1.
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
2.
Pericyclic reaction
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In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions, pericyclic reactions often have related stepwise radical processes associated with them. Some pericyclic reactions, such as the cycloaddition, are controversial because their mechanism is not definitively known to be concerted, a large photoinduced hydrogen sigmatropic shift was utilized in a corrin synthesis performed by Albert Eschenmoser containing a 16π system. Due to the principle of microscopic reversibility, there is a set of retro pericyclic reactions. Thus substituents in the diene can significantly affect the rate of the not only by their electronic character. Thus for example cis I -substituted butadiene I is less reactive than its trans isomer II since a bulky R disfavors the cisoid conformation, bulky 2-substituents in the diene favor the cisoid conformation more than the transoid and thus the diene in this case is more reactive
3.
Sigma bond
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In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals, sigma bonding is most simply defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis, by this definition, common forms of sigma bonds are s+s, pz+pz, s+pz and dz2+dz2. Quantum theory also indicates that molecular orbitals of identical symmetry actually mix or hybridize, as a practical consequence of this mixing of diatomic molecules, the wavefunctions s+s and pz+pz molecular orbitals become blended. The extent of this depends on the relative energies of the MOs of like symmetry. For homodiatomics, bonding σ orbitals have no nodal planes at which the wavefunction is zero, the corresponding antibonding, or σ* orbital, is defined by the presence of one nodal plane between the two bonded atoms. Sigma bonds are the strongest type of covalent bonds due to the overlap of orbitals. The symbol σ is the Greek letter sigma, when viewed down the bond axis, a σ MO has a circular symmetry, hence resembling a similarly sounding s atomic orbital. Typically, a bond is a sigma bond while a multiple bond is composed of one sigma bond together with pi or other bonds. A double bond has one sigma plus one pi bond, sigma bonds are obtained by head-on overlapping of atomic orbitals. The concept of bonding is extended to describe bonding interactions involving overlap of a single lobe of one orbital with a single lobe of another. For example, propane is described as consisting of ten sigma bonds, transition metal complexes that feature multiple bonds, such as the dihydrogen complex, have sigma bonds between the multiple bonded atoms. These sigma bonds can be supplemented with other bonding interactions, such as donation, as in the case of W32. Organic molecules are often cyclic compounds containing one or more rings, such as benzene, according to the sigma bond rule, the number of sigma bonds in a molecule is equivalent to the number of atoms plus the number of rings minus one. Nσ = Natoms + Nrings −1 A molecule with no rings can be represented as a tree with a number of equal to the number of atoms minus one. There is no more than 1 sigma bond between any two atoms, molecules with rings have additional sigma bonds, such as benzene rings, which have 6 C−C sigma bonds within the ring for 6 carbon atoms. The anthracene molecule, C14H10, has three rings so that the rule gives the number of bonds as 24 +3 −1 =26. In this case there are 16 C−C sigma bonds and 10 C−H bonds
4.
Intramolecular force
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Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule. Intramolecular hydride transfer intramolecular hydrogen bond In intramolecular organic reactions, two sites are contained within a single molecule. This creates a high effective concentration, and, therefore. Examples of intramolecular reactions are the Smiles rearrangement, the Dieckmann condensation, tethered Intramolecular reactions entail the formation of four-membered rings from two functional groups in the same molecule, each functional group contributes two atoms. The length of the effects the stereochemical outcome of the reaction. Longer tethers tend to generate the product where the terminal carbon of the alkene is linked to the α-carbon of the enone. When the tether consists only two carbons, the product is generated where the β-carbon of the enone is connected to the terminal carbon of the alkene. Tethered reactions have been used to synthesize organic compounds with interesting ring systems, for example, photocyclization was used to construct the tricyclic core structure in Ginkgolide B by E. J. Corey and co-workers in 1988. Popular choices of tether contain a carbonate ester, boronic ester, silyl ether, the main hurdle for this strategy to work is selecting the proper length for the tether and making sure reactive groups have an optimal orientation with respect to each other. No reaction takes place when these groups are replaced by smaller methyl groups. Another example is a cycloaddition with two alkene groups tethered through a silicon acetal group, which is subsequently cleaved by TBAF yielding the endo-diol. Without the tether, the exo isomer forms
5.
Compound (linguistics)
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In linguistics, a compound is a lexeme that consists of more than one stem. Compounding, composition or nominal composition is the process of formation that creates compound lexemes. That is, in terms, compounding occurs when two or more words are joined to make one longer word. The meaning of the compound may be similar to or different from the meanings of its components in isolation, with very few exceptions, English compound words are stressed on their first component stem. The process occurs readily in other Germanic languages for different reasons, words can be concatenated both to mean the same as the sum of two words or where an adjective and noun are compounded. The addition of affix morphemes to words should not be confused with nominal composition, compound formation rules vary widely across language types. In a synthetic language, the relationship between the elements of a compound may be marked with a case or other morpheme. This latter pattern is common throughout the Semitic languages, though in some it is combined with a genitive case. Agglutinative languages tend to very long words with derivational morphemes. Compounds may or may not require the use of derivational morphemes also, the longest compounds in the world may be found in the Finnic and Germanic languages. German examples include Farbfernsehgerät, Funkfernbedienung, and the jocular word Donaudampfschifffahrtsgesellschaftskapitänsmütze, in Finnish, although there is theoretically no limit to the length of compound words, words consisting of more than three components are rare. Even those with less than three components can look mysterious to non-Finnish such as hätäuloskäytävä, compounds can be rather long when translating technical documents from English to some other language, since the lengths of the words are theoretically unlimited, especially in chemical terminology. For example, the English compound doghouse, where house is the head, endocentric compounds tend to be of the same part of speech as their head, as in the case of doghouse. For example, the English compound white-collar is neither a kind of collar nor a white thing, in an exocentric compound, the word class is determined lexically, disregarding the class of the constituents. For example, a must-have is not a verb but a noun, the meaning of this type of compound can be glossed as whose B is A, where B is the second element of the compound and A the first. A bahuvrihi compound is one whose nature is expressed by neither of the words, copulative compounds are compounds with two semantic heads. Appositional compounds are lexemes that have two attributes that classify the compound, most natural languages have compound nouns. The positioning of the words according to the language
6.
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
7.
Rearrangement reaction
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A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule. In the example below the substituent R moves from carbon atom 1 to carbon atom 2, − C | R − C − C − ⟶ − C − C | R − C − Intermolecular rearrangements also take place. A rearrangement is not well represented by simple and discrete electron transfers, the actual mechanism of alkyl groups moving, as in Wagner-Meerwein rearrangement, probably involves transfer of the moving alkyl group fluidly along a bond, not ionic bond-breaking and forming. In pericyclic reactions, explanation by orbital interactions give a better picture than simple electron transfers. In allylic rearrangement, the reaction is indeed ionic, three key rearrangement reactions are 1, 2-rearrangements, pericyclic reactions and olefin metathesis. A1, 2-rearrangement is a reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible, examples are hydride shifts and the Claisen rearrangement, Olefin metathesis is a formal exchange of the alkylidene fragments in two alkenes. It is a reaction with carbene, or more accurately. In this example, a compound is dimerized with the expulsion of ethene
8.
Pi bond
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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
9.
Lewis acids and bases
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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
10.
