1.
Ether
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Ethers are a class of organic compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula R–O–R′, where R and R′ represent the alkyl or aryl groups, a typical example of the first group is the solvent and anesthetic diethyl ether, commonly referred to simply as ether. Ethers are common in chemistry and pervasive in biochemistry, as they are common linkages in carbohydrates. Ethers feature C–O–C linkage defined by an angle of about 110°. The barrier to rotation about the C–O bonds is low, the bonding of oxygen in ethers, alcohols, and water is similar. In the language of valence bond theory, the hybridization at oxygen is sp3, oxygen is more electronegative than carbon, thus the hydrogens alpha to ethers are more acidic than in simple hydrocarbons. They are far less acidic than hydrogens alpha to carbonyl groups, depending on the groups at R and R′, ethers are classified into two types, Simple ethers or symmetrical ethers, e. g. diethyl ether, dimethyl ether, etc. Mixed ethers or unsymmetrical ethers, e. g. methyl ethyl ether, methyl phenyl ether, in the IUPAC nomenclature system, ethers are named using the general formula alkoxyalkane, for example CH3–CH2–O–CH3 is methoxyethane. If the ether is part of a complex molecule, it is described as an alkoxy substituent. The simpler alkyl radical is written in front, so CH3–O–CH2CH3 would be given as methoxyethane, IUPAC rules are often not followed for simple ethers. The trivial names for simple ethers are a composite of the two followed by ether. For example, ethyl ether, diphenylether. As for other compounds, very common ethers acquired names before rules for nomenclature were formalized. Diethyl ether is called ether, but was once called sweet oil of vitriol. Methyl phenyl ether is anisole, because it was found in aniseed. Acetals are another class of ethers with characteristic properties, polyethers are compounds with more than one ether group. The crown ethers are examples of small polyethers, some toxins produced by dinoflagellates such as brevetoxin and ciguatoxin are extremely large and are known as cyclic or ladder polyethers. Polyether generally refers to polymers which contain the functional group in their main chain
2.
Equilateral triangle
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In geometry, an equilateral triangle is a triangle in which all three sides are equal. In the familiar Euclidean geometry, equilateral triangles are also equiangular and they are regular polygons, and can therefore also be referred to as regular triangles. Thus these are properties that are unique to equilateral triangles, the three medians have equal lengths. The three angle bisectors have equal lengths, every triangle center of an equilateral triangle coincides with its centroid, which implies that the equilateral triangle is the only triangle with no Euler line connecting some of the centers. For some pairs of triangle centers, the fact that they coincide is enough to ensure that the triangle is equilateral, in particular, A triangle is equilateral if any two of the circumcenter, incenter, centroid, or orthocenter coincide. It is also equilateral if its circumcenter coincides with the Nagel point, for any triangle, the three medians partition the triangle into six smaller triangles. A triangle is equilateral if and only if any three of the triangles have either the same perimeter or the same inradius. A triangle is equilateral if and only if the circumcenters of any three of the triangles have the same distance from the centroid. Morleys trisector theorem states that, in any triangle, the three points of intersection of the adjacent angle trisectors form an equilateral triangle, a version of the isoperimetric inequality for triangles states that the triangle of greatest area among all those with a given perimeter is equilateral. That is, PA, PB, and PC satisfy the inequality that any two of them sum to at least as great as the third. By Eulers inequality, the triangle has the smallest ratio R/r of the circumradius to the inradius of any triangle, specifically. The triangle of largest area of all those inscribed in a circle is equilateral. The ratio of the area of the incircle to the area of an equilateral triangle, the ratio of the area to the square of the perimeter of an equilateral triangle,1123, is larger than that for any other triangle. If a segment splits an equilateral triangle into two regions with equal perimeters and with areas A1 and A2, then 79 ≤ A1 A2 ≤97, in no other triangle is there a point for which this ratio is as small as 2. For any point P in the plane, with p, q, and t from the vertices A, B. For any point P on the circle of an equilateral triangle, with distances p, q. There are numerous triangle inequalities that hold with equality if and only if the triangle is equilateral, an equilateral triangle is the most symmetrical triangle, having 3 lines of reflection and rotational symmetry of order 3 about its center. Its symmetry group is the group of order 6 D3
3.
Ring strain
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In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, because of their high strain, the heat of combustion for these small rings is elevated. Ring strain results from a combination of angle strain, conformational strain or Pitzer strain, the simplest examples of angle strain are small cycloalkanes such as cyclopropane and cyclobutane. Furthermore, there is often eclipsing in cyclic systems which cannot be relieved, in alkanes, optimum overlap of atomic orbitals is achieved at 109. 5°. The most common compounds have five or six carbons in their ring. Adolf von Baeyer received a Nobel Prize in 1905 for the discovery of the Baeyer strain theory, angle strain occurs when bond angles deviate from the ideal bond angles to achieve maximum bond strength in a specific chemical conformation. Angle strain typically affects cyclic molecules, which lack the flexibility of acyclic molecules, angle strain destabilizes a molecule, as manifested in higher reactivity and elevated heat of combustion. Maximum bond strength results from effective overlap of orbitals in a chemical bond. A quantitative measure for angle strain is strain energy, angle strain and torsional strain combine to create ring strain that affects cyclic molecules. ΔHcombustion per CH2 -658.6 kJ = strain per CH2 The value 658.6 kJ per mole is obtained from an unstrained long-chain alkane, cyclic alkenes are subject to strain resulting from distortion of the sp2-hybridized carbon centers. Illustrative is C60 where the centres are pyramidalized. This distortion enhances the reactivity of this molecule, angle strain also is the basis of Bredts rule which dictates that bridgehead carbon centers are not incorporated in alkenes because the resulting alkene would be subject to extreme angle strain. In cycloalkanes, each carbon is bonded nonpolar covalently to two carbons and two hydrogen, the carbons have sp3 hybrization and should have ideal bond angles of 109. 5°. Due to the limitations of cyclic structure, however, the angle is only achieved in a six carbon ring — cyclohexane in chair conformation. For other cycloalkanes, the bond angles deviate from ideal, in cyclopropanes and cyclobutanes the C-C bonds are 60° and ~90° respectively. These molecules have bond angles between ring atoms which are more acute than the tetrahedral and trigonal planar bond angles required by their respective sp3. Because of the bond angles, the bonds have higher energy. In addition, the structures of cyclopropanes/enes and cyclclobutanes/enes offer very little conformational flexibility
4.
Functional group
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In organic chemistry, functional groups are specific groups of atoms or bonds within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of, however, its relative reactivity can be modified by other functional groups nearby. The atoms of functional groups are linked to other and to the rest of the molecule by covalent bonds. Any subgroup of atoms of a compound also may be called a radical, and if a covalent bond is broken homolytically, Functional groups can also be charged, e. g. in carboxylate salts, which turns the molecule into a polyatomic ion or a complex ion. Complexation and solvation is also caused by interactions of functional groups. In the common rule of thumb like dissolves like, it is the shared or mutually well-interacting functional groups give rise to solubility. For example, sugar dissolves in water because both share the functional group and hydroxyls interact strongly with each other. Combining the names of groups with the names of the parent alkanes generates what is termed a systematic nomenclature for naming organic compounds. In traditional nomenclature, the first carbon atom after the carbon that attaches to the group is called the alpha carbon, the second, beta carbon. IUPAC conventions call for numeric labeling of the position, e. g. 4-aminobutanoic acid, in traditional names various qualifiers are used to label isomers, for example isopropanol is an isomer is n-propanol. The following is a list of functional groups. In the formulas, the symbols R and R usually denote an attached hydrogen, or a side chain of any length. Functional groups, called hydrocarbyl, that only carbon and hydrogen. Each one differs in type of reactivity, there are also a large number of branched or ring alkanes that have specific names, e. g. tert-butyl, bornyl, cyclohexyl, etc. Hydrocarbons may form charged structures, positively charged carbocations or negative carbanions, examples are tropylium and triphenylmethyl cations and the cyclopentadienyl anion. Haloalkanes are a class of molecule that is defined by a carbon–halogen bond and this bond can be relatively weak or quite stable. In general, with the exception of fluorinated compounds, haloalkanes readily undergo nucleophilic substitution reactions or elimination reactions, the substitution on the carbon, the acidity of an adjacent proton, the solvent conditions, etc. all can influence the outcome of the reactivity. Compounds that contain nitrogen in this category may contain C-O bonds, compounds that contain sulfur exhibit unique chemistry due to their ability to form more bonds than oxygen, their lighter analogue on the periodic table
5.
Polymer
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A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Because of their range of properties, both synthetic and natural polymers play an essential and ubiquitous role in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure, Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. The units composing polymers derive, actually or conceptually, from molecules of low molecular mass. The term was coined in 1833 by Jöns Jacob Berzelius, though with a distinct from the modern IUPAC definition. The modern concept of polymers as covalently bonded macromolecular structures was proposed in 1920 by Hermann Staudinger, Polymers are studied in the fields of biophysics and macromolecular science, and polymer science. Polyisoprene of latex rubber is an example of a polymer. In biological contexts, essentially all biological macromolecules—i. e, proteins, nucleic acids, and polysaccharides—are purely polymeric, or are composed in large part of polymeric components—e. g. Isoprenylated/lipid-modified glycoproteins, where small molecules and oligosaccharide modifications occur on the polyamide backbone of the protein. The simplest theoretical models for polymers are ideal chains, Polymers are of two types, Natural polymeric materials such as shellac, amber, wool, silk and natural rubber have been used for centuries. A variety of natural polymers exist, such as cellulose. Most commonly, the continuously linked backbone of a used for the preparation of plastics consists mainly of carbon atoms. A simple example is polyethylene, whose repeating unit is based on ethylene monomer, however, other structures do exist, for example, elements such as silicon form familiar materials such as silicones, examples being Silly Putty and waterproof plumbing sealant. Oxygen is also present in polymer backbones, such as those of polyethylene glycol, polysaccharides. Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network, during the polymerization process, some chemical groups may be lost from each monomer. This is the case, for example, in the polymerization of PET polyester, the distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue. Laboratory synthetic methods are divided into two categories, step-growth polymerization and chain-growth polymerization. However, some methods such as plasma polymerization do not fit neatly into either category
6.
