polyimides and sometimes polyesters and polyamides, polyanhydrides are a class of polymers characterized by anhydride bonds that connect repeat units of the polymer backbone chain. Natural organic acid anhydrides are rare, because of the reactivity of the functional group, examples include cantharidin from species of blister beetle, including the Spanish fly, Lytta vesicatoria, and tautomycin, from the bacterium Streptomyces spiroverticillatus. Sulfur can replace oxygen, either in the group or in the bridge. In the former case, the name of the group is enclosed in parentheses to avoid ambiguity in the name
19.
Imide
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In organic chemistry, an imide is a functional group consisting of two acyl groups bound to nitrogen. These compounds are related to acid anhydrides, although imides are less reactive. In terms of applications, imides are best known as components of high-strength polymers. Most imides are cyclic compounds derived from acids, and their names reflect the parent acid. Examples are succinimide, derived from acid, and phthalimide. For imides derived from amines, the N-substituent is indicated by a prefix, for example, N-ethylsuccinimide is derived from succinic acid and ethylamine. Isoimides are isomeric with normal imides and have the formula RCOCR″ and they are often intermediates that convert to the more symmetrical imides. Organic compounds called carbodiimides have the formula RN=C=NR, the ligand in coordination chemistry known as imide has the formula NR. Imido ligands form multiple bond to the metal, in some the M-N-C angle is 180º but often the angle is decidedly bent. The parent imide is an intermediate in nitrogen fixation by synthetic catalysts, being highly polar, imides exhibit good solubility in polar media. The N–H center for imides derived from ammonia is acidic and can participate in hydrogen bonding, unlike the structurally related acid anhydrides, they resist hydrolysis and some can even be recrystallized from boiling water. Many high strength or electrically conductive polymers contain imide subunits, i. e. the polyimides, one example is Kapton where the repeat unit consists of two imide groups derived from aromatic tetracarboxylic acids. This technique is called reactive extrusion, a commercial polyglutarimide product based on the methylamine derivative of PMMA, called Kamax, was produced by the Rohm and Haas company. The toughness of these materials reflects the rigidity of the functional group. Interest in the bioactivity of imide-containing compounds was sparked by the discovery of the high bioactivity of the Cycloheximide as an inhibitor of protein biosynthesis in certain organisms. Thalidomide, famous for its effects, is one result of this research. A number of fungicides and herbicides contain the imide functionality, examples include Captan, which has been phased out because of its carcinogenic properties, and Procymidone. In the 21st century new interest arose in thalidomides immunomodulatory effects, most common imides are prepared by heating dicarboxylic acids or their anhydrides and ammonia or primary amines
20.
Urea
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Urea, also known as carbamide, is an organic compound with the chemical formula CO2. This amide has two groups joined by a carbonyl functional group. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals and it is a colorless, odorless solid, highly soluble in water, and practically non-toxic. Dissolved in water, it is neither acidic nor alkaline, the body uses it in many processes, most notably nitrogen excretion. The liver forms it by combining two molecules with a carbon dioxide molecule in the urea cycle. Urea is widely used in fertilizers as a source of nitrogen and is an important raw material for the chemical industry, Friedrich Wöhlers discovery in 1828 that urea can be produced from inorganic starting materials was an important conceptual milestone in chemistry. More than 90% of world production of urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use, therefore, it has the lowest transportation costs per unit of nitrogen nutrient. The standard crop-nutrient rating of urea is 46-0-0, many soil bacteria possess the enzyme urease, which catalyzes conversion of urea to ammonia or ammonium ion and bicarbonate ion. Thus urea fertilizers rapidly transform to the form in soils. Ammonium and nitrate are readily absorbed by plants, and are the dominant sources of nitrogen for plant growth, Urea is also used in many multi-component solid fertilizer formulations. Urea is highly soluble in water and is also very suitable for use in fertilizer solutions. For fertilizer use, granules are preferred over prills because of their particle size distribution. The most common impurity of synthetic urea is biuret, which impairs plant growth, Urea is usually spread at rates of between 40 and 300 kg/ha but rates vary. Smaller applications incur lower losses due to leaching, during summer, urea is often spread just before or during rain to minimize losses from volatilization. Because of the nitrogen concentration in urea, it is very important to achieve an even spread. The application equipment must be calibrated and properly used. Drilling must not occur on contact with or close to seed, Urea dissolves in water for application as a spray or through irrigation systems
21.
Carbamate
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A carbamate is an organic compound derived from carbamic acid. A carbamate group, carbamate ester, and carbamic acids are functional groups that are inter-related structurally, carbamate esters are also called urethanes. Carbamic acids are derived from amines, R2NH + CO2 → R2NCO2H Carbamic acid is about as acidic as acetic acid, n-terminal amino groups of valine residues in the α- and β-chains of deoxyhemoglobin exist as carbamates. They help to stabilise the protein, when it becomes deoxyhemoglobin and this stabilizing effect should not be confused with the Bohr effect. The ε-amino groups of the residues in urease and phosphotriesterase also feature carbamate. The carbamate derived from aminoimidazole is an intermediate in the biosynthesis of inosine, carbamoyl phosphate is generated from carboxyphosphate rather than CO2. Perhaps the most important carbamate is the one involved in the capture of CO2 by plants since this process is necessary for their growth, the enzyme ribulose 1, 5-bisphosphate carboxylase/oxygenase fixes a molecule of carbon dioxide as phosphoglycerate in the Calvin cycle. At the active site of the enzyme, a Mg2+ ion is bound to glutamate and aspartate residues as well as a lysine carbamate. The carbamate is formed when an uncharged lysine side chain near the ion reacts with a carbon dioxide molecule from the air, which renders it charged. The so-called carbamate insecticides feature the carbamate ester functional group, included in this group are aldicarb, carbofuran, carbaryl, ethienocarb, fenobucarb, oxamyl, and methomyl. These insecticides kill insects by reversibly inactivating the enzyme acetylcholinesterase, the organophosphate pesticides also inhibit this enzyme, although irreversibly, and cause a more severe form of cholinergic poisoning. Fenoxycarb has a group but acts as a juvenile hormone mimic. The insect repellent icaridin is a substituted carbamate, polyurethanes contain multiple carbamate groups as part of their structure. The urethane in the name refers to these carbamate groups. In contrast, the commonly called urethane, ethyl carbamate, is neither a component of polyurethanes. Urethanes are usually formed by reaction of an alcohol with an isocyanate, commonly, urethanes made by a non-isocyanate route are called carbamates. Polyurethane polymers have a range of properties and are commercially available as foams, elastomers. Typically, polyurethane polymers are made by combining diisocyanates, e. g. Urethane was once produced commercially in the United States as an antineoplastic agent and it was found to be toxic and largely ineffective
22.