Catalysis
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Catalysis is the increase in the rate of a chemical reaction due to the participation of an additional substance called a catalyst. In most cases, reactions occur faster with a catalyst because they require less activation energy, furthermore, since they are not consumed in the catalyzed reaction, catalysts can continue to act repeatedly. Often only tiny amounts are required in principle, in the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations, the effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons or promoters. Catalyzed reactions have an activation energy than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature. However, the mechanics of catalysis is complex. Usually, the catalyst participates in this slowest step, and rates are limited by amount of catalyst, in heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. A nanomaterial-based catalyst is an example of a heterogeneous catalyst, analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, in heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a system or sublimate in a solid–gas system. The production of most industrially important chemicals involves catalysis, similarly, most biochemically significant processes are catalysed. Research into catalysis is a field in applied science and involves many areas of chemistry, notably organometallic chemistry. Catalysis is relevant to aspects of environmental science, e. g. the catalytic converter in automobiles. Many transition metals and transition metal complexes are used in catalysis as well, Catalysts called enzymes are important in biology. A catalyst works by providing a reaction pathway to the reaction product. The rate of the reaction is increased as this route has a lower activation energy than the reaction route not mediated by the catalyst. The disproportionation of hydrogen peroxide creates water and oxygen, as shown below,2 H2O2 →2 H2O + O2 This reaction is preferable in the sense that the reaction products are more stable than the starting material, though the uncatalysed reaction is slow. In fact, the decomposition of hydrogen peroxide is so slow that hydrogen peroxide solutions are commercially available and this reaction is strongly affected by catalysts such as manganese dioxide, or the enzyme peroxidase in organisms
11.
Cope rearrangement
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The Cope rearrangement is an extensively studied organic reaction involving the -sigmatropic rearrangement of 1, 5-dienes. It was developed by Arthur C, for example, 3-methyl-1, 5-hexadiene heated to 300 °C yields 1, 5-heptadiene. The Cope rearrangement causes the fluxional states of the molecules in the bullvalene family, although the Cope rearrangement is concerted and pericyclic, it can also be considered to go via a transition state that is energetically and structurally equivalent to a diradical. This is an explanation which remains faithful to the uncharged nature of the Cope transition state. This also explains the energy requirement to perform a Cope rearrangement. Although illustrated in the conformation, the Cope can also occur with cyclohexadienes in the boat conformation. The above description of the state is not quite correct. It is currently accepted that the Cope rearrangement follows an allowed concerted route through a homoaromatic transition state. That is unless the energy surface is perturbed to favor the diradical. The rearrangement is used in organic synthesis. It is symmetry-allowed when it is suprafacial on all components, the transition state of the molecule passes through a boat or chair like transition state. A trans double bond in the ring would be too strained, the reaction occurs under thermal conditions. The driving force of the reaction is the loss of strain from the cyclobutane ring, another variation of the Cope rearrangement is the heteroatom Cope reactions such as the aza-Cope rearrangements. Another widely studied sigmatropic rearrangement is the Claisen rearrangement
12.
Claisen rearrangement
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The Claisen rearrangement is a powerful carbon–carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen. The heating of a vinyl ether will initiate a -sigmatropic rearrangement to give a γ. Discovered in 1912, the Claisen rearrangement is the first recorded example of a -sigmatropic rearrangement, the Claisen rearrangement is an exothermic, concerted pericyclic reaction. Woodward–Hoffmann rules show a suprafacial, stereospecific reaction pathway, the kinetics are of the first order and the whole transformation proceeds through a highly ordered cyclic transition state and is intramolecular. Crossover experiments eliminate the possibility of the rearrangement occurring via a reaction mechanism and are consistent with an intramolecular process. There are substantial solvent effects observed in the Claisen rearrangement, where polar solvents tend to accelerate the reaction to a greater extent, hydrogen-bonding solvents gave the highest rate constants. For example, ethanol/water solvent mixtures give rate constants 10-fold higher than sulfolane, trivalent organoaluminium reagents, such as trimethylaluminium, have been shown to accelerate this reaction. The first reported Claisen rearrangement is the rearrangement of an allyl phenyl ether to intermediate 1. Meta-substitution affects the regioselectivity of this rearrangement, for example, electron withdrawing groups at the meta-position direct the rearrangement to the ortho-position, while electron donating groups, direct rearrangement to the para-position. Additionally, presence of ortho-substituents exclusively leads to para-substituted rearrangement products, if an aldehyde or carboxylic acid occupies the ortho or para positions, the allyl side-chain displaces the group, releasing it as carbon monoxide or carbon dioxide, respectively. The Bellus–Claisen rearrangement is the reaction of ethers, amines, and thioethers with ketenes to give γ, δ-unsaturated esters, amides. This transformation was observed by Bellus in 1979 through their synthesis of a key intermediate of an insecticide. Halogen substituted ketenes are often used in reaction for their high electrophilicity. Numerous reductive methods for the removal of the resulting α-haloesters, amides, the Bellus-Claisen offers synthetic chemists a unique opportunity for ring expansion strategies. The Eschenmoser–Claisen rearrangement proceeds by heating allylic alcohols in the presence of N, N-dimethylacetamide dimethyl acetal to form γ and this was developed by Albert Eschenmoser in 1964. Eschenmoser-Claisen rearrangement was used as a key step in the synthesis of morphine. Mechanism, The Ireland–Claisen rearrangement is the reaction of a carboxylate with a strong base to give a γ. The rearrangement proceeds via silylketene acetal, which is formed by trapping the lithium enolate with chlorotrimethylsilane, like the Bellus-Claisen, Ireland-Claisen rearrangement can take place at room temperature and above
13.
Carroll rearrangement
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The Carroll rearrangement is a rearrangement reaction in organic chemistry and involves the transformation of a β-keto allyl ester into a α-allyl-β-ketocarboxylic acid. This organic reaction is accompanied by decarboxylation and the product is a γ. The Carroll rearrangement is an adaptation of the Claisen rearrangement and effectively a decarboxylative Allylation, the Carroll rearrangement in the presence of base and with high reaction temperature takes place through an intermediate enol which then rearranges in an electrocyclic Claisen rearrangement. With palladium as a catalyst, the reaction is much milder with an intermediate allyl cation / carboxylic acid anion organometallic complex, decarboxylation precedes allylation as evidenced by this reaction catalyzed by tetrakispalladium, By introducing suitable chiral ligands, the reaction becomes enantioselective. The first reported asymmetric rearrangement is catalyzed by trisdipalladium and the Trost ligand and this reaction delivers one enantiomer with 88% ee. It remains to be if this reaction will have a wide scope because the acetamido group appears to be a prerequisite. The same catalyst but a different ligand is employed in this enantioconvergent reaction, The scope is extended to asymmetric α-alkylation of ketones masked as their enol carbonate esters
14.
Fischer indole synthesis
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The Fischer indole synthesis is a chemical reaction that produces the aromatic heterocycle indole from a phenylhydrazine and an aldehyde or ketone under acidic conditions. The reaction was discovered in 1883 by Hermann Emil Fischer, today antimigraine drugs of the triptan class are often synthesized by this method. The choice of acid catalyst is very important, brønsted acids such as HCl, H2SO4, polyphosphoric acid and p-toluenesulfonic acid have been used successfully. Lewis acids such as boron trifluoride, zinc chloride, iron chloride, the reaction of a phenylhydrazine with a carbonyl initially forms a phenylhydrazone which isomerizes to the respective enamine. After protonation, a cyclic -sigmatropic rearrangement occurs producing an imine, the resulting imine forms a cyclic aminoacetal, which under acid catalysis eliminates NH3, resulting in the energetically favorable aromatic indole. Isotopic labelling studies show that the nitrogen of the starting phenylhydrazine is incorporated into the resulting indole. Via a palladium-catalyzed reaction, the Fischer indole synthesis can be effected by cross coupling aryl bromides and hydrazones and this result supports the previously proposed intermediacy as hydrazone intermediates in the classical Fischer indole synthesis. These N-arylhydrazones undergo exchange with other ketones, expanding the scope of this method, bartoli indole synthesis Japp-Klingemann indole synthesis Leimgruber-Batcho indole synthesis Larock indole synthesis Madelung synthesis Reissert synthesis Gassman synthesis Nenitzescu synthesis
15.
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
16.