Epoxy
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Epoxy is either any of the basic components or the cured end products of epoxy resins, as well as a colloquial name for the epoxide functional group. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups, Epoxy resins may be reacted either with themselves through catalytic homopolymerisation, or with a wide range of co-reactants including polyfunctional amines, acids, phenols, alcohols and thiols. These co-reactants are often referred to as hardeners or curatives, reaction of polyepoxides with themselves or with polyfunctional hardeners forms a thermosetting polymer, often with high mechanical properties, temperature and chemical resistance. Epoxy resins are low molecular weight pre-polymers or higher molecular weight polymers which contain at least two epoxide groups. The epoxide group is sometimes referred to as a glycidyl or oxirane group. A wide range of epoxy resins are produced industrially, the raw materials for epoxy resin production are today largely petroleum derived, although some plant derived sources are now becoming commercially available. Epoxy resins are polymeric or semi-polymeric materials, and as such rarely exist as pure substances, high purity grades can be produced for certain applications, e. g. using a distillation purification process. One downside of high purity liquid grades is their tendency to form crystalline solids due to their highly regular structure, an important criterion for epoxy resins is the epoxide content. Epoxies are typically cured with stoichiometric or near-stoichiometric quantities of curative to achieve maximum physical properties, use of blending, additives and fillers is often referred to as formulating. Important epoxy resins are produced from combining epichlorohydrin and bisphenol A to give bisphenol A diglycidyl ethers, as the molecular weight of the resin increases, the epoxide content reduces and the material behaves more and more like a thermoplastic. Very high molecular weight polycondensates form a class known as phenoxy resins and these resins do however contain hydroxyl groups throughout the backbone, which may also undergo other cross-linking reactions, e. g. with aminoplasts, phenoplasts and isocyanates. Bisphenol F may also undergo epoxidation in a fashion to bisphenol A. Compared to DGEBA, bisphenol F epoxy resins have lower viscosity and a higher mean epoxy content per gramme, reaction of phenols with formaldehyde and subsequent glycidylation with epichlorohydrin produces epoxidised novolacs, such as epoxy phenol novolacs and epoxy cresol novolacs. These are highly viscous to solid resins with typical mean epoxide functionality of around 2 to 6, the high epoxide functionality of these resins forms a highly crosslinked polymer network displaying high temperature and chemical resistance, but low flexibility. Aliphatic epoxy resins are typically formed by glycidylation of aliphatic alcohols or polyols, the resulting resins may be monofunctional, difunctional, or higher functionality. These resins typically display low viscosity at room temperature and are referred to as reactive diluents. They are rarely used alone, but are employed to modify the viscosity of other epoxy resins. This has led to the term ‘modified epoxy resin’ to denote those containing viscosity-lowering reactive diluents, a related class is cycloaliphatic epoxy resin, which contains one or more cycloaliphatic rings in the molecule
7.
Propylene oxide
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Propylene oxide is an organic compound with the molecular formula CH3CHCH2O. This colourless volatile liquid is produced on a large scale industrially and it is a chiral epoxide, although it is commonly used as a racemic mixture. This compound is sometimes called 1, 2-propylene oxide to distinguish it from its isomer 1, 3-propylene oxide, industrial production of propylene oxide starts from propylene. Two general approaches are employed, one involving hydrochlorination and the other involving oxidation, in 2005, about half of the world production was through chlorohydrin technology and one half via oxidation routes. The latter approach is growing in importance, for example, Lime is often used to absorb the HCl. The other general route to propylene oxide involves oxidation of propylene with an organic peroxide and this coproduct can be dehydrated to isobutene, converted to MTBE, an additive for gasoline. Ethylbenzene hydroperoxide, derived from oxygenation of ethylbenzene, which affords 1-phenylethanol and this coproduct can be dehydrated to give styrene, a useful monomer. Cumene hydroperoxide derived from oxygenation of cumene, which affords cumyl alcohol, via dehydration and hydrogenation this coproduct can be recycled back to cumene. This technology was commercialized by Sumitomo Chemical, in March 2009, BASF and Dow Chemical started up their new HPPO plant in Antwerp. In the HPPO-Process, propylene is oxidized with hydrogen peroxide, CH3CH=CH2 + H2O2 → CH3CHCH2O + H2O In this process no side products other than water are generated, between 60 and 70% of all propylene oxide is converted to polyether polyols by the process called alkoxylation. These polyols are building blocks in the production of polyurethane plastics, about 20% of propylene oxide is hydrolyzed into propylene glycol, via a process which is accelerated by acid or base catalysis. Other major products are polypropylene glycol, propylene glycol ethers, Propylene oxide was once used as a racing fuel, but that usage is now prohibited under the US NHRA rules for safety reasons. It has also used in glow fuel for model aircraft and surface vehicles, typically combined in small percentages of around 2% as an additive to the typical methanol, nitromethane. It is also used in weapons, and microbial fumigation. Pistachio nuts can also be subjected to propylene oxide to control Salmonella, Propylene oxide is commonly used in the preparation of biological samples for electron microscopy, to remove residual ethanol previously used for dehydration. In a typical procedure, the sample is first immersed in a mixture of equal volumes of ethanol and propylene oxide for 5 minutes, Propylene oxide is a probable human carcinogen, and listed as an IARC Group 2B carcinogen. In 2016 it was reported that propylene oxide was detected in Sagittarius B2 and it is the first chiral molecule to be detected in space. WebBook page for C3H6O Propylene oxide at the United States Environmental Protection Agency Propylene oxide - chemical product info, properties, production, Propylene oxide at the Technology Transfer Network Air Toxics Web Site CDC - NIOSH Pocket Guide to Chemical Hazards
8.
Oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the group on the periodic table and is a highly reactive nonmetal. By mass, oxygen is the third-most abundant element in the universe, after hydrogen, at standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. This is an important part of the atmosphere and diatomic oxygen gas constitutes 20. 8% of the Earths atmosphere, additionally, as oxides the element makes up almost half of the Earths crust. Most of the mass of living organisms is oxygen as a component of water, conversely, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone, strongly absorbs ultraviolet UVB radiation, but ozone is a pollutant near the surface where it is a by-product of smog. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft, the name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle, Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philos work by observing that a portion of air is consumed during combustion and respiration, Oxygen was discovered by the Polish alchemist Sendivogius, who considered it the philosophers stone. In the late 17th century, Robert Boyle proved that air is necessary for combustion, English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. From this he surmised that nitroaereus is consumed in both respiration and combustion, Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract De respiratione. Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, one part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. The fact that a substance like wood gains overall weight in burning was hidden by the buoyancy of the combustion products
9.
Stoichiometry
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Stoichiometry /ˌstɔɪkiˈɒmᵻtri/ is the calculation of relative quantities of reactants and products in chemical reactions. This means that if the amounts of the reactants are known. Conversely, if one reactant has a quantity and the quantity of product can be empirically determined. This is illustrated in the image here, where the equation is. Here, one molecule of methane reacts with two molecules of gas to yield one molecule of carbon dioxide and two molecules of water. Stoichiometry measures these quantitative relationships, and is used to determine the amount of products/reactants that are produced/needed in a given reaction, describing the quantitative relationships among substances as they participate in chemical reactions is known as reaction stoichiometry. In the example above, reaction stoichiometry measures the relationship between the methane and oxygen as they react to form carbon dioxide and water. Gas stoichiometry deals with reactions involving gases, where the gases are at a temperature, pressure. For gases, the ratio is ideally the same by the ideal gas law. In practice, due to the existence of isotopes, molar masses are used instead when calculating the mass ratio, the term stoichiometry was first used by Jeremias Benjamin Richter in 1792 when the first volume of Richters Stoichiometry or the Art of Measuring the Chemical Elements was published. The term is derived from the Greek words στοιχεῖον stoicheion element, in patristic Greek, the word Stoichiometria was used by Nicephorus to refer to the number of line counts of the canonical New Testament and some of the Apocrypha. In general, chemical reactions combine in definite ratios of chemicals, since chemical reactions can neither create nor destroy matter, nor transmute one element into another, the amount of each element must be the same throughout the overall reaction. Chemical reactions, as unit operations, consist of simply a very large number of elementary reactions. As the reacting molecules consist of a set of atoms in an integer ratio. A reaction may consume more than one molecule, and the number counts this number, defined as positive for products. Different elements have a different atomic mass, and as collections of atoms, molecules have a definite molar mass. By definition, carbon-12 has a mass of 12 g/mol. The mass ratios can be calculated by dividing each by the total in the whole reaction, Elements in their natural state are mixtures of isotopes of differing mass, thus atomic masses and thus molar masses are not exactly integers
10.
Heterogeneous catalysis
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In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, the great majority of practical heterogeneous catalysts are solids and the great majority of reactants are gases or liquids. Heterogeneous catalysis is of paramount importance in areas of the chemical. Heterogeneous catalysis has attracted Nobel prizes for Fritz Haber and Carl Bosch in 1918, Irving Langmuir in 1932, adsorption is commonly an essential first step in heterogeneous catalysis. Adsorption is when a molecule in the gas phase or in solution binds to atoms on the solid or liquid surface, the molecule that is binding is called the adsorbate, and the surface to which it binds is the adsorbent. The process of the binding to the adsorbent is called adsorption. The reverse of this process is called desorption, in terms of catalyst support, the catalyst is the adsorbate and the support is the adsorbent. Two types of adsorption are recognized in heterogeneous catalysis, although many processes fall into a range between the two extremes. In the first type, physisorption, induces only small changes to the structure of the adsorbate. Typical energies for physisorption are from 2 to 10 kcal/mol, the second type is chemisorption, in which the adsorbate is strongly perturbed, often with bond-breaking. Energies for typical chemisorptions range from 15 to 100 kcal/mol, for physisorption, adsorbate is attracted to the surface atoms by van der Waals forces. A mathematical model for physisorption was developed by London to predict the energies of basic physisorption of non-polar molecules, the analysis of physisorption for polar or ionic species is more complex. Chemisorption results in the sharing of electrons between the adsorbate and the adsorbent, chemisorption is traditionally described by the Lennard-Jones potential, which considers various cases, two of which are. Molecular adsorption, where the adsorbate remains intact, an example is alkene binding by platinum. In dissociation adsorption, one or more bonds break concomitantly with adsorption, in this case the barrier to dissociation affects the rate of adsorption. An example of this the binding of H2, where the H-H bond is broken upon adsorption by hydrogen spillover, with catalyst supports, the reaction that occurs often occurs on the surface of either the catalyst or the support. In terms of surface reactions there are three mechanisms, the two molecules A and B both adsorb to the surface. While adsorbed to the surface, the A and B meet, bond, one of the two molecules, A, adsorbs to the surface
11.