Acyl chloride
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In organic chemistry, an acyl chloride is an organic compound with the functional group -COCl. Their formula is usually written RCOCl, where R is a side chain and they are reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride, CH3COCl, Acyl chlorides are the most important subset of acyl halides. Where the acyl chloride moiety takes priority, acyl chlorides are named by taking the name of the parent carboxylic acid, for example, acetic acid boils at 118 °C, whereas acetyl chloride boils at 51 °C. Like most carbonyl compounds, infrared spectroscopy reveals a band near 1750 cm−1, the industrial route to acetyl chloride involves the reaction of acetic anhydride with hydrogen chloride. In this reaction, the dioxide and hydrogen chloride generated are both gases that can leave the reaction vessel, driving the reaction forward. Excess thionyl chloride is easily evaporated as well, the iminium intermediate reacts with the carboxylic acid, abstracting an oxide, and regenerating the DMF catalyst. Finally, methods that do not form HCl are also known, such as the Appel reaction, RCOOH + Ph3P + CCl4 → RCOCl + Ph3PO + HCCl3 and the use of cyanuric chloride, Acyl chlorides are very reactive. Consider the comparison to its RCOOH acid analogue, the ion is an excellent leaving group while the hydroxide is not under normal conditions. The use of a base, e. g. aqueous sodium hydroxide or pyridine, or excess amine is desirable to remove the hydrogen chloride byproduct, and to catalyze the reaction. While it is possible to obtain esters or amides from the carboxylic acid with alcohols or amines. In contrast, both involved in preparing esters and amides via acyl chlorides are fast and irreversible. This makes the route often preferable to the single step reaction with the carboxylic acid. With carbon nucleophiles such as Grignard reagents, acyl chlorides generally react first to give the ketone, a notable exception is the reaction of acyl halides with certain organocadmium reagents which stops at the ketone stage. The nucleophilic reaction with Gilman reagents also afford ketones, due to their lesser reactivity, acid chlorides of aromatic acids are generally less reactive those of alkyl acids and thus somewhat more rigorous conditions are required for reaction. Acyl chlorides are reduced by strong hydride donors such as lithium aluminium hydride, lithium tri-tert-butoxyaluminium hydride, a bulky hydride donor, reduces acyl chlorides to aldehydes, as does the Rosenmund reduction using hydrogen gas over a poisoned palladium catalyst. Because acyl chlorides are reactive compounds, precautions should be taken while handling them. They are lachrymatory because they can react with water at the surface of the eye producing hydrochloric and organic acids irritating to the eye, similar problems can result if one inhales acyl chloride vapors
23.
Chloroformate
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Not related to the similarly named chloroform. Chloroformates are a class of compounds which are esters of chloroformic acid. They are widely used as reagents in organic chemistry, for example, benzyl chloroformate is used to introduce the Cbz protecting group and fluorenylmethyloxycarbonyl chloride is used to introduce the FMOC protecting group. They are gaining popularity in the field of chromatography as versatile derivatization agents and they enable relatively simple transformation of large array of metabolites for analysis by gas chromatography / mass spectrometry
24.
Phosgene
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Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a weapon during World War I where it was responsible for about 85% of the 100,000 deaths caused by chemical weapons. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals, in low concentrations, its odor resembles freshly cut hay or grass. In addition to its production, small amounts occur from the breakdown. The chemical was named by combining the Greek words phos and genesis, Phosgene is a planar molecule as predicted by VSEPR theory. The C=O distance is 1.18 Å, the C−Cl distance is 1.74 Å and it is one of the simplest acid chlorides, being formally derived from carbonic acid. Typically, the reaction is conducted between 50 and 150 °C, above 200 °C, phosgene reverts to carbon monoxide and chlorine, Keq =0.05. World production of this compound was estimated to be 2.74 million tonnes in 1989, because of safety issues, phosgene is often produced and consumed within the same plant, and extraordinary measures are made to contain this toxic gas. It is listed on schedule 3 of the Chemical Weapons Convention, upon ultraviolet radiation in the presence of oxygen, chloroform slowly converts into phosgene by a radical reaction. To suppress this photodegradation, chloroform is often stored in brown-tinted glass containers, chlorinated compounds used to remove oil from metals, such as automotive brake cleaners, are converted to phosgene by the UV rays of arc welding processes. Phosgene may also be produced during testing for leaks of older-style refrigerant gases, chloromethanes were formerly leak-tested in situ by employing a small gas torch with a sniffer tube and a copper reaction plate in the flame nozzle of the torch. If any refrigerant gas was leaking from a pipe or joint, in the process, phosgene gas would be created due to the thermal reaction. Electronic sensing of refrigerant gases phased out the use of testing for leaks in the 1980s. Phosgene can be released during building fires, in one instance, a deputy fire chief was killed ten days after inhaling fumes that wafted down outside a burning restaurant. After a two-day hospitalization he had appeared to recover, but ultimately suffered cardiac arrest at home following from tracheobronchial inflammation, alveolar hemorrhage, the phosgene was produced by decomposing freon-22 after flames ducted up from a grease fire heated an air-conditioning unit on the roof and ruptured a hose. The great majority of phosgene is used in the production of isocyanates and these two isocyanates are precursors to polyurethanes. In the research laboratory phosgene still finds limited use in organic synthesis, a variety of substitutes have been developed, notably trichloromethyl chloroformate, a liquid at room temperature, and bis carbonate, a crystalline substance. Thionyl chloride is commonly and more safely employed for this application
25.