Hydrogen
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Hydrogen is a chemical element with chemical symbol H and atomic number 1. With a standard weight of circa 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form is the most abundant chemical substance in the Universe, non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton, the universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays an important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a charge when it is known as a hydride. The hydrogen cation is written as though composed of a bare proton, Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. Industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production, mostly for the fertilizer market, Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks. Hydrogen gas is flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol,2 H2 + O2 →2 H2O +572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%, the explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C, the detection of a burning hydrogen leak may require a flame detector, such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames, the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element, the ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 91 nm wavelength. The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, however, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. The most complicated treatments allow for the effects of special relativity
17.
Atomic orbital
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In quantum mechanics, an atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atoms nucleus. The term, atomic orbital, may refer to the physical region or space where the electron can be calculated to be present. Each such orbital can be occupied by a maximum of two electrons, each with its own quantum number s. The simple names s orbital, p orbital, d orbital and these names, together with the value of n, are used to describe the electron configurations of atoms. They are derived from the description by early spectroscopists of certain series of alkali metal spectroscopic lines as sharp, principal, diffuse, Orbitals for ℓ >3 continue alphabetically, omitting j because some languages do not distinguish between the letters i and j. Atomic orbitals are the building blocks of the atomic orbital model. In this model the electron cloud of an atom may be seen as being built up in an electron configuration that is a product of simpler hydrogen-like atomic orbitals. The lowest possible energy an electron can take is therefore analogous to the frequency of a wave on a string. Higher energy states are similar to harmonics of the fundamental frequency. The electrons are never in a point location, although the probability of interacting with the electron at a single point can be found from the wave function of the electron. Particle-like properties, There is always a number of electrons orbiting the nucleus. Electrons jump between orbitals in a particle-like fashion, for example, if a single photon strikes the electrons, only a single electron changes states in response to the photon. The electrons retain particle-like properties such as, each state has the same electrical charge as the electron particle. Each wave state has a single discrete spin and this can depend upon its superposition. Thus, despite the popular analogy to planets revolving around the Sun, in addition, atomic orbitals do not closely resemble a planets elliptical path in ordinary atoms. A more accurate analogy might be that of a large and often oddly shaped atmosphere, atomic orbitals exactly describe the shape of this atmosphere only when a single electron is present in an atom. This is due to the uncertainty principle, atomic orbitals may be defined more precisely in formal quantum mechanical language
18.
Stereochemistry
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Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. An important branch of stereochemistry is the study of chiral molecules, stereochemistry is also known as 3D chemistry because the prefix stereo- means three-dimensionality. The study of stereochemistry focuses on stereoisomers and spans the spectrum of organic, inorganic, biological, physical. This property, the physical property in which the two types of tartrate salts differed, is due to optical isomerism. In 1874, Jacobus Henricus van t Hoff and Joseph Le Bel explained optical activity in terms of the arrangement of the atoms bound to carbon. Cahn–Ingold–Prelog priority rules are part of a system for describing a molecules stereochemistry and they rank the atoms around a stereocenter in a standard way, allowing the relative position of these atoms in the molecule to be described unambiguously. A Fischer projection is a way to depict the stereochemistry around a stereocenter. An often cited example of the importance of stereochemistry relates to the thalidomide disaster, thalidomide is a pharmaceutical drug, first prepared in 1957 in Germany, prescribed for treating morning sickness in pregnant women. The drug was discovered to be teratogenic, causing genetic damage to early embryonic growth and development. Some of the proposed mechanisms of teratogenecity involve a different biological function for the -. In the human body however, thalidomide undergoes racemization, even if one of the two enantiomers is administered as a drug, the other enantiomer is produced as a result of metabolism. Accordingly, it is incorrect to state one of the stereoisomer is safe while the other is teratogenic. Thalidomide is currently used for the treatment of diseases, notably cancer. Strict regulations and controls have been enabled to avoid its use by pregnant women and this disaster was a driving force behind requiring strict testing of drugs before making them available to the public. Torsional strain results from resistance to twisting about a bond
19.
Antarafacial and suprafacial
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The reaction center can be a p-orbital, a conjugated system or even a sigma bond. This relationship is antarafacial when opposite faces are involved and it is suprafacial when both changes occur at the same face. Many sigmatropic reactions and cycloadditions can be either suprafacial or antarafacial, an example is the -hydride shift, in which the interacting frontier orbitals are the allyl free radical and the hydrogen 1s orbitals. The suprafacial shift is symmetry-forbidden because orbitals with opposite algebraic signs overlap, the symmetry allowed antarafacial shift would require a strained transition state and is also unlikely. In contrast a symmetry allowed and suprafacial -hydride shift is a common event
20.
Thermal radiation
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Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a greater than absolute zero emits thermal radiation. When the temperature of a body is greater than absolute zero, Thermal radiation is different from thermal convection and thermal conduction—a person near a raging bonfire feels radiant heating from the fire, even if the surrounding air is very cold. Sunlight is part of thermal radiation generated by the hot plasma of the Sun, the Earth also emits thermal radiation, but at a much lower intensity and different spectral distribution because it is cooler. The Earths absorption of radiation, followed by its outgoing thermal radiation are the two most important processes that determine the temperature and climate of the Earth. If a radiation-emitting object meets the physical characteristics of a body in thermodynamic equilibrium. Plancks law describes the spectrum of radiation, which depends only on the objects temperature. Wiens displacement law determines the most likely frequency of the radiation. Thermal radiation is one of the mechanisms of heat transfer. Thermal radiation is the emission of electromagnetic waves from all matter that has a greater than absolute zero. It represents a conversion of energy into electromagnetic energy. Thermal energy consists of the energy of random movements of atoms. All matter with a temperature by definition is composed of particles which have kinetic energy, and these atoms and molecules are composed of charged particles, i. e. protons and electrons, and kinetic interactions among matter particles result in charge-acceleration and dipole-oscillation. This results in the generation of coupled electric and magnetic fields, resulting in the emission of photons. Electromagnetic radiation, including light, does not require the presence of matter to propagate, the radiation is not monochromatic, i. e. it does not consist of just a single frequency, but comprises a continuous dispersion of photon energies, its characteristic spectrum. If the radiating body and its surface are in thermodynamic equilibrium, a black body is also a perfect emitter. The radiation of such perfect emitters is called black-body radiation, the ratio of any bodys emission relative to that of a black body is the bodys emissivity, so that a black body has an emissivity of unity. Absorptivity, reflectivity, and emissivity of all bodies are dependent on the wavelength of the radiation, the temperature determines the wavelength distribution of the electromagnetic radiation
21.