Propene
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Propene, also known as propylene or methyl ethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the class of hydrocarbons. At room temperature and atmospheric pressure, propene is a gas, propene has a higher density and boiling point than ethylene due to its greater mass. It has a lower boiling point than propane and is thus more volatile. It lacks strongly polar bonds, yet the molecule has a dipole moment due to its reduced symmetry. Propene has the empirical formula as cyclopropane but their atoms are connected in different ways. Propene is found in nature as a byproduct of vegetation and fermentation processes, propene is produced from fossil fuels—petroleum, natural gas, and, to a much lesser extent, coal. Propene is a byproduct of oil refining and natural gas processing, during oil refining, ethylene, propene, and other compounds are produced as a result of cracking larger hydrocarbon molecules to produce hydrocarbons more in demand. A major source of propene is naphtha cracking intended to produce ethylene, propene can be separated by fractional distillation from hydrocarbon mixtures obtained from cracking and other refining processes, refinery-grade propene is about 50 to 70%. A shift to lighter steam cracker feedstocks with relatively lower propene yields, on-purpose production methods are becoming increasingly significant. Propene yields of about 90 wt% are achieved and this option may also be used when there is no butene feedstock. In this case, part of the ethylene feeds an ethylene-dimerization unit that converts ethylene into butene, propane dehydrogenation converts propane into propene and by-product hydrogen. The propene from propane yield is about 85 m%, reaction by-products are usually used as fuel for the propane dehydrogenation reaction. As a result, propene tends to be the only product and this route is popular in regions, such as the Middle East, where there is an abundance of propane from oil/gas operations. In this region, the output is expected to be capable of supplying not only domestic needs, but also the demand from China. However, as natural gas offerings in the United States are significantly increasing due to the exploitation of shale gas. Chemical companies are planning to establish PDH plants in the USA to take advantage of the low price raw material. Numerous plants dedicated to propane dehydrogenation are currently under construction around the world, there are already five licensed technologies
12.
Alkene
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In organic chemistry, an alkene is an unsaturated hydrocarbon that contains at least one carbon–carbon double bond. The words alkene, olefin, and olefine are used often interchangeably, acyclic alkenes, with only one double bond and no other functional groups, known as mono-enes, form a homologous series of hydrocarbons with the general formula CnH2n. Alkenes have two hydrogen atoms less than the corresponding alkane, the simplest alkene, ethylene, with the International Union of Pure and Applied Chemistry name ethene, is the organic compound produced on the largest scale industrially. Aromatic compounds are drawn as cyclic alkenes, but their structure and properties are different. This double bond is stronger than a single covalent bond and also shorter, each carbon of the double bond uses its three sp2 hybrid orbitals to form sigma bonds to three atoms. The unhybridized 2p atomic orbitals, which lie perpendicular to the created by the axes of the three sp² hybrid orbitals, combine to form the pi bond. This bond lies outside the main C–C axis, with half of the bond on one side of the molecule, rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the p orbitals on the two carbon atoms. As a consequence, substituted alkenes may exist as one of two isomers, called cis or trans isomers, more complex alkenes may be named with the E–Z notation for molecules with three or four different substituents. For example, of the isomers of butene, the two groups of -but-2-ene appear on the same side of the double bond, and in -but-2-ene the methyl groups appear on opposite sides. These two isomers of butene are slightly different in their chemical and physical properties, a 90° twist of the C=C bond requires less energy than the strength of a pi bond, and the bond still holds. This contradicts a common assertion that the p orbitals would be unable sustain such a bond. In truth, the misalignment of the p orbitals is less than expected because pyramidalization takes place, trans-Cyclooctene is a stable strained alkene and the orbital misalignment is only 19° with a dihedral angle of 137° and a degree of pyramidalization of 18°. The trans isomer of cycloheptene is stable only at low temperatures, as predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes includes bond angles about each carbon in a double bond of about 120°. The angle may vary because of steric strain introduced by nonbonded interactions between groups attached to the carbons of the double bond. For example, the C–C–C bond angle in propylene is 123. 9°, for bridged alkenes, Bredts rule states that a double bond cannot occur at the bridgehead of a bridged ring system unless the rings are large enough. The physical properties of alkenes and alkanes are similar and they are colourless, nonpolar, combustable, and almost odorless. The physical state depends on mass, like the corresponding saturated hydrocarbons, the simplest alkenes, ethene, propene. Linear alkenes of approximately five to sixteen carbons are liquids, Alkenes are relatively stable compounds, but are more reactive than alkanes, either because of the reactivity of the carbon–carbon pi-bond or the presence of allylic CH centers
13.
Peroxide
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A peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion, O2−2. The O−O group is called the group or peroxo group. In contrast to oxide ions, the atoms in the peroxide ion have an oxidation state of −1. The simplest stable peroxide is hydrogen peroxide, superoxides, dioxygenyls, ozones and ozonides are considered separately. Peroxide compounds can be classified into organic and inorganic. Whereas the inorganic peroxides have an ionic, salt-like character, the peroxides are dominated by the covalent bonds. The oxygen–oxygen chemical bond of peroxide is unstable and easily split into reactive radicals via homolytic cleavage, for this reason, peroxides are found in nature only in small quantities, in water, atmosphere, plants, and animals. Peroxides have an effect on organic substances and therefore are added to some detergents. Other large-scale applications include medicine and chemical industry, where peroxides are used in synthesis reactions or occur as intermediate products. With an annual production of over 2 million tonnes, hydrogen peroxide is the most economically important peroxide, many peroxides are unstable and hazardous substances, they cannot be stored and therefore are synthesized in situ and used immediately. Peroxides are usually very reactive and thus occur in only in a few forms. These include, in addition to hydrogen peroxide, a few products such as ascaridole. Hydrogen peroxide occurs in water, groundwater and in the atmosphere. It forms upon illumination or natural catalytic action by substances containing in water, sea water contains 0.5 to 14 μg/L of hydrogen peroxide, freshwater 1 to 30 μg/L and air 0.1 to 1 parts per billion. Hydrogen peroxide is formed in human and animal organisms as a product in biochemical processes and is toxic to cells. The toxicity is due to oxidation of proteins, membrane lipids, the class of biological enzymes called SOD is developed in nearly all living cells as an important antioxidant agent. They promote the disproportionation of superoxide into oxygen and hydrogen peroxide,2 O2 − +2 H + → SOD H2 O2 ⏞ hydrogen peroxide + O2 ⏞ oxygen Formation of hydrogen peroxide by superoxide dismutase Peroxisomes are organelles found in virtually all eukaryotic cells. This reaction is important in liver and kidney cells, where the peroxisomes neutralize various toxic substances that enter the blood, some of the ethanol humans drink is oxidized to acetaldehyde in this way
14.
Hydrogen peroxide
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Hydrogen peroxide is a chemical compound with the formula H 2O2. In its pure form, it is a liquid, slightly more viscous than water. Hydrogen peroxide is the simplest peroxide and it is used as an oxidizer, bleaching agent and disinfectant. Concentrated hydrogen peroxide, or high-test peroxide, is an oxygen species and has been used as a propellant in rocketry. Its chemistry is dominated by the nature of its unstable peroxide bond, Hydrogen peroxide is unstable and slowly decomposes in the presence of base or a catalyst. Because of its instability, hydrogen peroxide is typically stored with a stabilizer in an acidic solution. Hydrogen peroxide is found in biological systems including the human body, enzymes that use or decompose hydrogen peroxide are classified as peroxidases. The boiling point of H 2O2 has been extrapolated as being 150.2 °C, in practice hydrogen peroxide will undergo potentially explosive thermal decomposition if heated to this temperature. It may be distilled at lower temperatures under reduced pressure. In aqueous solutions hydrogen peroxide differs from the material due to the effects of hydrogen bonding between water and hydrogen peroxide molecules. Hydrogen peroxide and water form a mixture, exhibiting freezing-point depression, pure water has a melting point of 0 °C. The boiling point of the same mixtures is also depressed in relation with the mean of both boiling points and this boiling point is 14 °C greater than that of pure water and 36.2 °C less than that of pure hydrogen peroxide. Hydrogen peroxide is a molecule with C2 symmetry. Although the O−O bond is a bond, the molecule has a relatively high rotational barrier of 2460 cm−1, for comparison. The increased barrier is ascribed to repulsion between the pairs of the adjacent oxygen atoms and results in hydrogen peroxide displaying atropisomerism. The molecular structures of gaseous and crystalline H 2O2 are significantly different and this difference is attributed to the effects of hydrogen bonding, which is absent in the gaseous state. Crystals of H 2O2 are tetragonal with the space group D4 4P4121, Hydrogen peroxide has several structural analogues with Hm−X−X−Hn bonding arrangements. It has the highest boiling point of this series, diphosphane and hydrogen disulfide exhibit only weak hydrogen bonding and have little chemical similarity to hydrogen peroxide
15.
Dimethyldioxirane
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Dimethyldioxirane, also referred to as Murrays reagent in reference to Robert W. Murray, is a dioxirane derived from acetone and can be considered as a monomer of acetone peroxide. It is a powerful yet selective oxidizing agent which finds use in organic synthesis and it is known only in the form of a dilute solution, usually in acetone, and hence the properties of the pure material are largely unknown. DMDO is not commercially available because of its instability and this is tolerable as preparation uses inexpensive substances, acetone, sodium bicarbonate, and potassium peroxymonosulfate. The solution can be stored at low temperatures and its concentration may be assayed immediately prior to its use, solutions are stable under refrigeration for up to a week. The rate of decomposition will increase exposure to light or heavy metals. The most common use for DMDO is the oxidation of alkenes to epoxides, one particular advantage of using DMDO is that the only byproduct of oxidation is acetone, a fairly innocuous and volatile compound. DMDO oxidations are particularly mild, sometimes allowing oxidations which might not otherwise be possible, in fact, DMDO is considered the reagent of choice for epoxidation, and in nearly all circumstances is as good as or better than peroxyacids such as meta-chloroperoxybenzoic acid. Despite its high reactivity, DMDO displays good selectivity for olefins, typically, electron deficient olefins are oxidized more slowly than electron rich ones. DMDO will also oxidize several other functional groups, for example, DMDO will oxidize primary amines to nitro compounds and sulfides to sulfoxides. In some cases, DMDO will even oxidize unactivated C-H bonds, DMDO can also be used to convert nitro compounds to carbonyl compounds
16.