Carbonate ester
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A carbonate ester is an ester of carbonic acid. This functional group consists of a carbonyl group flanked by two alkoxy groups, the general structure of these carbonates is R1OOR2 and they are related to esters R1OR and ethers R1OR2 and also to the inorganic carbonates. Monomers of polycarbonate are linked by carbonate groups and these polycarbonates are used in eyeglass lenses, compact discs, and bulletproof glass. Small carbonate esters like dimethyl carbonate, and ethylene and propylene carbonate are used as solvents, dimethyl carbonate is a mild methylating agent as well. The chemistry of carbonate esters has been reviewed, carbonate esters can be divided into three categories by their structures. The first and general case is the dialkyl or diaryl carbonate that comprises a group with two R substituents. For example, poly and poly, Alternatively, the groups can be linked by a 2- or 3-carbon bridge, such as ethylene carbonate and trimethylene carbonate, substituents. The most common carbonates have the structure, RO—CO—OR. R is a chain with 8 to 18 carbon atoms, saturated or with one double bond. They are miscible in organic solvents but insoluble in water, unsaturation or branching on the alkyl chain lowers their melting point. The condensation of phosgene with an alcohol appears the most commonly used procedure to synthesize oleochemical carbonates, the polar nature of the carbonate moiety enables it to adhere strongly to metal surfaces. Thus, they are used as lubricant components which have a property for metal corrosion. Some C8 to C18 carbonates have been exploited in personal-care products, extraction of metal ions is improved by the use of the chelating properties of oleochemical carbonates when mixed with the metal-containing aqueous phase. Future developments will ensure a growing interest in these molecules, there are two main industrial ways of preparing carbonate esters, the reaction of an alcohol with phosgene, and the reaction of an alcohol with carbon monoxide and an oxidizer. Other carbonate esters may subsequently be prepared by transesterification, alcohols react with phosgene to yield carbonate esters according to the following reaction,2 ROH + COCl2 → ROCO2R +2 HCl Phenols react similarly. Polycarbonate derived from bisphenol A is produced in this manner, however, toxic phosgene is used, and stoichiometric quantities of base are required to neutralize the hydrogen chloride that is cogenerated. Chloroformate esters are intermediates in this process, annual production of cyclic carbonates was estimated at 100,000 tonnes per year in 2010. Industrially, ethylene and propylene oxides readily react with carbon dioxide to give ethylene and propylene carbonates, for example, C2H4O + CO2 → C2H4O2CO Catalysts for this reaction have been reviewed, as have non-epoxide routes to these cyclic carbonates
26.
Thioester
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In chemistry thioesters are compounds with the functional group R–S–CO–R. They are the product of esterification between an acid and a thiol. In biochemistry, the best-known thioesters are derivatives of coenzyme A, e. g. acetyl-CoA, for example, thioacetate esters are commonly prepared by alkylation of potassium thioacetate, CH3COSK + RX → CH3COSR + KCl The analogous alkylation of an acetate salt is rarely practiced. Acid anhydrides and some also give thioesters upon treatment with thiols in the presence of a base. Thioesters can be prepared from alcohols by the Mitsunobu reaction. They also arise via carbonylation of alkynes and alkenes in the presence of thiols, the carbonyl center in thioesters is reactive toward nucleophiles, even water. A reaction unique to thioesters is the Fukuyama coupling, in which the thioester is coupled with an organozinc halide by a palladium catalyst to give a ketone, the C-H groups adjacent to the carbonyl in thioesters are mildly acidic and undergo aldol condensations. This kind of reaction occurs in the biosynthesis of fatty acids, thioesters are common intermediates in many biosynthetic reactions, including the formation and degradation of fatty acids and mevalonate, precursor to steroids. Examples include malonyl-CoA, acetoacetyl-CoA, propionyl-CoA, and cinnamoyl-CoA, acetogenesis proceeds via the formation of acetyl-CoA. The biosynthesis of lignin, which comprises a fraction of the Earths land biomass. These thioesters arise analogously to those prepared synthetically, the difference being that the agent is ATP. In addition, thioesters play an important role in the tagging of proteins with ubiquitin, oxidation of the sulfur atom in thioesters is postulated in the bioactivation of the antithrombotic prodrugs ticlopidine, clopidogrel, and prasugrel. As posited in a Thioester World, thioesters are possible precursors to life, as de Duve explains, It is revealing that thioesters are obligatory intermediates in several key processes in which ATP is either used or regenerated. Thioesters are involved in the synthesis of all esters, including those found in complex lipids and they also participate in the synthesis of a number of other cellular components, including peptides, fatty acids, sterols, terpenes, porphyrins, and others. In addition, thioesters are formed as key intermediates in several particularly ancient processes that result in the assembly of ATP, in both these instances, the thioester is closer than ATP to the process that uses or yields energy. In other words, thioesters could have played the role of ATP in a thioester world initially devoid of ATP. Eventually, thioesters could have served to usher in ATP through its ability to support the formation of bonds between phosphate groups, in a thionoester, sulfur replaces the carbonyl oxygen in an ester. Such compounds are prepared by the reaction of the thioacyl chloride with an alcohol
27.
Lactone
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Lactones are cyclic esters of hydroxycarboxylic acids, containing a 1-oxacycloalkan-2-one structure, or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring. Lactones are formed by esterification of the corresponding hydroxycarboxylic acids. Lactones with three- or four-membered rings are very reactive, making their isolation difficult, special methods are normally required for the laboratory synthesis of small-ring lactones as well as those that contain rings larger than six-membered. The first carbon atom after the carbon in the group on the parent compound is labelled α, the second will be labeled β. Therefore, the prefixes also indicate the size of the ring, α-lactone = 3-membered ring, β-lactone = 4-membered, γ-lactone = 5-membered. The other suffix used to denote a lactone is -olide, used in substance class names like butenolide, macrolide, cardenolide or bufadienolide, the name lactone derives from the ring compound called lactide, which is formed from the dehydration of 2-hydroxypropanoic acid CH3-CH-COOH. Lactic acid, in turn, derives its name from its isolation from soured milk. An internal dehydration within the molecule of lactic acid would have produced alpha-propiolactone. Naturally occurring lactones are mainly saturated and unsaturated γ- and δ-lactones, the γ- and δ-lactones are intramolecular esters of the corresponding hydroxy fatty acids. They contribute to the aroma of butter, cheese and various foods, cyclopentadecanolide is responsible for the musklike odor of angelica root oil. Of the naturally occurring bicyclic lactones, phthalides are responsible for the odors of celery and lovage oils, many methods in ester synthesis can also be applied to that of lactones. In one industrial synthesis of oxandrolone the key lactone-forming step is an organic reduction - esterification, in halolactonization, an alkene is attacked by a halogen via electrophilic addition with the cationic intermediate captured intramolecularly by an adjacent carboxylic acid. Specific methods include Yamaguchi esterification, Shiina macrolactonization, Baeyer–Villiger oxidation, the most stable structure for lactones are the 5-membered γ-lactones and 6-membered δ-lactones because, as in all organic cycles,5 and 6 membered rings minimize the strain of bond angles. γ-lactones are so stable that, in the presence of acids at room temperature. β-lactones do exist, but can only be made by special methods, α-lactones can be detected as transient species in mass spectrometry experiments. Like straight-chained esters, the reaction of lactones is a reversible reaction. However, the constant of the hydrolysis reaction of the lactone is lower than that of the straight-chained ester i. e. the products are less favored in the case of the lactones. This is because although the enthalpies of the hydrolysis of esters and lactones are about the same, straight-chained esters give two products upon hydrolysis, making the entropy change more favorable than in the case of lactones which give only a single product
28.