Topology
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In mathematics, topology is concerned with the properties of space that are preserved under continuous deformations, such as stretching, crumpling and bending, but not tearing or gluing. This can be studied by considering a collection of subsets, called open sets, important topological properties include connectedness and compactness. Topology developed as a field of study out of geometry and set theory, through analysis of such as space, dimension. Such ideas go back to Gottfried Leibniz, who in the 17th century envisioned the geometria situs, Leonhard Eulers Seven Bridges of Königsberg Problem and Polyhedron Formula are arguably the fields first theorems. The term topology was introduced by Johann Benedict Listing in the 19th century, by the middle of the 20th century, topology had become a major branch of mathematics. It defines the basic notions used in all branches of topology. Algebraic topology tries to measure degrees of connectivity using algebraic constructs such as homology, differential topology is the field dealing with differentiable functions on differentiable manifolds. It is closely related to geometry and together they make up the geometric theory of differentiable manifolds. Geometric topology primarily studies manifolds and their embeddings in other manifolds, a particularly active area is low-dimensional topology, which studies manifolds of four or fewer dimensions. This includes knot theory, the study of mathematical knots, Topology, as a well-defined mathematical discipline, originates in the early part of the twentieth century, but some isolated results can be traced back several centuries. Among these are certain questions in geometry investigated by Leonhard Euler and his 1736 paper on the Seven Bridges of Königsberg is regarded as one of the first practical applications of topology. On 14 November 1750 Euler wrote to a friend that he had realised the importance of the edges of a polyhedron and this led to his polyhedron formula, V − E + F =2. Some authorities regard this analysis as the first theorem, signalling the birth of topology, further contributions were made by Augustin-Louis Cauchy, Ludwig Schläfli, Johann Benedict Listing, Bernhard Riemann and Enrico Betti. Listing introduced the term Topologie in Vorstudien zur Topologie, written in his native German, in 1847, the term topologist in the sense of a specialist in topology was used in 1905 in the magazine Spectator. Their work was corrected, consolidated and greatly extended by Henri Poincaré, in 1895 he published his ground-breaking paper on Analysis Situs, which introduced the concepts now known as homotopy and homology, which are now considered part of algebraic topology. Unifying the work on function spaces of Georg Cantor, Vito Volterra, Cesare Arzelà, Jacques Hadamard, Giulio Ascoli and others, Maurice Fréchet introduced the metric space in 1906. A metric space is now considered a case of a general topological space. In 1914, Felix Hausdorff coined the term topological space and gave the definition for what is now called a Hausdorff space, currently, a topological space is a slight generalization of Hausdorff spaces, given in 1922 by Kazimierz Kuratowski
22.
Transition state
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The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the corresponding to the highest potential energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules always go on to form products and it is often marked with the double dagger ‡ symbol. The concept of a state has been important in many theories of the rates at which chemical reactions occur. This started with the state theory, which was first developed around 1935 by Eyring, Evans and Polanyi. A collision between reactant molecules may or may not result in a successful reaction, the outcome depends on factors such as the relative kinetic energy, relative orientation and internal energy of the molecules. Even if the partners form an activated complex they are not bound to go on and form products. Because of the rules of quantum mechanics, the state cannot be captured or directly observed. This is sometimes expressed by stating that the state has a fleeting existence. However, cleverly manipulated spectroscopic techniques can get us as close as the timescale of the technique allows, femtochemical IR spectroscopy was developed for precisely that reason, and it is possible to probe molecular structure extremely close to the transition point. Often along the reaction coordinate reactive intermediates are present not much lower in energy from a transition state making it difficult to distinguish between the two, Transition state structures can be determined by searching for first-order saddle points on the potential energy surface of the chemical species of interest. A first-order saddle point is a point of index one, that is. This is further described in the geometry optimization. The Hammond–Leffler Postulate states that the structure of the state more closely resembles either the products or the starting material. A transition state resembles the reactants more than the products is said to be early. Thus, the Hammond–Leffler Postulate predicts a transition state for an endothermic reaction. A dimensionless reaction coordinate that quantifies the lateness of a state can be used to test the validity of the Hammond–Leffler Postulate for a particular reaction. One demonstration of principle is found in the two bicyclic compounds depicted below
23.
Enol
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The terms enol and alkenol are portmanteaus deriving from -ene/alkene and the -ol suffix indicating the hydroxyl group of alcohols, dropping the terminal -e of the first term. Generation of enols often involves removal of an adjacent to the carbonyl group—i. e. Deprotonation, its removal as a proton, H+, when this proton is not returned at the end of the stepwise process, the result is an anion termed an enolate. The enolate structures shown are schematic, a modern representation considers the molecular orbitals that are formed and occupied by electrons in the enolate. Similarly, generation of the enol often is accompanied by trapping or masking of the group as an ether. The importance of enols in accomplishing nature and humankinds chemical transformations makes them irreplaceable, moreover, the substituents and conditions determine the preponderant conformations of these reactive species, and therefore dictate the stereochemical outcomes of their reactions. As indicated in the image above, carbonyl compounds that have an α-hydrogen atom adjacent to a carbonyl group—like organic esters, ketones. The examples of the 3-pentanone and 2, 4-pentanedione tautomerization equilibrium appear in the gallery of images above, in the case of ketones, it is formally called a keto-enol tautomerism, though this name is often more generally applied to all such tautomerizations. In organic compounds with two carbonyls, the constitutional isomer may be stabilized. Hence, while one α-hydrogen is required, the substituent in the α-position may be variable. Enol stabilization is due in part to the intramolecular hydrogen bonding that is available to it, as shown for the 2. In the case of malonaldehyde, over 99 mole% of the compound is in the enol form. While lower for 3-ketoaldehydes and 1, 3-diketones, the form still predominates, e. g. in the case of 2, 4-pentanedione. When keto-enol tautomerism occurs the keto or enol is deprotonated and an anion, enolates can exist in quantitative amounts in strictly Brønsted acid free conditions, since they are generally very basic. In enolates the anionic charge is delocalized over the oxygen and the carbon, enolate forms can be stabilized by this delocalization of the charge over three atoms. In valence bond theory, the structure and stability is explained by a phenomenon known as resonance. The two resonance structures shown here constitute the resonance hybrid, in molecular orbital theory, it is represented by three delocalized molecular orbitals, two of them filled. In ketones with α-hydrogens on both sides of the carbon, selectivity of deprotonation may be achieved to generate two different enolate structures
24.
Acid
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An acid is a molecule or ion capable of donating a hydron, or, alternatively, capable of forming a covalent bond with an electron pair. The first category of acids is the donors or Brønsted acids. In the special case of solutions, proton donors form the hydronium ion H3O+ and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents, acids form aqueous solutions with a sour taste, can turn blue litmus red, and react with bases and certain metals to form salts. The word acid is derived from the Latin acidus/acēre meaning sour, an aqueous solution of an acid has a pH less than 7 and is colloquially also referred to as acid, while the strict definition refers only to the solute. A lower pH means a higher acidity, and thus a higher concentration of hydrogen ions in the solution. Chemicals or substances having the property of an acid are said to be acidic, common aqueous acids include hydrochloric acid, acetic acid, sulfuric acid, and citric acid. As these examples show, acids can be solutions or pure substances, strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid. The second category of acids are Lewis acids, which form a covalent bond with an electron pair, however, hydrogen chloride, acetic acid, and most other Brønsted-Lowry acids cannot form a covalent bond with an electron pair and are therefore not Lewis acids. Conversely, many Lewis acids are not Arrhenius or Brønsted-Lowry acids, in modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists almost always refer to a Lewis acid explicitly as a Lewis acid. Modern definitions are concerned with the chemical reactions common to all acids. Most acids encountered in life are aqueous solutions, or can be dissolved in water, so the Arrhenius. The Brønsted-Lowry definition is the most widely used definition, unless otherwise specified, hydronium ions are acids according to all three definitions. Interestingly, although alcohols and amines can be Brønsted-Lowry acids, they can function as Lewis bases due to the lone pairs of electrons on their oxygen and nitrogen atoms. The Swedish chemist Svante Arrhenius attributed the properties of acidity to hydrogen ions or protons in 1884, an Arrhenius acid is a substance that, when added to water, increases the concentration of H+ ions in the water. Thus, an Arrhenius acid can also be described as a substance that increases the concentration of ions when added to water. Examples include molecular substances such as HCl and acetic acid, an Arrhenius base, on the other hand, is a substance which increases the concentration of hydroxide ions when dissolved in water. Thus, an Arrhenius acid could also be said to be one that decreases hydroxide concentration, in an acidic solution, the concentration of hydronium ions is greater than 10−7 moles per liter
25.