Diastereomer
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Diastereomers are a type of a stereoisomer. Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters and are not mirror images of each other, when two diastereoisomers differ from each other at only one stereocenter they are epimers. Each stereocenter gives rise to two different configurations and thus increases the number of stereoisomers by a factor of two, diastereomers differ from enantiomers in that the latter are pairs of stereoisomers that differ in all stereocenters and are therefore mirror images of one another. Enantiomers of a compound more than one stereocenter are also diastereomers of the other stereoisomers of that compound that are not their mirror image. Diastereomers have different physical properties and different chemical reactivity, diastereoselectivity is the preference for the formation of one or more than one diastereomer over the other in an organic reaction. When the single bond between the two centres is free to rotate, cis/trans descriptors are invalid, two widely accepted prefixes used to distinguish diastereomers on sp³-hybridised bonds in an open-chain molecule are syn and anti. Masamune proposed the descriptors which work even if the groups are not on adjacent carbons and it also works regardless of CIP priorities. Syn describes groups on the face while anti describes groups on opposite faces. The concept applies only to the Zigzag projection, the descriptors only describe relative stereochemistry rather than absolute stereochemistry. Two older prefixes still commonly used to distinguish diastereomers are threo, in the case of carbohydrates, when drawn in the Fischer projection the erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite sides. When drawn as a chain, the erythro isomer has two identical substituents on different sides of the plane. The names are derived from the diastereomeric aldoses erythrose and threose and these prefixes are not recommended for use outside of the realm of carbohydrates because their definitions can lead to conflicting interpretations. Another threo compound is threonine, one of the amino acids. The erythro diastereomer is called allo-threonine, if a molecule contains two asymmetric carbons, there are up to 4 possible configurations, and they cannot all be non-superimposable mirror images of each other. The possibilities continue to multiply as there are more asymmetric centers in a molecule, in general, the number of configurational isomers of a molecule can be determined by calculating 2n, where n = the number of chiral centers in the molecule. This holds true except in cases where the molecule has meso forms, for n =3, there are eight stereoisomers. There are four pairs of enantiomers, R, R, R and S, S, S, R, R, S and S, S, R, R, S, S and S, R, R, and R, S, R and S, R, S. There are four diastereomers, because each of the pairs of enantiomers is a diastereomer with respect to the other three, for n =4, there are sixteen stereoisomers, or eight pairs of enantiomers
17.
Ethylbenzene
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Ethylbenzene is an organic compound with the formula C6H5CH2CH3. It is a flammable, colorless liquid with an odor similar to that of gasoline. This monocyclic aromatic hydrocarbon is important in the industry as an intermediate in the production of styrene, the precursor to polystyrene. In 2012, more than 99% of ethylbenzene produced was consumed in the production of styrene, Ethylbenzene is also used to make other chemicals, in fuel, and as a solvent in inks, rubber adhesives, varnishes, and paints. Ethylbenzene exposure can be determined by testing for the products in urine. Ethylbenzene is a liquid that smells similar to gasoline with a sweet aroma. It has an odor with an odor threshold at 2.3 ppm and a melting point of −95 °C. It is classified as an aromatic hydrocarbon since it is a compound that contains one aromatic ring. Ethylbenzene occurs naturally in coal tar and petroleum, although this is not the source of this compound. The dominant application of ethylbenzene is role as an intermediate in the production of polystyrene. Catalytic dehydrogenation of ethylbenzene gives hydrogen and styrene, C 6H 5CH 2CH3 → C6H5CH=CH2 + H2 As of May 2012, Ethylbenzene is added to gasoline as an anti-knock agent, meaning it reduces engine knocking and increase the octane rating. Ethylbenzene is often found in manufactured products, including pesticides, cellulose acetate, synthetic rubber, paints. Used in the recovery of gas, ethylbenzene may be injected into the ground. Thus, manufacturers of ethylbenzene are the buyers of benzene. Small amounts of ethylbenzene are recovered from the mix of xylenes by superfractioning, in the 1980s a zeolite-based process using vapor phase alkylation, offered a higher purity and yield. Then a liquid phase process was introduced using zeolite catalysts and this offers low benzene-to-ethylene ratios, reducing the size of the required equipment and lowering byproduct production. Approximately 24,700,000 tons were produced in 1999, the acute toxicity of ethylbenzene is low, with an LD50 of about 4 grams per kilogram of body weight. The longer term toxicity and carcinogenicity is ambiguous, eye and throat sensitivity can occur when high level exposure to ethylbenzene in the air occurs
18.
Prilezhaev reaction
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The Prilezhaev reaction, also known as the Prileschajew reaction or Prilezhaev epoxidation, is the chemical reaction of an alkene with a peroxy acid to form epoxides. It is named after Nikolaus Prileschajew, who first reported this reaction in 1909, the most widely used peroxy acid for oxidation of the alkene is m-CPBA, due to its stability and good solubility in most organic solvents. An illustrative example is the epoxidation of styrene with perbenzoic acid to styrene oxide, the peroxide is viewed as an electrophile, and the alkene a nucleophile. The reaction is considered to be concerted, the butterfly mechanism allows ideal positioning of the O-O sigma star orbital for C-C Pi electrons to attack
19.
Peroxyacid
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A peroxy acid is an acid which contains an acidic –OOH group. The two main classes are derived from conventional mineral acids, especially sulfuric acid, and the organic derivatives of carboxylic acids. Peroxymonosulfuric acid is probably the most important inorganic peracid, at least in terms of the scale and it is used for the bleaching of pulp and for the detoxification of cyanide in the mining industry. It is produced by treating sulfuric acid with hydrogen peroxide, several organic peroxyacids are commercially useful. They can be prepared in several ways, the third method involves treatment of acid chlorides, RCCl + H2O2 → RCO3H + HCl meta-chloroperoxybenzoic acid is prepared in this way. Peroxycarboxylic acids are about 1000× weaker than the parent carboxylic acid, for similar reasons, their pKa values tend also to be relatively insensitive to the substituent R. The largest use of organic acids is for the conversion of alkenes to epoxides. Certain cyclic ketones are converted to the ring-expanded esters using peracids in a Baeyer-Villiger oxidation and they are also used for the oxidation of amines and thioethers to amine oxides and sulfoxides. The laboratory applications of the valued reagent mCPBA illustrate these reactions and it is used as a reagent in the Baeyer-Villiger oxidation and in oxidation of carbon-carbon double bonds in alkenes to generate epoxides. Reaction of peroxycarboxylic acids with acid chlorides affords diacyl peroxides, RCCl + RCO2H → 2O2 + HCl Organic peroxide Meta-Chloroperoxybenzoic acid Peracetic acid Peroxyacyl nitrates
20.
Meta-Chloroperoxybenzoic acid
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Meta-Chloroperoxybenzoic acid is a peroxycarboxylic acid used widely as an oxidant in organic synthesis. MCPBA is often preferred to other peroxy acids because of its ease of handling. MCPBA is an oxidizing agent that may cause fire upon contact with flammable material. MCPBA can be prepared by reacting m-chlorobenzoyl chloride with hydrogen peroxide in the presence of sulfate, aqueous sodium hydroxide. As a pure substance, mCPBA can be detonated by shock or by sparks and it is therefore sold commercially as a much more stable mixture that is less than 72% mCPBA, with the balance made up of m-chlorobenzoic acid and water. The peroxyacid can be purified by washing the commercial material with a slightly basic buffer solution, peroxyacids are generally slightly less acidic than their carboxylic acid counterparts, so one can extract the acid impurity by careful control of pH. The purified material is stable against decomposition if stored at low temperatures in a plastic container. In reactions where the amount of mCPBA must be controlled. The following scheme shows the reaction of cyclohexene with mCPBA to give an epoxide, the epoxidation mechanism is concerted, the cis or trans geometry of the alkene starting material is retained in the epoxide ring of the product
21.
Styrene
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Styrene, also known as ethenylbenzene, vinylbenzene, and phenylethene, is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is an oily liquid that evaporates easily and has a sweet smell. Styrene is the precursor to polystyrene and several copolymers, approximately 25 million tonnes of styrene were produced in 2010. Styrene is named for balsam, the resin of Liquidambar trees of the Altingiaceae plant family. Styrene occurs naturally in small quantities in some plants and foods, in 1839, the German apothecary Eduard Simon isolated a volatile oil from the resin of the American sweetgum tree. He also noticed that when Styrol was exposed to air, light, or heat, it transformed into a hard, rubber-like substance. By 1845, the German chemist August Hofmann and his student John Blyth had determined Styrols empirical formula and they had also determined that Simons Styroloxyd — which they renamed Metastyrol — had the same empirical formula as Styrol. Furthermore, they could obtain Styrol by dry distilling Metastyrol, in 1865, the German chemist Emil Erlenmeyer found that Styrol could form a dimer, and in 1866 the French chemist Marcelin Berthelot stated that Metastyrol was a polymer of Styrol. Meanwhile, other chemists had been investigating another component of storax, namely and they had found that cinnamic acid could be decarboxylated to form cinnamène, which appeared to be Styrol. In 1845, French chemist Emil Kopp suggested that the two compounds were identical, and in 1866, Erlenmeyer suggested that both cinnamol and Styrol might be vinyl benzene. However, the Styrol that was obtained from cinnamic acid seemed different from the Styrol that was obtained by distilling storax resin, the latter was optically active. Eventually, in 1876, the Dutch chemist van t Hoff resolved the ambiguity, the modern method for production of styrene by dehydrogenation of ethylbenzene was first achieved in the 1930s. The production of styrene increased dramatically during the 1940s, when it was popularized as a feedstock for synthetic rubber, because it is produced on such a large scale, ethylbenzene in turn prepared on a prodigious scale. Ethylbenzene is mixed in the gas phase with 10–15 times its volume in high-temperature steam, most ethylbenzene dehydrogenation catalysts are based on iron oxide, promoted by several percent potassium oxide or potassium carbonate. Steam serves several roles in this reaction and it is the source of heat for powering the endothermic reaction, and it removes coke that tends to form on the iron oxide catalyst through the water gas shift reaction. The potassium promoter enhances this decoking reaction, the steam also dilutes the reactant and products, shifting the position of chemical equilibrium towards products. A typical styrene plant consists of two or three reactors in series, which operate under vacuum to enhance the conversion and selectivity, typical per-pass conversions are ca. 65% for two reactors and 70-75% for three reactors. The main byproducts are benzene and toluene, because styrene and ethylbenzene have similar boiling points, their separation requires tall distillation towers and high return/reflux ratios
22.