Lactam
–
A lactam is a cyclic amide. The term is a portmanteau of the words lactone + amide, general synthetic methods exist for the organic synthesis of lactams. Lactams form by the rearrangement of oximes in the Beckmann rearrangement. Lactams form from cyclic ketones and hydrazoic acid in the Schmidt reaction, lactams form from cyclisation of amino acids. Lactams form from intramolecular attack of linear acyl derivatives from the nucleophilic abstraction reaction, in iodolactamization an iminium ion reacts with an halonium ion formed in situ by reaction of an alkene with iodine. At lower temp, β-lactam is the preferred product, at optimum temperatures, a highly useful γ-lactam known as Vince Lactam is obtained. Lactim is a cyclic carboximidic acid compound characterized by an endocyclic carbon-nitrogen double bond and it is formed when lactam undergoes tautomerization. β-Lactam β-Lactam antibiotics, which includes penicillins 2-Pyrrolidone 2-Piperidinone Caprolactam
29.
Hydroxamic acid
–
A hydroxamic acid is a class of organic compounds bearing the functional group RCNR, with R and R as organic residues and CO as a carbonyl group. They are amides wherein the NH center has an OH substitution and they are often used as metal chelators. Hydroxamic acids are prepared from either esters or acid chlorides by a reaction with hydroxylamine salts. For the synthesis of benzohydroxamic acid, the equations is. A well-known hydroxamic acid reaction is the Lossen rearrangement, sample gallery In the area of coordination chemistry, hydroxamic acids are excellent ligands. They deprotonate to give hydroxamates, which bind to metals ions as bidentate ligands, so high is the affinity of hydroxamates for ferric ions that nature has evolved families of hydroxamic acids to function as iron-binding compounds in bacteria. The resulting complexes are transported into the cell, where the iron is extracted and utilized metabolically, Hydroxamic acids are used extensively in flotation of rare earth minerals during the concentration and extraction of ores to be subjected to further processing. Some hydroxamic acids are HDAC inhibitors with anti-cancer properties, fosmidomycin is a natural hydroxamic acid inhibitor of 1-deoxy-D-xylulose-5-phosphate reductoisomerase. Hydroxamic acids have also investigated for reprocessing of irradiated fuel. Fouché, K. F. H. J. le Roux, complex formation of Zr and Hf with hydroxamic acids in acidic solutions. Journal of Inorganic and Nuclear Chemistry, barocas, A. F. Baroncelli, G. B. Journal of Inorganic and Nuclear Chemistry, journal of Inorganic and Nuclear Chemistry. Al-Jarrah, R. H. A. R. Al-Karaghouli, S. A. Al-Assaf, solvent extraction of uranium and some other metal ions with 2-N-butyl-2-ethyl octanohydroxamic acid. Journal of Inorganic and Nuclear Chemistry, Gopalan, Aravamudan S. Vincent J. Huber, Orhan Zincircioglu, Paul H. Smith. Novel tetrahydroxamate chelators for actinide complexation, synthesis and binding studies, journal of the Chemical Society, Chemical Communications, 1266–1268. Koshti, Nirmal, Vincent Huber, Paul Smith, Aravamudan S. Gopalan, design and synthesis of actinide specific chelators, Synthesis of new cyclam tetrahydroxamate and cyclam tetraacetonylacetone chelators
30.
Isocyanate
–
Isocyanate is the functional group with the formula R–N=C=O. Organic compounds that contain a group are referred to as isocyanates. An isocyanate that has two groups is known as a di-isocyanate. Di-isocyanates are manufactured for reactions with polyols in the production of polyurethanes, isocyanates should not be confused with cyanate esters and isocyanides, whose behaviors are very different. The cyanate functional group is arranged differently from the isocyanate group, isocyanides have the connectivity R-N≡C, lacking the oxygen of the cyanate groups. Isocyanates are produced by treating amines with phosgene, RNH2 + COCl2 → RNCO +2 HCl These reactions proceed via the intermediacy of a carbamoyl chloride, owing to the hazards associated with phosgene, the production of isocyanates requires special precautions. Isocyanates are electrophiles, and as such they are reactive toward a variety of nucleophiles including alcohols, amines, isocyanates react with water to form carbon dioxide, RNCO + H2O → RNH2 + CO2 This reaction is exploited in tandem with the production of polyurethane to give polyurethane foams. The carbon dioxide functions as a blowing agent, isocyanates also can react with themselves. Aliphatic di-isocyanates can form trimers, which are related to cyanuric acid. Isocyanates participate in Diels-Alder reactions, functioning as dienophiles, schmidt reaction, a reaction where a carboxylic acid is treated with ammonia and hydrazoic acid yielding an isocyanate. Curtius rearrangement degradation of an azide to an isocyanate and nitrogen gas. Lossen rearrangement, the conversion of an acid to an isocyanate via the formation of an O-acyl, sulfonyl. A monofunctional isocyanate of industrial significance is methyl isocyanate, which is used in the manufacture of pesticides, lD50s are typically several hundred milligrams per kilogram. Isocyanates are potentially dangerous irritants to the eyes and respiratory tract, methyl isocyanate was the chemical released in the Bhopal disaster causing the death of nearly 4000 people. Polyurethanes have variable curing times, and the presence of free isocyanates in foams vary accordingly, prepared by the Institute of Occupational Medicine for the Health and Safety Executive
31.
Carbonyl sulfide
–
Carbonyl sulfide is the organic compound with the formula OCS. Commonly written as COS, it is a flammable gas with an unpleasant odor. It is a molecule consisting of a carbonyl group double bonded to a sulfur atom. Carbonyl sulfide can be considered to be intermediate between carbon dioxide and carbon disulfide, both of which are isoelectronic with it. Carbonyl sulfide decomposes in the presence of humidity and bases to carbon dioxide and this compound is found to catalyze the formation of peptides from amino acids. This finding is an extension of the Miller–Urey experiment and it is suggested that carbonyl sulfide played a significant role in the origin of life. Carbonyl sulfide is the most abundant sulfur compound naturally present in the atmosphere, at 0. 5±0.05 ppb, because it is emitted from oceans, volcanoes, as such, it is a significant compound in the global sulfur cycle. Some carbonyl sulfide that is transported into the stratospheric sulfate layer is oxidized to sulfuric acid, sulfuric acid forms particulate which affects energy balance due to light scattering. The long atmospheric lifetime of COS makes it the source of stratospheric sulfate. Carbonyl sulfide is removed from the atmosphere by terrestrial vegetation by enzymes associated with the uptake of carbon dioxide during photosynthesis. Loss processes, such as these, limit the persistence of a molecule of COS in the atmosphere to a few years. Coal-fired power plants, biomass combustion, fish processing, combustion of refuse and plastics, petroleum manufacture, and manufacture of synthetic fibers, starch, and rubber. The average total worldwide release of carbonyl sulfide to the atmosphere has been estimated at about 3 million tons/year and it is also a significant sulfur-containing impurity in synthesis gas. Carbonyl sulfide is present in foodstuffs, such as cheese and prepared vegetables of the cabbage family, traces of COS are naturally present in grains and seeds in the range of 0. 05–0.1 mg·kg−1. Carbonyl sulfide is used as an intermediate in the production of thiocarbamate herbicides, Carbonyl sulfide is a potential alternative fumigant to methyl bromide and phosphine. In some cases, however, residues on the result in flavours that are unacceptable to consumers. Carbonyl sulfide is converted to the gaseous signaling molecule hydrogen sulfide by carbonic anhydrase enzymes in plants. Because of this chemistry, the release of carbonyl sulfide from small molecules has been identified as a strategy for delivering hydrogen sulfide in different biological contexts
32.