Alkyl
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In organic chemistry, an alkyl substituent is an alkane missing one hydrogen. The term alkyl is intentionally unspecific to include many possible substitutions, an acyclic alkyl has the general formula CnH2n+1. A cycloalkyl is derived from a cycloalkane by removal of an atom from a ring and has the general formula CnH2n-1. Typically an alkyl is a part of a larger molecule, in structural formula, the symbol R is used to designate a generic alkyl group. The smallest alkyl group is methyl, with the formula CH3−, the word root alkyl is encountered in several contexts. Alkylation is an important operation in refineries, for example in the production of high-octane gasoline, alkylating antineoplastic agents refer to a class of compounds that are used to treat cancer. In such case, the alkyl is used loosely. For example, nitrogen mustards are well-known alkylating agents, but they are more complex than a mere hydrocarbon, in chemistry, alkyl refers to a group, a substituent, that is attached to other molecular fragments. For example, alkyl lithium reagents have the empirical formula Li, a dialkyl ether is an ether with two alkyl groups, e. g. diethyl ether. In medicinal chemistry, the incorporation of alkyl chains into some chemical compounds increases their lipophilicity and this strategy has been used to increase the antimicrobial activity of flavanones and chalcones. Usually alkyl groups are attached to atoms or groups of atoms. Free alkyls occur as neutral compounds, as anions, or as cations, the neutral alkyl free radicals have no special name. Such species are encountered only as transient intermediates, but some are quite stable. Typically alkyl cations are generated using super acids, alkyl anions are observed in the presence of strong bases, alkyls are commonly observed in mass spectrometry of organic compounds. The simplest series have the general formula CnH2n+1, alkyls include methyl, CH3·, ethyl, propyl, butyl, pentyl, and so on. Alkyl groups that one ring have the formula CnH2n−1, e. g. cyclopropyl and cyclohexyl. First, five atoms comprise the longest straight chain of carbon centers, the parent five-carbon compound is named pentane. The methyl substituent or group is highlighted red, according to the usual rules of nomenclature, alkyl groups are included in the name of the molecule before the root, as in methylpentane
26.
Cyclic compound
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A cyclic compound is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon, none of the atoms are carbon, cyclic compound examples, All-carbon and more complex natural cyclic compounds. Indeed, the development of important chemical concept arose, historically. A cyclic compound or ring compound is a compound at least some of whose atoms are connected to form a ring, rings vary in size from 3 to many tens or even hundreds of atoms. Examples of ring compounds readily include cases where, all the atoms are carbon, none of the atoms are carbon, common atoms can form varying numbers of bonds, and many common atoms readily form rings. As a consequence of the variability that is thermodynamically possible in cyclic structures. IUPAC nomenclature has extensive rules to cover the naming of cyclic structures, the term macrocycle is used when a ring-containing compound has a ring of 8 or more atoms. The term polycyclic is used more than one ring appears in a single molecule. Naphthalene is formally a polycyclic, but is specifically named as a bicyclic compound. Several examples of macrocyclic and polycyclic structures are given in the gallery below. The atoms that are part of the structure are called annular atoms. The vast majority of compounds are organic, and of these. Inorganic atoms form cyclic compounds as well, examples include sulfur, silicon, phosphorus, and boron. Hantzsch–Widman nomenclature is recommended by the IUPAC for naming heterocycles, cyclic compounds may or may not exhibit aromaticity, benzene is an example of an aromatic cyclic compound, while cyclohexane is non-aromatic. As a result of their stability, it is difficult to cause aromatic molecules to break apart. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, nevertheless, many non-benzene aromatic compounds exist. In living organisms, for example, the most common aromatic rings are the bases in RNA and DNA. A functional group or other substituent that is aromatic is called an aryl group, the earliest use of the term “aromatic” was in an article by August Wilhelm Hofmann in 1855
27.
Aromatic hydrocarbon
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An aromatic hydrocarbon or arene is a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle. In contrast, aliphatic hydrocarbons lack this delocalization, the term aromatic was assigned before the physical mechanism determining aromaticity was discovered, the term was coined as such simply because many of the compounds have a sweet or pleasant odour. The configuration of six carbon atoms in compounds is known as a benzene ring, after the simplest possible such hydrocarbon. Aromatic hydrocarbons can be monocyclic or polycyclic, some non-benzene-based compounds called heteroarenes, which follow Hückels rule, are also called aromatic compounds. In these compounds, at least one atom is replaced by one of the heteroatoms oxygen, nitrogen. Benzene, C6H6, is the simplest aromatic hydrocarbon, and it was the first one named as such, the nature of its bonding was first recognized by August Kekulé in the 19th century. Each carbon atom in the cycle has four electrons to share. One goes to the atom, and one each to the two neighbouring carbons. The structure is alternatively illustrated as a circle around the inside of the ring to show six electrons floating around in delocalized molecular orbitals the size of the ring itself. This depiction represents the equivalent nature of the six carbon–carbon bonds all of bond order 1.5, the electrons are visualized as floating above and below the ring with the electromagnetic fields they generate acting to keep the ring flat. The proper use of the symbol is debated, it is used to describe any cyclic π system in some publications, jensen argues that, in line with Robinsons original proposal, the use of the circle symbol should be limited to monocyclic 6 π-electron systems. In this way the symbol for a six-center six-electron bond can be compared to the Y symbol for a three-center two-electron bond. A reaction that forms a compound from an unsaturated or partially unsaturated cyclic precursor is simply called an aromatization. Many laboratory methods exist for the synthesis of arenes from non-arene precursors. Many methods rely on cycloaddition reactions, alkyne trimerization describes the cyclization of three alkynes, in the Dötz reaction an alkyne, carbon monoxide and a chromium carbene complex are the reactants. Diels–Alder reactions of alkynes with pyrone or cyclopentadienone with expulsion of carbon dioxide or carbon monoxide also form arene compounds, in Bergman cyclization the reactants are an enyne plus a hydrogen donor. Arenes are reactants in many organic reactions, in aromatic substitution one substituent on the arene ring, usually hydrogen, is replaced by another substituent. The two main types are electrophilic aromatic substitution when the reagent is an electrophile and nucleophilic aromatic substitution when the reagent is a nucleophile
28.
Reaction rate
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The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how quickly or slowly a reaction takes place. Chemical kinetics is the part of chemistry that studies reaction rates. The concepts of chemical kinetics are applied in many disciplines, such as engineering, enzymology. Consider a typical reaction, a A + b B → p P + q Q The lowercase letters represent stoichiometric coefficients, while the capital letters represent the reactants. Reaction rate usually has the units of mol L−1 s−1, the rate of a reaction is always positive. A negative sign is present to indicate that the reactant concentration is decreasing. )The IUPAC recommends that the unit of time should always be the second. The rate of reaction differs from the rate of increase of concentration of a product P by a constant factor and for a reactant A by minus the reciprocal of the stoichiometric number. The stoichiometric numbers are included so that the rate is independent of which reactant or product species is chosen for measurement. For example, if a =1 and b =3 then B is consumed three times more rapidly than A, but v = -d/dt = -d/dt is uniquely defined. The above definition is valid for a single reaction, in a closed system of constant volume. If water is added to a pot containing salty water, the concentration of salt decreases, although there is no chemical reaction. When applied to the system at constant volume considered previously, this equation reduces to, r = d d t. Here N0 is the Avogadro constant, for a single reaction in a closed system of varying volume the so-called rate of conversion can be used, in order to avoid handling concentrations. It is defined as the derivative of the extent of reaction with respect to time, also V is the volume of reaction and Ci is the concentration of substance i. When side products or reaction intermediates are formed, the IUPAC recommends the use of the rate of appearance and rate of disappearance for products and reactants. Reaction rates may also be defined on a basis that is not the volume of the reactor, when a catalyst is used the reaction rate may be stated on a catalyst weight or surface area basis. If the basis is a specific catalyst site that may be counted by a specified method. The nature of the reaction, Some reactions are faster than others
29.
