Alkenes (or olefins, olefines in old-fashioned parlance) – unsaturated hydrocarbons that contain one or more carbon-carbon double bonds. In IUPAC nomenclature alkenes have the -ene suffix. The mono-unsaturated alkenes ethene, propene, butene . . form a homologous series with the general formula CnH2n. As an example of ethylene, each of the carbon atoms is sp2-hybridized, and the double bond possesses a ? component and a ? component. Homologues with n greater than or equal to 4 exhibit geometric isomerism (E-Z national system).
The physical properties of alkenes: the dipole moments of most alkenes are quite small, a methyl group releases electrons to a double bond in much the same way that it releases electrons to the positively charged carbon of a carbocation-by an inductive effect and by hyperconjugation. In general, alkenes with more highly substituted double bonds are more stable than isomers with less substituted double bonds. Van der Waals strain. Alkenes are more stable when large substituents are trans to each other than when they are cis. Alkenes are very important in industry, daily life and so many others fields.
We can use the alkenes as fuel in aerospace, industry and so on, which not does only protect the natural environment, but it is also economic. In biological and curatorial fields, they can make much more contribution. Therefore people is trying to make alkenes in different ways, and trying their best to find the easiest and the most economic methods. 1 Making the alkene There are so many ways can make an alkene, usually we make the alkenes by elimination reactions. <1>. i?? elimination reaction: Ethylene and propene are prepared on an industrial scale by the high temperature dehydrogenation of ethane and propane.
Both reactions involve i?? elimination of H2. Many reactions classified as dehydrogenations occur within the cells of living systems at 25 ?. (The enzyme indicated is a special kind, known as flavoprotein in reaction) Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes. The principal methods by which alkenes are prepared in the laboratory are two other i?? eliminations: the dehydration of alcohols and the dehydrohalogenation of alkyl halides. (i) Dehydration of Alcohols: In the dehydration of alcohols, the H and OH are lost from adjacent carbons.
An acid catalyst is necessary. Before dehydrogenation of ethane became the dominant method, ethylene was prepared heating ethyl alcohol with sulfuric acid. CH3CH2OH H2SO4 CH2=CH2 + H2O Other alcohols behave similarly. Secondary alcohols undergo elimination at lower temperatures than primary alcohols, In 1875 Alexander M. Zaitsev of the University of Kazan set forth a generalization describing the regioselective to be expected in i?? elimination reactions. Zaitsev’s rule is an empirical one and summarizes the results of numerous experiments in which alkene mixtures were produced by i?? elimination.
In its original form Zaitsev’s rule state that the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the i?? carbon having the fewest hydrogen sustituents. Zaitsev’s rule as applied to the acid-catalyzed dehydration of alcohols is now more often expressed in a different way: i?? elimination reactions of alcohols yield the most highly substituted alkene as the major product. Zaitsev’s rule is sometimes expressed in terms of a preference for predominant formation of the most stable alkene that could arise by i?? elimination. In addition to being regioselective, alcohol dehydrations are stereoselective.
A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other (cis-, trans-). The biological dehydrogenation of succinic acid is 100% stereoselective. The mechanism of acid-catalysed dehydration of alcohols. The overall reaction: (CH3)3COH (CH3)3C=CH2 + H2O Step 1: protonation of tert-butyl alcohol: Step 2: Dissociation of tert-butyloxonium ion: Step 3: Deprotation of tert-butyl cation: (ii) Dehydrohalogenation of alkyl Halides: Dehydrohalogenation is loss of a hydrogen and a halogen from an alkyl halide.