Meldrum's acid
–
Meldrums acid or 2, 2-dimethyl-1, 3-dioxane-4, 6-dione is an organic compound. The compound was first made in 1908 by Andrew Norman Meldrum by a reaction of malonic acid with acetone in acetic anhydride. Meldrum misidentified the structure as a β-lactone of β-hydroxyisopropylmalonic acid, the correct structure is shown on this page. As an alternative to its preparation, Meldrums acid can be synthesized from malonic acid, isopropenyl acetate. Meldrums acid has a high acidity with a pKa of 4.97, Meldrums acid high acidity was long considered anomalous given it is 8 orders of magnitude more acidic than the highly related compound, dimethyl malonate. In 2004, Ohwada and coworkers resolved the Meldrum acid anomaly by performing several calculations, because of its great acidity, Meldrums acid, like malonic acid, can serve as a reactant in Knoevenagel condensations. At high temperatures, Meldrums acid undergoes a pericyclic reaction that releases acetone and carbon dioxide, ketene intermediates can be isolated using flash vacuum pyrolysis of Meldrums acid at high temperatures. These highly electrophilic ketenes allow various chemical reactions to take place by reacting the unstable ketene with other chemicals, alternately, the pyrolysis can be performed in solution, allowing a one-pot reaction with chemicals already present rather than isolating the unstable ketene intermediate. With relative simplicity, is possible to form new C–C bonds, rings, amides, esters, the ability to form such diverse products makes Meldrums acid a very useful reagent for synthetic chemists. The position between the two groups in Meldrums acid is acidic, allowing simple alkylation and acylation at this position. For example, deprotonation and reaction with an alkyl halide allows formation of products containing carbon chains attached to the ring. Alternatively, this position can be acylated with an acid chloride. These two reactions allow Meldrums acid to serve as a scaffold for the synthesis of many different structures with various functional groups. Paired with the pyrolysis and nucleophilic trapping reaction, above, the products can be carried on to produce a range of different amide. Heating the acyl product in the presence of a leads to ester exchange. The reactive nature of the cyclic-diester allows good reactivity even for alcohols as hindered as t-butanol, ketoesters of this type are useful in the Knorr pyrrole synthesis. One hundred years of Meldrums acid, advances in the synthesis of pyridine and pyrimidine derivatives
33.
Acetylacetone
–
Acetylacetone is an organic compound that exists in two tautomeric forms that interconvert rapidly and are treated as a single compound in most applications. Although the compound is named as the diketone form, pentane-2, 4-dione. It is a liquid that is a precursor to acetylacetonate. It is also a block for the synthesis of heterocyclic compounds. The keto and enol forms of acetylacetone coexist in solution, these forms are tautomers, the enol form has C2v symmetry, meaning the hydrogen atom is shared equally between the two oxygen atoms. In the gas phase, the constant, Kketo→enol, is 11.7. The two tautomeric forms can easily be distinguished by NMR spectroscopy, IR spectroscopy, and other methods, the equilibrium constant tends to remain high in nonpolar solvents, the keto form becomes more favorable in polar, hydrogen-bonding solvents, such as water. The enol form is a analogue of a carboxylic acid. Acetylacetone is an acid, C5H8O2 ⇌ C 5H 7O−2 + H+ IUPAC recommended pKa values for this equilibrium in aqueous solution at 25 °C are 8.99 ±0.04,8.83 ±0.02 and 9.00 ±0.03. Values for mixed solvents are available, very strong bases, such as organolithium compounds, will deprotonate acetylacetone twice. The resulting dilithio species can then be alkylated at C-1, acetylacetone is prepared industrially by the thermal rearrangement of isopropenyl acetate. CH2COCMe → MeCCH2CMe Laboratory routes to acetylacetone begin also with acetone, some examples include C6H5CCH2CC6H5 and 3CCCH2CCC3. Hexafluoroacetylacetonate is also used to generate volatile metal complexes. Acetylacetone is a versatile precursor to heterocycles because both keto groups undergo condensation. Condensation with two aryl- and alkylamines to gives NacNacs, wherein the oxygen atoms in acetylacetone are replaced by NR, the acetylacetonate anion, acac−, forms complexes with many transition metal ions. Both oxygen atoms bind to the metal to form a six-membered chelate ring, in some cases the chelate effect is so strong that no added base is needed to form the complex. Since the metal complex carries no charge, it is often insoluble in water. Enzymatic breakdown, The enzyme acetylacetone dioxygenase cleaves the bond of acetylacetone, producing acetate
34.
Electronegativity
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Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom to attract electrons towards itself. An atoms electronegativity is affected by both its number and the distance at which its valence electrons reside from the charged nucleus. The higher the associated electronegativity number, the more an element or compound attracts electrons towards it, the term electronegativity was introduced by Jöns Jacob Berzelius in 1811, though the concept was known even before that and was studied by many chemists including Avogadro. It has been shown to correlate with a number of chemical properties. Electronegativity cannot be measured and must be calculated from other atomic or molecular properties. The most commonly used method of calculation is that proposed by Linus Pauling. This gives a quantity, commonly referred to as the Pauling scale. When other methods of calculation are used, it is conventional to quote the results on a scale that covers the range of numerical values. As it is calculated, electronegativity is not a property of an atom alone. Properties of a free atom include ionization energy and electron affinity, on the most basic level, electronegativity is determined by factors like the nuclear charge and the number/location of other electrons present in the atomic shells. The opposite of electronegativity is electropositivity, a measure of an ability to donate electrons. Caesium is the least electronegative element in the table, while fluorine is most electronegative. According to valence bond theory, of which Pauling was a notable proponent, hydrogen was chosen as the reference, as it forms covalent bonds with a large variety of elements, its electronegativity was fixed first at 2.1, later revised to 2.20. It is also necessary to decide which of the two elements is the more electronegative, to calculate Pauling electronegativity for an element, it is necessary to have data on the dissociation energies of at least two types of covalent bond formed by that element. A. L.32 e V This is an approximate equation, Pauling obtained it by noting that a bond can be approximately represented as a quantum mechanical superposition of a covalent bond and two ionic bond-states. e. The geometric mean is equal to the arithmetic mean - which is applied in the first formula above - when the energies are of the similar value. The square root of this energy, Pauling notes, is approximately additive. Thus, it is this semi-empirical formula for energy that underlies Pauling electronegativity concept
35.