Styrene oxide
–
Styrene oxide is an epoxide derived from styrene. It can be prepared by epoxidation of styrene with peroxybenzoic acid, in the Prilezhaev reaction, trace amount of acid in water causes hydrolysis to racemic phenylethyleneglycol via aryl cation. If the amount of water is not sufficient, acid-catalyzed isomerization for phenylacetaldehyde will occur, Styrene oxide in the body is metabolized to mandelic acid, phenyl glyoxylic acid, benzoic acid and hippuric acid. Since styrene oxide has a center at the benzylic carbon atom, there are -styrene oxide. In addition, nucleophiles attack the secondary carbon of the oxirane ring because of stability of the intermediate. If optically pure reagent is used, only one optically pure compound will be obtained, Styrene oxide is a main metabolite of styrene in humans or animals, resulting from oxidation by cytochrome P450. It is considered possibly carcinogenic from gavaging significant amounts into mice, Styrene oxide is subsequently hydrolyzed in vivo to styrene glycol by epoxide hydrolase. Styrene oxide has a center and thus two enantiomers. It has been reported that the two enantiomers had different toxicokinetics and toxicity and it was reported that the -styrene oxide was preferentially formed in mice, especially in the lung, whereas the -styrene oxide was preferentially generated in rats. In human volunteers, the excretion of the -enantiomer of styrene glycol. In human liver microsomes, cytochrome P450-mediated styrene oxidation showed the production of more S enantiomer relative to the R enantiomer and it was also found that -styrene oxide was preferentially hydrolyzed than the R enantiomer in human liver microsomes. Animal studies have shown that the -enantiomer of styrene oxide was more toxic than the -enantiomer in mice
23.
Electrophile
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In chemistry, an electrophile is a reagent attracted to electrons. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to a rich centre. It participates in a reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids, most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons. The electrophiles are attacked by the most electron-populated part of one nucleophile and these occur between alkenes and electrophiles, often halogens as in halogen addition reactions. Common reactions include use of water to titrate against a sample to deduce the number of double bonds present. Forming of a bromonium ion The alkene is working as an electron donor. The three-membered bromonium ion 2 consisted with two atoms and a bromine atom forms with a release of Br−. Attacking of bromide ion The bromonium ion is opened by the attack of Br− from the back side and this yields the vicinal dibromide with an antiperiplanar configuration. When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether and this process is called AdE2 mechanism. Iodine, chlorine, sulfenyl ion, mercury cation, and dichlorocarbene also react through similar pathways, the direct conversion of 1 to 3 will appear when the Br− is large excess in the reaction medium. A β-bromo carbenium ion intermediate may be predominant instead of 3 if the alkene has a cation-stabilizing substituent like phenyl group, there is an example of the isolation of the bromonium ion 2. Hydrogen halides such as hydrogen chloride adds to alkenes to give alkyl halides in hydrohalogenation, for example, the reaction of Hcl with ethylene furnishes chloroethane. The reaction proceeds with an intermediate, being different from the above halogen addition. An example is shown below, Proton adds to one of the atoms on the alkene to form cation 1. Chloride ion combines with the cation 1 to form the adducts 2 and 3, in this manner, the stereoselectivity of the product, that is, from which side Cl− will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H+ is usually decided by Markovnikovs rule, thus, H+ attacks the carbon atom that carries fewer substituents so as the more stabilized carbocation will form. This process is called A-SE2 mechanism, hydrogen fluoride and hydrogen iodide react with alkenes in a similar manner, and Markovnikov-type products will be given
24.
Nucleophile
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A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. All molecules or ions with a pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases, Nucleophilic describes the affinity of a nucleophile to the nuclei. Nucleophilicity, sometimes referred to as strength, refers to a substances nucleophilic character and is often used to compare the affinity of atoms. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis, nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge. The terms nucleophile and electrophile were introduced by Christopher Kelk Ingold in 1933, the word nucleophile is derived from nucleus and the Greek word φιλος, philos for love. In general, in a row across the table, the more basic the ion the more reactive it is as a nucleophile. g. The iodide ion is more nucleophilic than the fluoride ion, many schemes attempting to quantify relative nucleophilic strength have been devised. The following empirical data have been obtained by measuring reaction rates for a number of reactions involving a large number of nucleophiles and electrophiles. Nucleophiles displaying the so-called alpha effect are usually omitted in this type of treatment. This treatment results in the values for typical nucleophilic anions, acetate 2.7, chloride 3.0, azide 4.0, hydroxide 4.2, aniline 4.5, iodide 5.0. Typical substrate constants are 0.66 for ethyl tosylate,0.77 for β-propiolactone,1.00 for 2, 3-epoxypropanol,0.87 for benzyl chloride, and 1.43 for benzoyl chloride. The equation predicts that, in a nucleophilic displacement on benzyl chloride, the Ritchie equation, derived in 1972, is another free-energy relationship, log 10 = N + where N+ is the nucleophile dependent parameter and k0 the reaction rate constant for water. In this equation, a substrate-dependent parameter like s in the Swain–Scott equation is absent, the equation states that two nucleophiles react with the same relative reactivity regardless of the nature of the electrophile, which is in violation of the Reactivity–selectivity principle. For this reason this equation is called the constant selectivity relationship. Many other reaction types have since been described. Typical Ritchie N+ values are,0.5 for methanol,5.9 for the anion,7.5 for the methoxide anion,8.5 for the azide anion. The values for the relative cation reactivities are -0.4 for the malachite green cation, +2.6 for the benzenediazonium cation, the constant s is defined as 1 with 2-methyl-1-pentene as the nucleophile
25.
Molecular orbital
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In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region, the term orbital was introduced by Robert S. Mulliken in 1932 as an abbreviation for one-electron orbital wave function. At an elementary level, it is used to describe the region of space in which the function has a significant amplitude, Molecular orbitals are usually constructed by combining atomic orbitals or hybrid orbitals from each atom of the molecule, or other molecular orbitals from groups of atoms. They can be calculated using the Hartree–Fock or self-consistent field methods. A molecular orbital can be used to represent the regions in a molecule where an electron occupying that orbital is likely to be found, Molecular orbitals are obtained from the combination of atomic orbitals, which predict the location of an electron in an atom. A molecular orbital can specify the configuration of a molecule. Most commonly a MO is represented as a combination of atomic orbitals. They are invaluable in providing a model of bonding in molecules. Most present-day methods in computational chemistry begin by calculating the MOs of the system, a molecular orbital describes the behavior of one electron in the electric field generated by the nuclei and some average distribution of the other electrons. In the case of two electrons occupying the orbital, the Pauli principle demands that they have opposite spin. Necessarily this is an approximation, and highly accurate descriptions of the electronic wave function do not have orbitals. Molecular orbitals arise from allowed interactions between orbitals, which are allowed if the symmetries of the atomic orbitals are compatible with each other. Efficiency of atomic orbital interactions is determined from the overlap between two atomic orbitals, which is significant if the atomic orbitals are close in energy. Finally, the number of molecular orbitals that form must equal the number of orbitals in the atoms being combined to form the molecule. Here, the orbitals are expressed as linear combinations of atomic orbitals. Molecular orbitals were first introduced by Friedrich Hund and Robert S. Mulliken in 1927 and 1928, the linear combination of atomic orbitals or LCAO approximation for molecular orbitals was introduced in 1929 by Sir John Lennard-Jones. His ground-breaking paper showed how to derive the electronic structure of the fluorine and this qualitative approach to molecular orbital theory is part of the start of modern quantum chemistry. Linear combinations of atomic orbitals can be used to estimate the molecular orbitals that are formed upon bonding between the constituent atoms
26.
Enantioselective
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A single chiral atom or similar structural feature in a compound causes that compound to have two possible structures which are non-superposable, each a mirror image of the other. Each member of the pair is termed an enantiomorph, the property is termed enantiomerism. The presence of multiple features in a given compound increases the number of geometric forms possible. Enantiopure compounds refer to samples having, within the limits of detection and they are sometimes called optical isomers for this reason. Enantiomer members often have different chemical reactions with other enantiomer substances, since many biological molecules are enantiomers, there is sometimes a marked difference in the effects of two enantiomers on biological organisms. Owing to this discovery, drugs composed of one enantiomer can be developed to enhance the pharmacological efficacy. An example is eszopiclone, which is enantiopure and therefore administered in doses that are exactly 1/2 of the older, in the case of eszopiclone, the S enantiomer is responsible for all the desired effects, while the other enantiomer seems to be inactive. A dose of 2 mg of zopiclone must be administered to produce the therapeutic effect as 1 mg of eszopiclone. In chemical synthesis of enantiomeric substances, non-enantiomeric precursors inevitably produce racemic mixtures, in the absence of an effective enantiomeric environment, separation of a racemic mixture into its enantiomeric components is impossible. The R/S system is an important nomenclature system for denoting distinct enantiomers, another system is based on prefix notation for optical activity, - and - or d- and l-. The Latin for left and right is laevus and dexter, respectively, left and right have always had moral connotations, and the Latin words for these are sinister and rectus. The English word right is a cognate of rectus and this is the origin of the D, L and S, R notations, and the employment of prefixes levo- and dextro- in common names. Most compounds that one or more asymmetric carbon atoms show enantiomerism. There are a few compounds that do have asymmetric carbon atoms. An example of such an enantiomer is the sedative thalidomide, which was sold in a number of countries across the world from 1957 until 1961 and it was withdrawn from the market when it was found to cause of birth defects. One enantiomer caused the desirable effects, while the other, unavoidably present in equal quantities. The herbicide mecoprop is a mixture, with the --enantiomer possessing the herbicidal activity. Another example is the antidepressant drugs escitalopram and citalopram, citalopram is a racemate, escitalopram is a pure enantiomer
27.