Carbonyl group
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In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom, C=O. It is common to several classes of compounds, as part of many larger functional groups. A compound containing a group is often referred to as a carbonyl compound. The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex, the remainder of this article concerns itself with the organic chemistry definition of carbonyl, where carbon and oxygen share a double bond. A carbonyl group characterizes the types of compounds, Note that the most specific labels are usually employed. For example, ROR structures are known as acid anhydride rather than the more generic ester, other organic carbonyls are urea and the carbamates, the derivatives of acyl chlorides chloroformates and phosgene, carbonate esters, thioesters, lactones, lactams, hydroxamates, and isocyanates. Examples of inorganic compounds are carbon dioxide and carbonyl sulfide. A special group of compounds are 1, 3-dicarbonyl compounds that have acidic protons in the central methylene unit. Examples are Meldrums acid, diethyl malonate and acetylacetone, because oxygen is more electronegative than carbon, carbonyl compounds often have resonance structures which affect their reactivity. This relative electronegativity draws electron density away from carbon, increasing the bonds polarity, carbon can then be attacked by nucleophiles or a negatively charged part of another molecule. During the reaction, the double bond is broken. This reaction is known as addition-elimination or condensation, the electronegative oxygen also can react with an electrophile, for example a proton in an acidic solution or with Lewis acids to form an oxocarbenium ion. The polarity of oxygen also makes the alpha hydrogens of carbonyl compounds much more acidic than typical sp3 C-H bonds, for example, the pKa values of acetaldehyde and acetone are 16.7 and 19 respectively, while the pKa value of methane is extrapolated to be approximately 50. This is because a carbonyl is in resonance with an enol. The deprotonation of the enol with a base produces an enolate. Amides are the most stable of the carbonyl couplings due to their high resonance stabilization between the nitrogen-carbon and carbon-oxygen bonds, carbonyl groups can be reduced by reaction with hydride reagents such as NaBH4 and LiAlH4, with bakers yeast, or by catalytic hydrogenation. Ketones give secondary alcohols while aldehydes, esters and carboxylic acids give primary alcohols, carbonyls can be alkylated in nucleophilic addition reactions using organometallic compounds such as organolithium reagents, Grignard reagents, or acetylides. Carbonyls also may be alkylated by enolates as in aldol reactions, carbonyls are also the prototypical groups with vinylogous reactivity
30.
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
31.
Phenyl group
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In organic chemistry, the phenyl group or phenyl ring is a cyclic group of atoms with the formula C6H5. Phenyl groups are related to benzene and can be viewed as a benzene ring, minus a hydrogen. Phenyl groups have six carbon atoms bonded together in a planar ring, five of which are bonded to individual hydrogen atoms. Phenyl groups are commonplace in organic chemistry, although often depicted with alternating double and single bonds, phenyl groups are chemically aromatic and show nearly equal bond lengths between carbon atoms in the ring. Usually, a group is synonymous to C6H5– and is represented by the symbol Ph or, archaically. Benzene is sometimes denoted as PhH, Phenyl groups are generally attached to other atoms or groups. For example, triphenylmethane has three groups attached to the same carbon center. Many or even most phenyl compounds are not described with the term phenyl, for example, the chloro derivative C6H5Cl is normally called chlorobenzene, although it could be called phenyl chloride. For example, O2NC6H4 is nitrophenyl and F5C6 is pentafluorophenyl, monosubstituted phenyl groups are associated with electrophilic aromatic substitution reactions and the products follow the arene substitution pattern. So, a substituted phenyl compound has three isomers, ortho, meta and para. A disubstituted phenyl compound may be, for example,1,3, 5-trisubstituted, or 1,2, higher degrees of substitution, of which the pentafluorophenyl group is an example, exist, and are named according to IUPAC nomenclature. Phenyl compounds are derived from benzene, at least conceptually and often in terms of their production, in terms of its electronic properties, the phenyl group is related to a vinyl group. Phenyl groups tend to resist oxidation and reduction, Phenyl groups have enhanced stability in comparison to equivalent bonding in aliphatic groups. This increased stability is due to the properties of aromatic molecular orbitals. The bond lengths between carbon atoms in a group are approximately 1.4 Å. In 1H-NMR spectroscopy, protons of a group typically have chemical shifts around 7.27 ppm. These chemical shifts are influenced by aromatic ring current and may change depending on substituents, Phenyl groups are usually introduced using reagents that behave as sources of the phenyl anion or the phenyl cation. Representative reagents include phenyllithium and phenylmagnesium bromide, electrophiles attack benzene to give phenyl derivatives, C6H6 + E+ → C6H5E + H+ where E+ = Cl+, NO2+, SO3
32.
Vinyl group
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In chemistry, vinyl or ethenyl is the functional group −CH=CH2, namely the ethylene molecule minus one hydrogen atom. The name is used for any compound containing that group. An industrially important example is vinyl chloride, precursor to PVC, vinyl is one of the alkenyl functional groups. On a carbon skeleton, sp2-hybridized carbons or positions are often called vinylic, allyls, acrylates and styrenics contain vinyl groups. The etymology of vinyl is the Latin vinum = wine, because of its relationship with alcohol, the term vinyl was coined by the German chemist Hermann Kolbe in 1851. Vinyl groups can polymerize with the aid of an initiator or a catalyst. In these polymers, the bonds of the vinyl monomers turn into single bonds. Vinyl groups do not exist in polymer, the term refers to the precursor. It is sometimes important to ascertain the absence of unreacted vinyl monomer in the product when the monomer is toxic or reduces the performance of the plastic. The following table gives examples of vinyl polymers. The vinylidene and vinylene derivatives can polymerize in the same manner, allyl Grignard reagents can attack with the vinyl end first. If next to a group, conjugate addition occurs. Vinyl organometallics, e. g. vinyl lithium, participate in coupling reactions such as in Negishi coupling
33.