Resonance (chemistry)
–
In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. A molecule or ion with such delocalized electrons is represented by several contributing structures, each contributing structure can be represented by a Lewis structure, with only an integer number of covalent bonds between each pair of atoms within the structure. Several Lewis structures are used collectively to describe the molecular structure. Electron delocalization lowers the energy of the substance and thus makes it more stable than any of the contributing structures. The difference between the energy of the actual structure and that of the contributing structure with the lowest potential energy is called the resonance energy or delocalization energy. An isomer is a molecule with the chemical formula but with different arrangements of atoms in space. Resonance contributors of a molecule, on the contrary, can differ by the arrangements of electrons. Therefore, the resonance hybrid cannot be represented by a combination of isomers, benzene undergoes substitution reactions, rather than addition reactions as typical for alkenes. He proposed that the bond in benzene is intermediate of a single and double bond. The mechanism of resonance was introduced into quantum mechanics by Werner Heisenberg in 1926 in a discussion of the states of the helium atom. He compared the structure of the atom with the classical system of resonating coupled harmonic oscillators. Linus Pauling used this mechanism to explain the partial valence of molecules in 1928, the alternative term mesomerism popular in German and French publications with the same meaning was introduced by C. K. Ingold in 1938, but did not catch on in the English literature. The current concept of effect has taken on a related. The double headed arrow was introduced by the German chemist Fritz Arndt who preferred the German phrase zwischenstufe or intermediate stage, the real structure is an intermediate of these structures represented by a resonance hybrid. The contributing structures are not isomers and they differ only in the position of electrons, not in the position of nuclei. Each Lewis formula must have the number of valence electrons. Bonds that have different bond orders in different contributing structures do not have typical bond lengths, the real structure has a lower total potential energy than each of the contributing structures would have. This means that it is more stable than each separate contributing structure would be and it is a common misconception that resonance structures are actual transient states of the molecule, with the molecule oscillating between them or existing as an equilibrium between them
36.
Electrophile
–
In chemistry, an electrophile is a reagent attracted to electrons. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to a rich centre. It participates in a reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids, most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons. The electrophiles are attacked by the most electron-populated part of one nucleophile and these occur between alkenes and electrophiles, often halogens as in halogen addition reactions. Common reactions include use of water to titrate against a sample to deduce the number of double bonds present. Forming of a bromonium ion The alkene is working as an electron donor. The three-membered bromonium ion 2 consisted with two atoms and a bromine atom forms with a release of Br−. Attacking of bromide ion The bromonium ion is opened by the attack of Br− from the back side and this yields the vicinal dibromide with an antiperiplanar configuration. When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether and this process is called AdE2 mechanism. Iodine, chlorine, sulfenyl ion, mercury cation, and dichlorocarbene also react through similar pathways, the direct conversion of 1 to 3 will appear when the Br− is large excess in the reaction medium. A β-bromo carbenium ion intermediate may be predominant instead of 3 if the alkene has a cation-stabilizing substituent like phenyl group, there is an example of the isolation of the bromonium ion 2. Hydrogen halides such as hydrogen chloride adds to alkenes to give alkyl halides in hydrohalogenation, for example, the reaction of Hcl with ethylene furnishes chloroethane. The reaction proceeds with an intermediate, being different from the above halogen addition. An example is shown below, Proton adds to one of the atoms on the alkene to form cation 1. Chloride ion combines with the cation 1 to form the adducts 2 and 3, in this manner, the stereoselectivity of the product, that is, from which side Cl− will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H+ is usually decided by Markovnikovs rule, thus, H+ attacks the carbon atom that carries fewer substituents so as the more stabilized carbocation will form. This process is called A-SE2 mechanism, hydrogen fluoride and hydrogen iodide react with alkenes in a similar manner, and Markovnikov-type products will be given
37.
Nucleophile
–
A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. All molecules or ions with a pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases, Nucleophilic describes the affinity of a nucleophile to the nuclei. Nucleophilicity, sometimes referred to as strength, refers to a substances nucleophilic character and is often used to compare the affinity of atoms. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis, nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge. The terms nucleophile and electrophile were introduced by Christopher Kelk Ingold in 1933, the word nucleophile is derived from nucleus and the Greek word φιλος, philos for love. In general, in a row across the table, the more basic the ion the more reactive it is as a nucleophile. g. The iodide ion is more nucleophilic than the fluoride ion, many schemes attempting to quantify relative nucleophilic strength have been devised. The following empirical data have been obtained by measuring reaction rates for a number of reactions involving a large number of nucleophiles and electrophiles. Nucleophiles displaying the so-called alpha effect are usually omitted in this type of treatment. This treatment results in the values for typical nucleophilic anions, acetate 2.7, chloride 3.0, azide 4.0, hydroxide 4.2, aniline 4.5, iodide 5.0. Typical substrate constants are 0.66 for ethyl tosylate,0.77 for β-propiolactone,1.00 for 2, 3-epoxypropanol,0.87 for benzyl chloride, and 1.43 for benzoyl chloride. The equation predicts that, in a nucleophilic displacement on benzyl chloride, the Ritchie equation, derived in 1972, is another free-energy relationship, log 10 = N + where N+ is the nucleophile dependent parameter and k0 the reaction rate constant for water. In this equation, a substrate-dependent parameter like s in the Swain–Scott equation is absent, the equation states that two nucleophiles react with the same relative reactivity regardless of the nature of the electrophile, which is in violation of the Reactivity–selectivity principle. For this reason this equation is called the constant selectivity relationship. Many other reaction types have since been described. Typical Ritchie N+ values are,0.5 for methanol,5.9 for the anion,7.5 for the methoxide anion,8.5 for the azide anion. The values for the relative cation reactivities are -0.4 for the malachite green cation, +2.6 for the benzenediazonium cation, the constant s is defined as 1 with 2-methyl-1-pentene as the nucleophile
38.