Sharpless epoxidation
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The Sharpless epoxidation reaction is an enantioselective chemical reaction to prepare 2, 3-epoxyalcohols from primary and secondary allylic alcohols. The stereochemistry of the epoxide is determined by the diastereomer of the chiral tartrate diester employed in the reaction. The oxidizing agent is tert-butyl hydroperoxide, enantioselectivity is achieved by a catalyst formed from titanium tetra and diethyl tartrate. Only 5–10 mol% of the catalyst in the presence of 3Å molecular sieves is necessary, the Sharpless epoxidations success is due to five major reasons by Martijn Patist. First, epoxides can be converted into diols, aminoalcohols or ethers. Second, the Sharpless epoxidation reacts with primary and secondary allylic alcohols. Third, the products of the Sharpless epoxidation frequently have enantiomeric excesses above 90%, fourth, the products of the Sharpless epoxidation are predictable using the Sharpless Epoxidation model discovered by Debbie van Basten. Finally, the reactants for the Sharpless epoxidation are commercially available, K. Barry Sharpless shared the 2001 Nobel prize in Chemistry for his work on asymmetric oxidations. The prize was shared with William S. Knowles and Ryōji Noyori, the structure of the catalyst is uncertain. The chirality of the product of a Sharpless epoxidation is sometimes predicted with the following mnemonic, in this orientation, the diester tartrate preferentially interacts with the top half of the molecule, and the diester tartrate preferentially interacts with the bottom half of the molecule. This model seems to be valid despite substitution on the olefin, selectivity decreases with larger R1, but increases with larger R2 and R3. However, this method incorrectly predicts the product of allylic 1, the Sharpless epoxidation can also give kinetic resolution of a racemic mixture of secondary 2, 3-epoxyalcohols. While the yield of a resolution process cannot be higher than 50%. The Sharpless epoxidation is viable with a range of primary and secondary olefinic alcohols. Furthermore, with the exception noted above, a given dialkyl tartrate will preferentially add to the same independent of the substitution on the olefin. As one of the few, highly enantioselective reactions during its time, many manipulations of the 2, the Sharpless epoxidation has been used for the total synthesis of various carbohydrates, terpenes, leukotrienes, pheromones, and antibiotics. The main drawback of this protocol is the necessity of the presence of an allylic alcohol, the Jacobsen epoxidation, an alternative method to enantioselectively oxidise alkenes, overcomes this issue and tolerates a wider array of functional groups. Early work showed that allylic alcohols give facial selectivity when using m-CPBA as an oxidant and this selectivity was reversed when the allylic alcohol was acetylated
28.
Jacobsen epoxidation
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It is complementary to the Sharpless epoxidation. The Jacobsen epoxidation gains its stereoselectivity from a C2 symmetric manganese salen-like ligand, the manganese atom transfers an oxygen atom from chlorine bleach or similar oxidant. The reaction is named after its inventor, Eric Jacobsen, chiral-directing catalysts are useful to organic chemists trying to control the stereochemistry of biologically active compounds and develop enantiopure drugs. Several improved procedures have been developed, in 1991, Jacobsen published work where he attempted to perfect the catalyst. He was able to obtain ee values above 90% for a variety of ligands, also, the amount of catalyst used was no more than 15% of the amount of alkene used in the reaction. The degree of enantioselectivity depends on numerous factors, namely the structure of the olefin, the nature of the axial donor ligand on the active oxomanganese species, furthermore, the enantioselective epoxidation of conjugated dienes is much higher than that of the nonconjugated dienes. The enantioselectivity is explained by either a top-on approach or by an approach of the olefin. The mechanism of the Jacobsen–Katsuki epoxidation is not fully understood, the concerted pathway, the metalla oxetane pathway and the radical pathway. The most accepted mechanism is the concerted pathway mechanism, after the formation of the Mn complex, the catalyst is activated and therefore can form epoxides with alkenes. The alkene comes in from the approach and the oxygen atom now is bonded to the two carbon atoms and is still bonded to the manganese metal. Then, the Mn–O bond breaks and the epoxide is formed, the Mn-salen complex is regenerated, which can then be oxidized again to form the Mn complex. The radical intermediate accounts for the formation of mixed epoxides when conjugated olefins are used as substrates
29.
Shi epoxidation
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The Shi epoxidation is a chemical reaction described as the asymmetric epoxidation of alkenes with oxone and a fructose-derived catalyst. This reaction is thought to proceed via an intermediate, generated from the catalyst ketone by oxone. The addition of the group by the oxone facilitates the formation of the dioxirane by acting as a good leaving group during ring closure. It is notable for its use of a non-metal catalyst, the reaction was first discovered by Yian Shi, of Colorado State University in 1996. Many attempts at the synthesis of an efficient non-metal catalyst were made before one was discovered, the problem with previous catalysts was the rapid decomposition/oxidation of the dioxirane intermediate and lack of electrophilicity of the reactive ketone. Enantioselectivity of these early catalysts were also lowered because of large distances between the subunits and reaction centers, yielding less than 10 percent in enantiomeric excess. Oxidation by the active dioxirane catalyst takes place from the si-face and this catalyst functions efficiently as an asymmetric catalyst for unfunctionalized trans-olefins. Under normal pH conditions, an excess of 3 stoichiometirc amounts of ketone catalyst are needed due to a rate of decomposition. At basic pH conditions greater than 10 substoichiometric amounts are needed for epoxidations, higher temperatures result in further decomposition, thus a low temperature of zero degrees Celsius is used. Decomposition of reagents is bimolecular, so low amounts of oxone, the reaction is mediated by a D-fructose derived catalyst, which produces the enantiomer of the resulting epoxide. Solubilities of olefin organic substrate and oxidant differ, and thus a medium is needed. The first step in the catalytic reaction is the nucleophilic addition reaction of the oxone with the ketone group on the catalyst. This forms the reactive intermediate number 2 species, the Criegee intermediate that can lead to unwanted side reactions. The generation of species number 3 occurs under basic conditions. The sulfate group facilitates the subsequent formation of the dioxirane, intermediate species number 4, the activated dioxirane catalytic species then transfers an oxygen atom to the alkene, leading to a regeneration of the original catalyst. A potential side reaction that may occur is the Baeyer-Villiger reaction of intermediate 2, there are two proposed transition states, whose geometries are speculated and not corroborated by experimental evidence, but are attributed to stereoelectronic effects. Donation of these electrons into the forming C-O σ bonds of the bonds also encourages the formation of the spiro-product. The planar configuration is disfavored due to lack of pi-backbonding and steric hindrance of the groups with large alkyl functional groups of the catalytic ring
30.
Oxaziridine
–
An oxaziridine is an organic molecule that features a three-membered heterocycle containing oxygen, nitrogen, and carbon. In their largest application, oxazidines are intermediates in the production of hydrazine. Oxaziridines also serve as precursors to amides and participate in cycloadditions with various heterocumulenes to form substituted five-membered heterocycles, chiral oxaziridine derivatives effect asymmetric oxygen transfer to prochiral enolates as well as other substrates. Some oxaziridines also have the property of a barrier to inversion of the nitrogen. Oxaziridine derivatives were first reported in the mid-1950s by Emmons and subsequently by Krimm and Horner, whereas oxygen and nitrogen typically act as nucleophiles due to their high electronegativity, oxaziridines allow for electrophilic transfer of both heteroatoms. This unusual reactivity is due to the presence of the strained three membered ring and the relatively weak N-O bond. Nucleophiles tend to attack at the aziridine nitrogen when the substituent is small. The Peroxide process for the production of hydrazine through the oxidation of ammonia with hydrogen peroxide in the presence of ketones was developed in the early 1970s. Chiral camphorsulfonyloxaziridines proved useful in the syntheses of natural product. Both the Holton Taxol total synthesis and the Wender Taxol total synthesis feature asymmetric α-hydroxylation with camphorsulfonyloxaziridine, the two main approaches to synthesis of N-H, N-alkyl, and N-aryloxaziridines are oxidation of imines with peracids and amination of carbonyls. Additionally, oxidation of imines and oxidation of imines with chiral peracids may yield enantiopure oxaziridines. Some oxaziridines have the property of configurationally stable nitrogen atoms at room temperature due to an inversion barrier of 24 to 31 kcal/mol. Enantiopure oxaziridines where stereochemistry is entirely due to configurationally stable nitrogen are reported, while originally synthesized with mCPBA and the phase transfer catalyst benzyltrimethylammonium chloride, an improved synthesis using oxone as the oxidant is now most prevalent. Many N-sulfonyloxaziridines are used today, each slightly different properties. These reagents are summarized in the table below, with highly electron withdrawing perfluoroalkyl substituents, oxaziridines exhibit reactivity more similar to that of dioxiranes than typical oxaziridines. Notably, perfluoroalkyloxaziridines hydroxylate certain C-H bonds with high selectivity, perfluorinated oxaziridines may be synthesized by subjecting a perfluorinated imine to perfluoromethyl fluorocarbonyl peroxide and a metal fluoride to act as an HF scavenger. Oxaziridines are intermediates in the Peroxide process for the production of hydrazine, α-Hydroxyketones have been synthesized in many ways, including reduction of α-diketones, substitution of a hydroxyl for a leaving group and direct oxidation of an enolate. Oxodiperoxymolybdenum- and N-sulfonyloxaziridines are the most common sources of oxygen implemented in this process
31.
Cytochrome P450
–
Cytochromes P450 are proteins of the superfamily containing heme as a cofactor and, therefore, are hemoproteins. CYPs use a variety of small and large molecules as substrates in enzymatic reactions and they are, in general, the terminal oxidase enzymes in electron transfer chains, broadly categorized as P450-containing systems. The term P450 is derived from the peak at the wavelength of the absorption maximum of the enzyme when it is in the reduced state. CYP enzymes have been identified in all kingdoms of life, animals, plants, fungi, protists, bacteria, archaea, however, they are not omnipresent, for example, they have not been found in Escherichia coli. More than 200,000 distinct CYP proteins are known, most CYPs require a protein partner to deliver one or more electrons to reduce the iron. Cytochrome b5 can also contribute reducing power to this system after being reduced by cytochrome b5 reductase, mitochondrial P450 systems, which employ adrenodoxin reductase and adrenodoxin to transfer electrons from NADPH to P450. Bacterial P450 systems, which employ a ferredoxin reductase and a ferredoxin to transfer electrons to P450, cYB5R/cyb5/P450 systems, in which both electrons required by the CYP come from cytochrome b5. FMN/Fd/P450 systems, originally found in Rhodococcus species, in which a FMN-domain-containing reductase is fused to the CYP, P450 only systems, which do not require external reducing power. Notable ones include thromboxane synthase, prostacyclin synthase, and CYP74A, the most common reaction catalyzed by cytochromes P450 is a monooxygenase reaction, e. g. The convention is to italicise the name referring to the gene. For example, CYP2E1 is the gene encodes the enzyme CYP2E1—one of the enzymes involved in paracetamol metabolism. The CYP nomenclature is the naming convention, although occasionally CYP450 or CYP450 is used synonymously. However, some gene or enzyme names for CYPs may differ from this nomenclature, denoting the catalytic activity, examples include CYP5A1, thromboxane A2 synthase, abbreviated to TBXAS1, and CYP51A1, lanosterol 14-α-demethylase, sometimes unofficially abbreviated to LDM according to its substrate and activity. The current nomenclature guidelines suggest that members of new CYP families share at least 40% amino acid identity, there are nomenclature committees that assign and track both base gene names and allele names. The active site of cytochrome P450 contains a heme-iron center, the iron is tethered to the protein via a cysteine thiolate ligand. This cysteine and several flanking residues are conserved in known CYPs and have the formal PROSITE signature consensus pattern - - x - - - - - C - -. Because of the vast variety of reactions catalyzed by CYPs, the activities and properties of the many CYPs differ in many aspects. In general, the P450 catalytic cycle proceeds as follows, Substrate binds in proximity to the heme group, Substrate binding induces electron transfer from NADH via cytochrome P450 reductase or another associated reductase
32.