1,3-Cyclohexadiene
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1, 3-Cyclohexadiene is an organic compound with the formula 24. It is a colorless, flammable liquid, a naturally occurring derivative of 1, 3-cyclohexadiene is terpinene, a component of pine oil. Useful reactions of this diene are cycloadditions, such as the Diels-Alder reaction,1, 3-Cyclohexadiene could in principle be used as a hydrogen donor in transfer hydrogenation, since its conversion to benzene + hydrogen is exothermic by about 25 kJ/mol
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Electrocyclic reaction
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In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement where the net result is one pi bond being converted into one sigma bond or vice versa. These reactions are categorized by the following criteria, Reactions can be either photochemical or thermal. Reactions can be either ring-opening or ring-closing, depending on the type of reaction and the number of pi electrons, the reaction can happen through either a conrotatory or disrotatory mechanism. The type of rotation determines whether the cis or trans isomer of the product will be formed, the Nazarov cyclization reaction is a named electrocyclic reaction converting divinylketones to cyclopentenones. A classic example is the thermal ring-opening reaction of 3, 4-dimethylcyclobutene, the only way to accomplish this is through a conrotatory ring-opening which results in opposite signs for the terminal lobes. When performing an electrocyclic reaction it is desirable to predict the cis/trans geometry of the reactions product. The first step in process is to determine whether a reaction proceeds through conrotation or disrotation. The table below shows the selectivity rules for thermal and photochemical electrocyclic reactions, for the example given below, the thermal reaction of -octa-2,4, 6-triene will happen through a disrotatory mechanism. After determining the type of rotation, whether the product will be cis or trans can be determined by examining the starting molecule, in the example below, the disrotation causes both methyls to point upwards, causing the product to be cis-dimethylcyclohexadiene. In addition, the torquoselectivity in an electrocyclic reaction refers to the direction of rotation, for example, a reaction that is conrotatory can still rotate in two directions, producing enantiomeric products. A reaction that is torquoselective restricts one of these directions of rotation to produce a product in enantiomeric excess, correlation diagrams, which connect the molecular orbitals of the reactant to those of the product having the same symmetry, can then be constructed for the two processes. This is because only in cases would maximum orbital overlap occur in the transition state. Also, the product would be in a ground state rather than an excited state. According to the Frontier Molecular Orbital Theory, the bond in the ring will open in such a way that the resulting p-orbitals will have the same symmetry as the HOMO of the product. For the 5, 6-dimethylcyclohexa-1, 3-diene, only a disrotatory mode would result in p-orbitals having the symmetry as the HOMO of hexatriene. For the 3, 4-dimethylcyclobutene, on the hand, only a conrotatory mode would result in p-orbitals having the same symmetry as the HOMO of butadiene. Only a disrotatory mode, in which symmetry about a plane is maintained throughout the reaction. Also, once again, this would result in the formation of a product that is in a state of comparable stability to the excited state of the reactant compound
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Ergocalciferol
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Ergocalciferol, also known as vitamin D2 and calciferol, is a type of vitamin D found in food and used as a dietary supplement. As a supplement it is used to prevent and treat vitamin D deficiency and this includes vitamin D deficiency due to poor absorption by the intestines or liver disease. It may also be used for low blood calcium due to hypoparathyroidism and it is used by mouth or injection into a muscle. Excessive doses can result in increased production, high blood pressure, kidney stones, kidney failure, weakness. If high doses are taken for a period of time tissue calcification may occur. It is recommended that people on high doses have their blood calcium levels regularly checked, normal doses are safe in pregnancy. It works by increasing the amount of absorbed by the intestines. Food in which it is found include some mushrooms, ergocalciferol was first described in 1936. It is on the World Health Organizations List of Essential Medicines, ergocalciferol is available as a generic medication and over the counter. In the United Kingdom a typical dose costs the NHS less than 10 pounds a month, certain foods such as breakfast cereal and margarine have ergocalciferol added to them in some countries. Both forms appear to have efficacy in ameliorating rickets and reducing the incidence of falls in elderly patients. Ergocalciferol is a formed by a photochemical bond breaking of a steroid, specifically. The ranges for provitamin D2 and vitamin D2 were 89-146 and 0. 22-0.55 μg/g dry matter, respectively, human bioavailability of vitamin D2 from vitamin D2-enhanced button mushrooms via UV-B irradiation is effective in improving vitamin D status and not different from a vitamin D2 supplement. Vitamin D2 from UV-irradiated yeast baked into bread is bioavailable, by visual assessment or using a chromometer, no significant discoloration of irradiated mushrooms, as measured by the degree of whiteness, was observed. Viosterol, the given to early preparations of irradiated ergosterol, is essentially synonymous with ergocalciferol. Ergocalciferol is manufactured and marketed under various names, including Deltalin, NIST Chemistry WebBook page for ergocalciferol
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Methyl group
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A methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms — CH3. In formulas, the group is often abbreviated Me, such hydrocarbon groups occur in many organic compounds. It is a stable group in most molecules. While the methyl group is part of a larger molecule. The anion has eight electrons, the radical seven and the cation six. All three forms are highly reactive and rarely observed, the methylium cation exists in the gas phase, but is otherwise not encountered. Some compounds are considered to be sources of the CH3+ cation, the methanide anion exists only in rarefied gas phase or under exotic conditions. It can be produced by electrical discharge in ketene at low pressure, such reagents are generally prepared from the methyl halides, M + CH3X → MCH3 where M is an alkali metal. The methyl radical has the formula CH3 and it exists in dilute gases, but in more concentrated form it readily dimerizes to ethane. It can be produced by decomposition of only certain compounds. The reactivity of a methyl group depends on the adjacent substituents, methyl groups can be quite unreactive. For example, in compounds, the methyl group resists attack by even the strongest acids. The oxidation of a group occurs widely in nature and industry. The oxidation products derived from methyl are CH2OH, CHO, for example, permanganate often converts a methyl group to a carboxyl group, e. g. the conversion of toluene to benzoic acid. Ultimately oxidation of methyl groups gives protons and carbon dioxide, as seen in combustion, demethylation is a common process, and reagents that undergo this reaction are called methylating agents. Common methylating agents are dimethyl sulfate, methyl iodide, and methyl triflate, methanogenesis, the source of natural gas, arises via a demethylation reaction. Certain methyl groups can be deprotonated, for example, the acidity of the methyl groups in acetone is about 1020 more acidic than methane. The resulting carbanions are key intermediates in many reactions in organic synthesis and biosynthesis, fatty acids are produced in this way
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Valence (chemistry)
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In chemistry, the valence or valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. The concept of valence was developed in the half of the 19th century and was successful in explaining the molecular structure of inorganic and organic compounds. The combining power or affinity of an atom of an element was determined by the number of atoms that it combined with. In methane, carbon has a valence of 4, in ammonia, nitrogen has a valence of 3, in water, oxygen has a valence of 2, and in hydrogen chloride, chlorine has a valence of 1. Chlorine, as it has a valence of one, can be substituted for hydrogen, so phosphorus has a valence of 5 in phosphorus pentachloride, PCl5. Valence diagrams of a compound represent the connectivity of the elements, examples are, Valence only describes connectivity, it does not describe the geometry of molecular compounds, or what are now known to be ionic compounds or giant covalent structures. A line between atoms does not represent a pair of electrons as it does in Lewis diagrams and this definition differs from the IUPAC definition as an element can be said to have more than one valence. It is in manner, according to Frankland, that their affinities are best satisfied. Following these examples and postulates, Frankland declares how obvious it is that This “combining power” was afterwards called quantivalence or valency. In 1857 August Kekulé proposed fixed valences for many elements, such as 4 for carbon, and used them to propose structural formulas for many organic molecules, which are still accepted today. Most 19th-century chemists defined the valence of an element as the number of its bonds without distinguishing different types of valence or of bond. For main-group elements, in 1904 Richard Abegg considered positive and negative valences, in 1916, Gilbert N. Lewis explained valence and chemical bonding in terms of a tendency of atoms to achieve a stable octet of 8 valence-shell electrons. According to Lewis, covalent bonding leads to octets by the sharing of electrons, the term covalence is attributed to Irving Langmuir, who stated in 1919 that the number of pairs of electrons which any given atom shares with the adjacent atoms is called the covalence of that atom. The prefix co- means together, so that a co-valent bond means that the share a valence. Pauling also considered hypervalent molecules, in which main-group elements have apparent valences greater than the maximal of 4 allowed by the octet rule. Similar calculations on transition-metal molecules show that the role of p orbitals is minor, so that one s, for elements in the main groups of the periodic table, the valence can vary between 1 and 7. Many elements have a common valence related to their position in the periodic table, the Latin/Greek prefixes uni-/mono-, bi-/di-, ter-/tri-, quadri-/tetra- and quinque-/penta- are used to describe ions in the charge states 1,2,3,4,5 respectively. Polyvalence or multivalence refers to species that are not restricted to a number of valence bonds
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Bicyclic molecule