Ion
–
An ion is an atom or a molecule in which the total number of electrons is not equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge. Ions can be created, by chemical or physical means. In chemical terms, if an atom loses one or more electrons. If an atom gains electrons, it has a net charge and is known as an anion. Ions consisting of only a single atom are atomic or monatomic ions, because of their electric charges, cations and anions attract each other and readily form ionic compounds, such as salts. In the case of ionization of a medium, such as a gas, which are known as ion pairs are created by ion impact, and each pair consists of a free electron. The word ion comes from the Greek word ἰόν, ion, going and this term was introduced by English physicist and chemist Michael Faraday in 1834 for the then-unknown species that goes from one electrode to the other through an aqueous medium. Faraday also introduced the words anion for a charged ion. In Faradays nomenclature, cations were named because they were attracted to the cathode in a galvanic device, arrhenius explanation was that in forming a solution, the salt dissociates into Faradays ions. Arrhenius proposed that ions formed even in the absence of an electric current, ions in their gas-like state are highly reactive, and do not occur in large amounts on Earth, except in flames, lightning, electrical sparks, and other plasmas. These gas-like ions rapidly interact with ions of charge to give neutral molecules or ionic salts. These stabilized species are commonly found in the environment at low temperatures. A common example is the present in seawater, which are derived from the dissolved salts. Electrons, due to their mass and thus larger space-filling properties as matter waves, determine the size of atoms. Thus, anions are larger than the parent molecule or atom, as the excess electron repel each other, as such, in general, cations are smaller than the corresponding parent atom or molecule due to the smaller size of its electron cloud. One particular cation contains no electrons, and thus consists of a single proton - very much smaller than the parent hydrogen atom. Since the electric charge on a proton is equal in magnitude to the charge on an electron, an anion, from the Greek word ἄνω, meaning up, is an ion with more electrons than protons, giving it a net negative charge. A cation, from the Greek word κατά, meaning down, is an ion with fewer electrons than protons, there are additional names used for ions with multiple charges
39.
Cyanide ion
–
A cyanide is any chemical compound that contains monovalent combining group CN. This group, known as the group, consists of a carbon atom triple-bonded to a nitrogen atom. The cyanide ion is isoelectronic with carbon monoxide and with molecular nitrogen, organic cyanides are usually called nitriles, in these, the CN group is linked by a covalent bond to a carbon-containing group, such as methyl in methyl cyanide. Because they do not release cyanide ions, nitriles are generally toxic, or in the case of insoluble polymers such as acrylic fiber. Hydrocyanic acid, also known as hydrogen cyanide, or HCN, is a volatile liquid used to prepare acrylonitrile, which is used in the production of acrylic fibers, synthetic rubber. Cyanides are employed in a number of processes, including fumigation, case hardening of iron and steel, electroplating. In nature, substances yielding cyanide are present in seeds, such as the pit of the cherry. In IUPAC nomenclature, organic compounds that have a functional group are called nitriles. An example of a nitrile is CH3CN, acetonitrile, also known as methyl cyanide, nitriles usually do not release cyanide ions. A functional group with a hydroxyl and cyanide bonded to the carbon is called cyanohydrin. Unlike nitriles, cyanohydridins do release hydrogen cyanide, in inorganic chemistry, salts containing the C≡N− ion are referred to as cyanides. The word is derived from the Greek kyanos, meaning dark blue, cyanides are produced by certain bacteria, fungi, and algae and are found in a number of plants. Cyanides are found in substantial amounts in certain seeds and fruit stones, e. g. those of apricots, apples, in plants, cyanides are usually bound to sugar molecules in the form of cyanogenic glycosides and defend the plant against herbivores. Cassava roots, an important potato-like food grown in tropical countries, the Madagascar bamboo Cathariostachys madagascariensis produces cyanide as a deterrent to grazing. In response, the bamboo lemur, which eats the bamboo, has developed a high tolerance to cyanide. The cyanide radical CN· has been identified in interstellar space, the cyanide radical is used to measure the temperature of interstellar gas clouds. Hydrogen cyanide is produced by the combustion or pyrolysis of certain materials under oxygen-deficient conditions, for example, it can be detected in the exhaust of internal combustion engines and tobacco smoke. Certain plastics, especially derived from acrylonitrile, release hydrogen cyanide when heated or burnt
40.
Lone pair
–
In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom and is sometimes called a non-bonding pair. Lone pairs are found in the outermost electron shell of atoms and they can be identified by using a Lewis structure. Electron pairs are therefore considered lone pairs if two electrons are paired but are not used in chemical bonding, thus, the number of lone pair electrons plus the number of bonding electrons equals the total number of valence electrons around an atom. Lone pair is a used in VSEPR theory which explains the shapes of molecules. They are also referred to in the chemistry of Lewis acids, however, not all non-bonding pairs of electrons are considered by chemists to be lone pairs. Examples are the transition metals where the non-bonding pairs do not influence molecular geometry and are said to be stereochemically inactive, in VSEPR theory the electron pairs on the oxygen atom in water form the vertices of a tetrahedron with the lone pairs on two of the four vertices. The H–O–H bond angle is with 104. 5°, less than the 109° predicted for an angle. The pairs often exhibit a negative polar character with their charge density and are located closer to the atomic nucleus on average compared to the bonding pair of electrons. The presence of a lone pair decreases the angle between the bonding pair of electrons, due to their high electric charge which causes great repulsion between the electrons. They are also used in the formation of a dative bond, for example, the creation of the hydronium ion occurs when acids are dissolved in water and is due to the oxygen atom donating a lone pair to the hydrogen ion. This can be more clearly when looked at it in two more common molecules. For example, in carbon dioxide, the atoms are on opposite sides of the carbon. Due to the force of the oxygen atoms lone pairs. This is an illustration of the VSEPR theory, lone pairs can make a contribution to a molecules dipole moment. NH3 has a moment of 1.47 D. There is also an associated with the lone pair and this reinforces the contribution made by the polar covalent N-H bonds to ammonias dipole moment. In contrast to NH3, NF3 has a lower dipole moment of 0.24 D. A stereochemically active lone pair is expected for divalent lead
41.