Naphthalene
–
Naphthalene is an organic compound with formula C 10H8. It is the simplest polycyclic aromatic hydrocarbon, and is a crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalenes structure consists of a pair of benzene rings. It is best known as the ingredient of traditional mothballs. In the early 1820s, two separate reports described a white solid with a pungent odor derived from the distillation of coal tar, in 1821, John Kidd cited these two disclosures and then described many of this substances properties and the means of its production. He proposed the name naphthaline, as it had been derived from a kind of naphtha, naphthalenes chemical formula was determined by Michael Faraday in 1826. The structure of two fused benzene rings was proposed by Emil Erlenmeyer in 1866, and confirmed by Carl Gräbe three years later, a naphthalene molecule can be viewed as the fusion of a pair of benzene rings. As such, naphthalene is classified as a polycyclic aromatic hydrocarbon. There are two sets of equivalent hydrogen atoms, the positions are numbered 1,4,5, and 8, and the beta positions,2,3,6. Unlike benzene, the bonds in naphthalene are not of the same length. The bonds C1−C2, C3−C4, C5−C6 and C7−C8 are about 1.37 Å in length and this difference, established by X-ray diffraction, is consistent with the valence bond model in naphthalene and in particular, with the theorem of cross-conjugation. This theorem would describe naphthalene as an aromatic benzene unit bonded to a diene, as such, naphthalene possesses several resonance structures. Two isomers are possible for mono-substituted naphthalenes, corresponding to substitution at an alpha or beta position, bicyclodecapentaene is a structural isomer with a fused 4–8 ring system. In electrophilic aromatic substitution reactions, naphthalene reacts more readily than benzene, for example, chlorination and bromination of naphthalene proceeds without a catalyst to give 1-chloronaphthalene and 1-bromonaphthalene, respectively. In terms of regiochemistry, electrophiles attack occurs at the alpha position, for beta substitution, the intermediate has only six resonance structures, and only two of these are aromatic. Sulfonation, however, gives a mixture of the alpha product 1-naphthalenesulfonic acid, the 1-isomer forms predominantly at 25 °C, and the 2-isomer at 160 °C. Sulfonation to give the 1- and 2-sulfonic acid occurs readily, H 2SO4 + C 10H8 → C 10H 7−SO 3H + H 2O Further sulfonation occurs to give di-, tri- and these 1, 8-dilithio derivatives are precursors to a host of peri-naphthalene derivatives. With alkali metals, naphthalene forms the dark blue-green radical anion salts such as sodium naphthalenide, the naphthalenide salts are strong reducing agents
33.
Toluene
–
Toluene /ˈtɒljuːiːn/, also known as toluol /ˈtɒljuːɒl/, is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a benzene derivative, consisting of a CH3 group attached to a phenyl group. As such, its IUPAC systematic name is methylbenzene, Toluene is widely used as an industrial feedstock and a solvent. In 2013, worldwide sales of toluene amounted to about 24.5 billion US-dollars, as the solvent in some types of paint thinner, contact cement and model airplane glue, toluene is sometimes used as a recreational inhalant and has the potential of causing severe neurological harm. The compound was first isolated in 1837 through a distillation of oil by a Polish chemist named Filip Walter. In 1843, Jöns Jacob Berzelius recommended the name toluin, in 1850, French chemist Auguste Cahours isolated from a distillate of wood a hydrocarbon which he recognized as similar to Devilles benzoène and which Cahours named toluène. Toluene reacts as an aromatic hydrocarbon in electrophilic aromatic substitution. Because the methyl group has greater electron-releasing properties than an atom in the same position. It undergoes sulfonation to give p-toluenesulfonic acid, and chlorination by Cl2 in the presence of FeCl3 to give ortho, importantly, the methyl side chain in toluene is susceptible to oxidation. Toluene reacts with Potassium permanganate to yield benzoic acid, and with chromyl chloride to yield benzaldehyde, the methyl group undergoes halogenation under free radical conditions. For example, N-bromosuccinimide heated with toluene in the presence of AIBN leads to benzyl bromide, the same conversion can be effected with elemental bromine in the presence of UV light or even sunlight. Toluene may also be brominated by treating it with HBr and H2O2 in the presence of light. C6H5CH3 + Br2 → C6H5CH2Br + HBr C6H5CH2Br + Br2 → C6H5CHBr2 + HBr The methyl group in toluene undergoes deprotonation only with strong bases. Catalytic hydrogenation of toluene gives methylcyclohexane, the reaction requires a high pressure of hydrogen and a catalyst. Final separation and purification is done by any of the distillation or solvent extraction processes used for BTX aromatics, Toluene is so inexpensively produced industrially that it is not prepared in the laboratory. In principle it could be prepared by a variety of methods, Toluene is mainly used as a precursor to benzene via hydrodealkylation, C6H5CH3 + H2 → C6H6 + CH4 The second ranked application involves its disproportionation to a mixture of benzene and xylene. When oxidized it yields benzaldehyde and benzoic acid, two important intermediates in chemistry, in addition to the synthesis of benzene and xylene, toluene is a feedstock for toluene diisocyanate, trinitrotoluene, and a number of synthetic drugs. Toluene is a solvent, e. g. for paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives, lacquers, leather tanners
34.
Benzopyrene
–
A benzopyrene is an organic compound with the formula C20H12. Structurally speaking, the isomers of benzopyrene are pentacyclic hydrocarbons and are fusion products of pyrene. Two isomeric species of benzopyrene are benzopyrene and the less common benzopyrene and they belong to the chemical class of polycyclic aromatic hydrocarbons. Related compounds include cyclopentapyrenes, dibenzopyrenes, indenopyrenes and naphthopyrenes, Benzopyrene is a component of pitch and occurs together with other related pentacyclic aromatic species such as picene, benzofluoranthenes, and perylene. It is naturally emitted by forest fires and volcanic eruptions and can also be found in tar, cigarette smoke, wood smoke. Fumes that develop from fat dripping on blistering charcoal are rich in benzopyrene, benzopyrenes are harmful because they form carcinogenic and mutagenic metabolites which intercalate into DNA, interfering with transcription. They are considered pollutants and carcinogens, the mechanism of action of benzopyrene-related DNA modification has been investigated extensively and relates to the activity of cytochrome P450 subclass 1A1. Seemingly, the activity of CYP1A1 in the intestinal mucosa prevents major amounts of ingested benzopyrene from entering portal blood. The intestinal detoxification mechanism seems to depend on receptors that recognize bacterial surface components, evidence exists to link benzopyrene to the formation of lung cancer. In February 2014, NASA announced an upgraded database for tracking polycyclic aromatic hydrocarbons, including benzopyrene. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets. Media related to Benzopyrene at Wikimedia Commons
35.
Enantiomer
–
A single chiral atom or similar structural feature in a compound causes that compound to have two possible structures which are non-superposable, each a mirror image of the other. Each member of the pair is termed an enantiomorph, the property is termed enantiomerism. The presence of multiple features in a given compound increases the number of geometric forms possible. Enantiopure compounds refer to samples having, within the limits of detection and they are sometimes called optical isomers for this reason. Enantiomer members often have different chemical reactions with other enantiomer substances, since many biological molecules are enantiomers, there is sometimes a marked difference in the effects of two enantiomers on biological organisms. Owing to this discovery, drugs composed of one enantiomer can be developed to enhance the pharmacological efficacy. An example is eszopiclone, which is enantiopure and therefore administered in doses that are exactly 1/2 of the older, in the case of eszopiclone, the S enantiomer is responsible for all the desired effects, while the other enantiomer seems to be inactive. A dose of 2 mg of zopiclone must be administered to produce the therapeutic effect as 1 mg of eszopiclone. In chemical synthesis of enantiomeric substances, non-enantiomeric precursors inevitably produce racemic mixtures, in the absence of an effective enantiomeric environment, separation of a racemic mixture into its enantiomeric components is impossible. The R/S system is an important nomenclature system for denoting distinct enantiomers, another system is based on prefix notation for optical activity, - and - or d- and l-. The Latin for left and right is laevus and dexter, respectively, left and right have always had moral connotations, and the Latin words for these are sinister and rectus. The English word right is a cognate of rectus and this is the origin of the D, L and S, R notations, and the employment of prefixes levo- and dextro- in common names. Most compounds that one or more asymmetric carbon atoms show enantiomerism. There are a few compounds that do have asymmetric carbon atoms. An example of such an enantiomer is the sedative thalidomide, which was sold in a number of countries across the world from 1957 until 1961 and it was withdrawn from the market when it was found to cause of birth defects. One enantiomer caused the desirable effects, while the other, unavoidably present in equal quantities. The herbicide mecoprop is a mixture, with the --enantiomer possessing the herbicidal activity. Another example is the antidepressant drugs escitalopram and citalopram, citalopram is a racemate, escitalopram is a pure enantiomer
36.
Chemical reaction
–
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
37.
Allyl alcohol
–
Allyl alcohol is an organic compound with the structural formula CH2=CHCH2OH. Like many alcohols, it is a water-soluble, colourless liquid, allyl alcohol is used as a raw material for the production of glycerol, but is also used as a precursor to many specialized compounds such as flame-resistant materials, drying oils, and plasticizers. Today allyl alcohol can be obtained by many methods, allyl alcohol is the smallest representative of the allylic alcohols. Allyl alcohol was first prepared in 1856 by Auguste Cahours and August Hofmann by saponification of allyl iodide, the advantage of this method relative to the allyl chloride route is that it does not generate salt. Also avoiding chloride-containing intermediates is the acetoxylation of propylene to allyl acetate, in alternative fashion, propylene can be oxidized to acrolein, which upon hydrogenation gives the alcohol. In principle, allyl alcohol can be obtained by dehydrogenation of propanol, in the laboratory, it has been prepared by the reaction of glycerol with oxalic or formic acids. Allyl alcohols in general can be prepared by oxidation of allyl compounds by selenium dioxide. Allyl alcohol is converted mainly to glycidol, which is an intermediate in the synthesis of glycerol, glycidyl ethers, esters. Also, a variety of polymerizable esters are prepared from allyl alcohol, allyl alcohol is more toxic than related alcohols. Its threshold limit value is 2 ppm and it is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U. S. Emergency Planning and Community Right-to-Know Act, and is subject to reporting requirements by facilities which produce, store. International Chemical Safety Card 0095 NIOSH Pocket Guide to Chemical Hazards #0017, national Institute for Occupational Safety and Health. Institut national de recherche et de sécurité, state of Michigan public information on allyl alcohol Occupational exposure guidelines Datasheet regulatory information Propargyl alcohol
38.