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A bicyclic molecule is a molecule that features two joined rings. Bicyclic structures occur widely, for example in many important molecules like α-thujene. A bicyclic compound can be carbocyclic, or heterocyclic, like DABCO, moreover, the two rings can both be aliphatic, or can be aromatic, or a combination of aliphatic and aromatic. There are three modes of ring junction for a bicyclic compound, In spirocyclic compounds, the two rings share only one single atom, the spiro atom, which is usually a quaternary carbon. An example of a compound is the photochromic switch spiropyran. In fused bicyclic compounds, two rings share two adjacent atoms, in other words, the rings share one covalent bond, i. e. the so-called bridgehead atoms are directly connected. In bridged bicyclic compounds, the two rings share three or more atoms, separating the two atoms by a bridge containing at least one atom. For example, norbornane, also known as bicycloheptane, can be thought of as a pair of cyclopentane rings each sharing three of their five carbon atoms, the structure of camphor has the same basis as norbornane, but with some substituents added. Bicyclic molecules have a strict nomenclature, the root of the compound name depends on the total number of atoms in all rings together, possibly followed by a suffix denoting the functional group with the highest priority. Numbering of the carbon chain always begins at one bridgehead atom and follows the carbon chain along the longest path, then numbering is continued along the second longest path and so on. Fused and bridged bicyclic compounds get the prefix bicyclo, whereas spirocyclic compounds get the prefix spiro, in between the prefix and the suffix, a pair of brackets with numerals denotes the number of carbon atoms between each of the bridgehead atoms. These numbers are arranged in descending order and are separated by periods, for example, the carbon frame of norbornane contains a total of 7 atoms, hence the root name heptane. This molecule has two paths of 2 carbon atoms and a path of 1 carbon atom between the two bridgehead carbons, so the brackets are filled in descending order. Addition of the prefix bicyclo gives the total name bicycloheptane, the carbon frame of camphor also counts 7 atoms, but is substituted with a carbonyl in this case, hence the suffix heptanone. Equal to norbornane, this also has two paths of 2 carbon atoms and one path of 1 carbon atom between the two bridgehead carbons, so the numbers within the brackets stay. Combining the brackets and suffix gives us heptan-2-one, besides bicyclo, the prefix should also specify the positions of all methyl substituents so the complete, official name becomes 1,7, 7-trimethylbicycloheptan-2-one. When naming simple fused bicyclic compounds, the method as for bridged bicyclic compounds is applied. Therefore, fused bicyclic compounds have a 0 included in the brackets, for example, decalin is named bicyclodecane
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Electron
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The electron is a subatomic particle, symbol e− or β−, with a negative elementary electric charge. Electrons belong to the first generation of the lepton particle family, the electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the include a intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant. As it is a fermion, no two electrons can occupy the same state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of particles and waves, they can collide with other particles and can be diffracted like light. Since an electron has charge, it has an electric field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law, electrons radiate or absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields, special telescopes can detect electron plasma in outer space. Electrons are involved in applications such as electronics, welding, cathode ray tubes, electron microscopes, radiation therapy, lasers, gaseous ionization detectors. Interactions involving electrons with other particles are of interest in fields such as chemistry. The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms, ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of a quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge electron in 1891, electrons can also participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of isotopes and in high-energy collisions. The antiparticle of the electron is called the positron, it is identical to the electron except that it carries electrical, when an electron collides with a positron, both particles can be totally annihilated, producing gamma ray photons. The ancient Greeks noticed that amber attracted small objects when rubbed with fur, along with lightning, this phenomenon is one of humanitys earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electricus, both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον
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Rainer Ludwig Claisen
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Rainer Ludwig Claisen was a German chemist best known for his work with condensations of carbonyls and sigmatropic rearrangements. He was born in Cologne as the son of a jurist and studied chemistry at the university of Bonn and he served in the army as a nurse in 1870–1871 and continued his studies at Göttingen University. He returned to the University of Bonn in 1872 and started his career at the same university in 1874. He died in 1930 in Godesberg am Rhein, described the condensation of aromatic aldehydes with aliphatic aldehydes or ketones in 1881. This variation of the now well-known aldol condensation reaction is called the Claisen–Schmidt condensation, discovered the condensation reaction of an ester with an activated methylene group, now known as the Claisen condensation. Synthesis of cinnamates by reacting aldehydes with esters. The reaction is known as the Claisen reaction and was described by Claisen for the first time in 1890, discovered the thermally induced rearrangement of allyl phenyl ether in 1912. He details its reaction mechanism in his last scientific publication, in his honor, the reaction has been named the Claisen rearrangement. Synthesis of isatin via a process known as the Claisen isatin synthesis, designer of a special distillation flask, now known as the Claisen flask
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Chemical bond
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A chemical bond is a lasting attraction between atoms that enables the formation of chemical compounds. The bond may result from the force of attraction between atoms with opposite charges, or through the sharing of electrons as in the covalent bonds. Since opposite charges attract via an electromagnetic force, the negatively charged electrons that are orbiting the nucleus. An electron positioned between two nuclei will be attracted to both of them, and the nuclei will be attracted toward electrons in this position and this attraction constitutes the chemical bond. This phenomenon limits the distance between nuclei and atoms in a bond, in general, strong chemical bonding is associated with the sharing or transfer of electrons between the participating atoms. All bonds can be explained by quantum theory, but, in practice, simplification rules allow chemists to predict the strength, directionality, the octet rule and VSEPR theory are two examples. Electrostatics are used to describe bond polarities and the effects they have on chemical substances, a chemical bond is an attraction between atoms. This attraction may be seen as the result of different behaviors of the outermost or valence electrons of atoms and these behaviors merge into each other seamlessly in various circumstances, so that there is no clear line to be drawn between them. However it remains useful and customary to differentiate different types of bond, which result in different properties of condensed matter. In the simplest view of a covalent bond, one or more electrons are drawn into the space between the two atomic nuclei, energy is released by bond formation. This is not as a reduction in energy, because the attraction of the two electrons to the two protons is offset by the electron-electron and proton-proton repulsions. In a polar covalent bond, one or more electrons are shared between two nuclei. Such weak intermolecular bonds give organic molecular substances, such as waxes and oils, their soft bulk character, also, the melting points of such covalent polymers and networks increase greatly. In a simplified view of a bond, the bonding electron is not shared at all. In this type of bond, the atomic orbital of one atom has a vacancy which allows the addition of one or more electrons. These newly added electrons potentially occupy a lower energy-state than they experience in a different atom, thus, one nucleus offers a more tightly bound position to an electron than does another nucleus, with the result that one atom may transfer an electron to the other. This transfer causes one atom to assume a net charge. The bond then results from electrostatic attraction between atoms and the atoms become positive or negatively charged ions, ionic bonds may be seen as extreme examples of polarization in covalent bonds
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Chemical reaction
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A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Nuclear chemistry is a sub-discipline of chemistry that involves the reactions of unstable. The substance initially involved in a reaction are called reactants or reagents. Chemical reactions are characterized by a chemical change, and they yield one or more products. Reactions often consist of a sequence of individual sub-steps, the elementary reactions. Chemical reactions are described with chemical equations, which present the starting materials, end products. Chemical reactions happen at a characteristic reaction rate at a given temperature, typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium, Reactions that proceed in the forward direction to approach equilibrium are often described as spontaneous, requiring no input of free energy to go forward. Non-spontaneous reactions require input of energy to go forward. Different chemical reactions are used in combinations during chemical synthesis in order to obtain a desired product, in biochemistry, a consecutive series of chemical reactions form metabolic pathways. These reactions are catalyzed by protein enzymes. Chemical reactions such as combustion in fire, fermentation and the reduction of ores to metals were known since antiquity, in the Middle Ages, chemical transformations were studied by Alchemists. They attempted, in particular, to lead into gold, for which purpose they used reactions of lead. The process involved heating of sulfate and nitrate minerals such as sulfate, alum. In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid, further optimization of sulfuric acid technology resulted in the contact process in the 1880s, and the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle, the phlogiston theory was proposed in 1667 by Johann Joachim Becher. It postulated the existence of an element called phlogiston, which was contained within combustible bodies. This proved to be false in 1785 by Antoine Lavoisier who found the explanation of the combustion as reaction with oxygen from the air
![[1,3] Alkyl shifts](/wiki/File:1,3alkylfixed.png)
![[1,5] Hydride shift in a cyclic system](/wiki/File:1,5hydridecyclicfixed.png)
![Antarafacial [1,5] Hydride shift](/wiki/File:1,5hantarafacialfixed.png)
![[5,5] shift of phenyl pentadienyl ether](/wiki/File:5,5shiftfixeds.png)