Pi bond
–
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital. Each of these orbitals is zero at a shared nodal plane. The same plane is also a plane for the molecular orbital of the pi bond. The Greek letter π in their name refers to p orbitals, P orbitals often engage in this sort of bonding. D orbitals also engage in pi bonding, and form part of the basis for metal-metal multiple bonding, Pi bonds are usually weaker than sigma bonds. From the perspective of quantum mechanics, this weakness is explained by significantly less overlap between the component p-orbitals due to their parallel orientation. This is contrasted by sigma bonds which form bonding orbitals directly between the nuclei of the atoms, resulting in greater overlap and a strong sigma bond. Pi bonds result from overlap of atomic orbitals that are in contact through two areas of overlap, pi-bonds are more diffuse bonds than the sigma bonds. Electrons in pi bonds are referred to as pi electrons. Molecular fragments joined by a pi bond cannot rotate about that bond without breaking the pi bond, for homonuclear diatomic molecules, bonding π molecular orbitals have only the one nodal plane passing through the bonded atoms, and no nodal planes between the bonded atoms. The corresponding antibonding, or π* molecular orbital, is defined by the presence of a nodal plane between these two bonded atoms. A typical double bond consists of one bond and one pi bond, for example. A typical triple bond, for example in acetylene, consists of one sigma bond, two pi bonds are the maximum that can exist between a given pair of atoms. Quadruple bonds are rare and can be formed only between transition metal atoms, and consist of one sigma bond, two pi bonds and one delta bond. A pi bond is weaker than a bond, but the combination of pi. The enhanced strength of a multiple bond versus a single is indicated in many ways, for example, in organic chemistry, carbon–carbon bond lengths are about 154 pm in ethane,134 pm in ethylene and 120 pm in acetylene. More bonds make the total bond shorter and stronger, a pi bond can exist between two atoms that do not have a net sigma-bonding effect between them. In certain metal complexes, pi interactions between an atom and alkyne and alkene pi antibonding orbitals form pi-bonds
42.
Addition reaction
–
An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one. Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with double bonds. Molecules containing carbon—hetero double bonds like carbonyl groups, or imine groups, can undergo addition, an addition reaction is the reverse of an elimination reaction. For instance, the hydration of an alkene to an alcohol is reversed by dehydration, there are two main types of polar addition reactions, electrophilic addition and nucleophilic addition. Two non-polar addition reactions exist as well, called free-radical addition, addition reactions are also encountered in polymerizations and called addition polymerization. In the related addition-elimination reaction an addition reaction is followed by an elimination reaction, in the majority of reactions it involves addition of nucleophiles to carbonyl compounds in what is called nucleophilic acyl substitution. Other addition-elimination reactions are the reaction of an amine to an imine. The hydrolysis of nitriles to carboxylic acids is also a form of addition-elimination
43.
Condensation reaction
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A condensation reaction is a chemical reaction in which two molecules or moieties, often functional groups, combine to form a larger molecule, together with the loss of a small molecule. Possible small molecules that are lost include water, acetic acid, hydrogen chloride, or methanol, condensations producing water as a byproduct are the opposite reaction of transformations involving hydrolysis, which split a reactant into two new species through addition of a water molecule. Condensation can be intermolecular or intramolecular, a simple example of an intermolecular condensation is the joining of two amino acids in the peptide bond, as is characteristic of all proteins. The condensation reaction-speed can be catalyzed, by adding a concentrated acid to the reaction. It effects it by acidifying the environment whereas the reaction takes place the acid thereby binds with the water molecules, condensation reactions can follow a variety of different reaction mechanisms, depending on the groups reacting and the conditions employed to perform the reaction. Many artificial, man-made chemical reactions, and many biological transformations are condensation reactions, condensation polymerization produces many important polymers, for example, nylon, polyester, and other condensation polymers and various epoxies. It is also the basis for the formation of silicates and polyphosphates. In condensation polymerization or step-growth polymerization, multiple reactions take place, joining monomers. It occurs for example in the synthesis of polyesters or nylons and it can be homopolymerization of a single monomer A-B with two different end groups that condense, or copolymerization of two co-monomers A-A and B-B. Condensation polymerization releases multiple small molecules, in contrast to polyaddition reactions, in general, condensation polymers form more slowly than addition polymers, often requiring heat. They are generally lower in molecular weight, monomers are consumed early in the reaction, the terminal functional groups remain active throughout, and short chains combine to form longer chains. A high conversion rate is required to achieve high molecular weights, anabolism Hydrolysis, the opposite of a condensation reaction Condensed tannins
44.
Lewis acids and bases
–
A Lewis acid is a chemical species that reacts with a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that donates a pair of electrons to a Lewis acid to form a Lewis adduct, for example, OH− and NH3 are Lewis bases, because they can donate a lone pair of electrons. In the adduct, the Lewis acid and base share an electron pair furnished by the Lewis base, usually the terms Lewis acid and Lewis base are defined within the context of a specific chemical reaction. For example, in the reaction of Me3B and NH3 to give Me3BNH3, Me3B acts as a Lewis acid, the terminology refers to the contributions of Gilbert N. Lewis. Another example is boron trifluoride etherate, BF3•Et2O, the center dot is also used to represent hydrate coordination in various crystals, as in MgSO4•7H2O for hydrated magnesium sulfate. In general, however, the bond is viewed as simply somewhere along a continuum between idealized covalent bonding and ionic bonding. Classically, the term Lewis acid is restricted to trigonal planar species with an empty p orbital, for the purposes of discussion, even complex compounds such as Et3Al2Cl3 and AlCl3 are treated as trigonal planar Lewis acids. Other reactions might simply be referred to as acid-catalyzed reactions, some compounds, such as H2O, are both Lewis acids and Lewis bases, because they can either accept a pair of electrons or donate a pair of electrons, depending upon the reaction. Simplest are those that react directly with the Lewis base, but more common are those that undergo a reaction prior to forming the adduct. Again, the description of a Lewis acid is used loosely. For example, in solution, bare protons do not exist, BF3 + OMe2 → BF3OMe2 Both BF4− and BF3OMe2 are Lewis base adducts of boron trifluoride. Well known cases are the aluminium trihalides, which are viewed as Lewis acids. Aluminium trihalides, unlike the boron trihalides, do not exist in the form AlX3, a simpler case is the formation of adducts of borane. Monomeric BH3 does not exist appreciably, so the adducts of borane are generated by degradation of diborane, B2H6 +2 H− →2 BH4− In this case, an intermediate B2H7− can be isolated. Many metal complexes serve as Lewis acids, but usually only after dissociating a more weakly bound Lewis base, 2+ +6 NH3 → 2+ +6 H2O The proton is one of the strongest but is also one of the most complicated Lewis acids. It is convention to ignore the fact that a proton is heavily solvated, the key step is the acceptance by AlCl3 of a chloride ion lone-pair, forming AlCl4− and creating the strongly acidic, that is, electrophilic, carbonium ion. RCl +AlCl3 → R+ + AlCl4− A Lewis base is an atomic or molecular species where the highest occupied molecular orbital is highly localized, typical Lewis bases are conventional amines such as ammonia and alkyl amines. Other common Lewis bases include pyridine and its derivatives, some of the main classes of Lewis bases are amines of the formula NH3−xRx where R = alkyl or aryl