Epichlorohydrin
–
Epichlorohydrin is an organochlorine compound and an epoxide. Despite its name, it is not a halohydrin and it is a colorless liquid with a pungent, garlic-like odor, moderately soluble in water, but miscible with most polar organic solvents. It is a chiral molecule generally existing as a mixture of right-handed and left-handed enantiomers. Epichlorohydrin is a reactive compound and is used in the production of glycerol, plastics, epoxy glues and resins. In contact with water, epichlorohydrin hydrolyzes to 3-MCPD, a found in food. Epichlorohydrin was first described in 1848 by Marcellin Berthelot, the compound was isolated during studies on reactions between glycerol and gaseous hydrogen chloride. Glycerol is a co-product of biodiesel produced on a large scale, the conversion of glycerol into other building block chemicals is of interest because glycerol is otherwise hard to dispose of. Dow and Solvay are building glycerol-to-epichlorohydrin plants in Shanghai and Thailand, in Dows process, glycerol is dichlorinated with hydrogen chloride with the help of a carboxylic acid catalyst. The mixture of dichlorohydroxypropanes is treated with base to form epichlorohydrin, epichlorohydrin is mainly converted to bisphenol A diglycidyl ether, a building block in the manufacture of epoxy resins. It is also a precursor to monomers for other resins and polymers, synthetic glycerol is now used only in sensitive pharmaceutical, technical and personal care applications where quality standards are very high. Epichlorohydrin is a precursor in the synthesis of many organic compounds. For example, it is converted to nitrate, an energetic binder used in explosive. The epichlorohydrin is reacted with an alkali nitrate, such as nitrate, producing glycidyl nitrate. It is used as a solvent for cellulose, resins, and paints, an important biochemical application of epichlorohydrin is its use as crosslinking agent for the production of Sephadex size-exclusion chromatographic resins from dextrans. Epichlorohydrin is classified by several health research agencies and groups as a probable or likely carcinogen in humans. Prolonged oral consumption of high levels of epichlorohydrin could result in stomach problems, occupational exposure to epichlorohydrin via inhalation could result in lung irritation and an increased risk of lung cancer
39.
Dehydrohalogenation
–
Dehydrohalogenation is a chemical reaction that involves removal of a hydrogen halide from a substrate. The reaction is associated with the synthesis of alkenes. Traditionally, alkyl halides are substrates for this dehydrohalogenations, the alkyl halide must be able to form an alkene, thus methyl and benzy halides are not suitable substrates. Upon treatment with base, chlorobenzene dehydrohalogenates to give phenol via a benzyne intermediate. When treated with a strong base many alkyl chlorides convert to corresponding alkene and it is also called a β-Elimination reaction and is a type of elimination reaction. Zaitsevs rule helps to predict regioselectivity for this reaction type, likewise, 1-chloropropane and 2-chloropropane give propene. In general, the reaction of haloalkane with potassium hydroxide can compete with an SN2 nucleophilic substitution reaction by OH− a strong, alcohols are however generally minor products. Often dehydrohalogenations employ strong bases such as potassium tert-butoxide, on an industrial scale, base-promoted dehydrohalogenations as described above are disfavored. The disposal of the halide salt is problematic. One example is provided by the production of vinyl chloride by heating 1, 2-dichloroethane, thermally induced dehydrofluorinations are employed in the production of fluoroolefins and hydrofluoroolefins. This reaction is employed industrially to produce millions of tons of propylene oxide annually from propylene chlorohydrin, CH3CHCH2Cl + KOH → CH3CHCH2 + H2O + KCl Dehydrohalogenation
40.
Williamson ether synthesis
–
The Williamson ether synthesis is an organic reaction, forming an ether from an organohalide and a deprotonated alcohol. This reaction was developed by Alexander Williamson in 1850, typically it involves the reaction of an alkoxide ion with a primary alkyl halide via an SN2 reaction. This reaction is important in the history of chemistry because it helped prove the structure of ethers. Both symmetrical and asymmetrical ethers are easily prepared, the intramolecular reaction of halohydrins in particular, gives epoxides. In the case of asymmetrical ethers there are two possibilities for the choice of reactants, and one is usually either on the basis of availability or reactivity. The Williamson reaction is frequently used to prepare an ether indirectly from two alcohols. One of the alcohols is first converted to a leaving group, the alkoxide may be primary, secondary or tertiary. The alkylating agent, on the hand is most preferably primary. Secondary alkylating agents also react, but tertiary ones are too prone to side reactions to be of practical use. The leaving group is most often a halide or a sulfonate ester synthesized for the purpose of the reaction. In laboratory chemistry, in situ generation is most often accomplished by the use of a base or potassium hydroxide. A wide range of solvents can be used, but protic solvents and apolar solvents tend to slow the reaction rate strongly, for this reason, acetonitrile and N, N-dimethylformamide are particularly commonly used. A typical Williamson reaction is conducted at 50–100 °C and is complete in 1–8 hours, often the complete disappearance of the starting material is difficult to achieve, and side reactions are common. Yields of 50–95% are generally achieved in laboratory syntheses, while near-quantitative conversion can be achieved in industrial procedures, catalysis is not usually necessary in laboratory syntheses. However, if an unreactive alkylating agent is used then the rate of reaction can be improved by the addition of a catalytic quantity of a soluble iodide salt. In extreme cases, silver salts may be added for example silver oxide, finally, phase transfer catalysts are sometimes used in order to increase the solubility of the alkoxide by offering a softer counter-ion. In particular, some structures of alkylating agent can be prone to elimination. When the nucleophile is an ion, the Williamson reaction can also compete with alkylation on the ring since the aroxide is an ambident nucleophile
41.
Alkoxide
–
An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO−, where R is the organic substituent, alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands. Alkoxides, although generally not stable in protic solvents such as water, occur widely as intermediates in various reactions, transition metal alkoxides are widely used for coatings and as catalysts. Enolates are unsaturated alkoxides derived by deprotonation of a C-H bond adjacent to a ketone or aldehyde, the nucleophilic center for simple alkoxides is located on the oxygen, whereas the nucleophilic site on enolates is delocalized onto both carbon and oxygen sites. Phenoxides are close relatives of the alkoxides, in which the group is replaced by a derivative of benzene. Phenol is more acidic than a typical alcohol, thus, phenoxides are correspondingly less basic and they are, however, often easier to handle, and yield derivatives that are more crystalline than those of the alkoxides. Alkoxides can be produced by several routes starting from an alcohol, highly reducing metals react directly with alcohols to give the corresponding metal alkoxide. The alcohol serves as an acid, and hydrogen is produced as a by-product, many other metal and main group halides can be used instead of titanium, for example SiCl4, ZrCl4, and PCl3. Many alkoxides can be prepared by dissolution of the corresponding metals in water-free alcohols in the presence of electroconductive additive. The metals may be Co, Ga, Ge, Hf, Fe, Ni, Nb, Mo, La, Re, Sc, Si, Ti, Ta, W, Y, Zr, etc. The conductive additive may be lithium chloride, quaternary ammonium halide, some examples of metal alkoxides obtained by this technique, Ti4, Nb210, Ta210,2, Re2O36, Re4O612, and Re4O610. Other alcohols can be employed in place of water, in this way one alkoxide can be converted to another, and the process is properly referred to as alcoholysis. The position of the equilibrium can be controlled by the acidity of the alcohol, for example phenols typically react with alkoxides to release alcohols, more simply, the alcoholysis can be controlled by selectively evaporating the more volatile component. In this way, ethoxides can be converted to butoxides, since ethanol is more volatile than butanol, in the transesterification process, metal alkoxides react with esters to bring about an exchange of alkyl groups between metal alkoxide and ester. Sodium methoxide, for example, is used for this purpose. Many metal alkoxide compounds also feature oxo-ligands, oxo-ligands typically arise via the hydrolysis, often accidentally, and via ether elimination,2 LnMOR → 2O + R2O Additionally, low valent metal alkoxides are susceptible to oxidation by air. Characteristically, transition metal alkoxides are polynuclear, that is they contain more than one metal, alkoxides are sterically undemanding and highly basic ligands that tend to bridge metals. Upon the isomorphic substitution of metal atoms close in properties crystalline complexes of variable composition are formed, the metal ratio in such compounds can vary over a broad range
42.
Halohydrin
–
In organic chemistry a halohydrin is a functional group in which a halogen and a hydroxyl are bonded to adjacent carbon atoms, which otherwise bear only hydrogen or hydrocarbyl groups. The term only applies to saturated motifs, as such compounds like 2-chlorophenol would not normally be considered halohydrins, megatons of some chlorohydrins, e. g. propylene chlorohydrin, are produced annually as precursors to polymers. Halohydrins may be categorized as chlorohydrins, bromohydrins, fluorohydrins or iodohydrins depending on the halogen present, halohydrins are usually prepared by treatment of an alkene with a halogen, in the presence of water. The reaction is a form of addition, similar to the halogen addition reaction and proceeds with anti addition, leaving the newly added X. Halohydrins may also be prepared from the reaction of an epoxide with a hydrohalic acid, the reaction is produced on an industrial scale for the production of chlorohydrin precursors to two important epoxides, epichlorohydrin and propylene oxide. In presence of a base a halohydrin undergo internal SN2 reaction to form an epoxides, industrially, the base is calcium hydroxide, whereas in the laboratory, potassium hydroxide is often used. This reaction is the reverse of the reaction from an epoxide. Most of the supply of propylene oxide arises via this route. Such reactions can form the basis of more complicated processes, for example epoxide formation is one of the key steps in the Darzens reaction. Compounds such as 2,2, 2-trichloroethanol, which contain several geminal halogens adjacent to a group may be considered halohydrins as they possess similar chemistry. In particular they also undergo intramolecular cyclisation to form dihaloepoxy groups and these species are both highly reactive and synthetically useful, forming the basis of the Jocic-Reeve, Bargellini and Corey–Link reactions. In general, simpler low molecular compounds are often toxic and carcinogenic by virtue of being alkylating agents. This reactivity can be put to use, for instance in the anti-cancer drug mitobronitol. A number of synthetic corticosteroids exist baring a fluorohydrin motif, despite their rather suggestive names epichlorohydrin and sulfuric chlorohydrin are not halohydrins
