CH3CH3 750 ℃ CH2=CH2 + H2
CH3CH2CH3 750 ℃ CH3CH=CH2 + H2
Both reactions involve β 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 βeliminations: the dehydration of alcohols and the dehydrohalogenation of alkyl halides.
- 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 β elimination reactions. Zaitsev’s rule is an empirical one and summarizes the results of numerous experiments in which alkene mixtures were produced by β 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 β 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: β 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 β 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:
- Dehydrohalogenation of alkyl Halides:
Dehydrohalogenation is loss of a hydrogen and a halogen from an alkyl halide. It is one of the most useful methods for preparing alkenes by β elimination.
When applied to the preparation of alkenes, the reaction is carried out in the presence of a strong base, such as NaOCH2CH3 in ethyl alcohol as solvent.
Mechanism of the dehydrohalogenation of alkyl halides: The E2 mechanism
1.The dehydrohalogenation reaction exhibits second-order kinetics; it is first-order in alkyl halide and first-order in base
2. The rate of elimination depends on the halogen, the reactivity of alkyl halides increasing with decreasing strength of the carbon-carbon bond.
Increasing rate of dehaydrohalogenation
RF < RCl< RBr< RI.
<2> The Witting reaction:
Alkenes may be formed by dehydrating alcohols with conc. sulphuric acid or by passing over hot Al2O3. A powerful method of synthesizing alkenes is the Wittig reaction.
the reaction of an α-phosphorus-stabilised anion with a carbonyl derivative, is undoubtedly one of the most useful methods for selectively constructing carbon-carbon double bonds.
Phosphorus is a second row element - in Group 5 - like nitrogen - but unlike nitrogen can expand its valency from 3 to 4, 5, or even 6. The stable 5 valency is met in compounds like phosphoric acid and PCl5. What Wittig discovered was that phosphines (the P equivalent of amines) easily form phosphonium salts with alkyl halides and that these salts readily lose HX with a strong base. The product is called an ylide or a phosphorane.
Alkene is made up by linking an aldehyde or ketone to an alkyl halide replacing the O and the halide by a double bond. Just snip the alkene at the double bond and make one half an aldehyde or ketone and the other half an alkyl halide.
Another variant on the Wittig reaction that is much easier and cheaper to carry out is called the Wittig-Horner reaction. This uses a phosphite ester instead of a phosphine:
(How to Tackle Organic Syntheses, A Beginner's Guide by Prof Otto Meth-Cohn, writing reaction, center for chemistry study, Ol’spidey, 1997)
<3> Reduction of Alkynes:
The conversion of alkynes into alkenes, by addition of two hydrogen atoms across the carbon-carbon triple bond, is invaluable in organic synthesis. There are several methods of performing this addition. Such as the partial hydrogenation of the triple bond by homogeneous catalysis which, in general affords predominantly (Z)-alkenes, and using of hydride reagents. Also alkynes can be reduced using dissolving metal reactions which have an essential role to play I the production of (E)-alkenes.
- Reduction of alkynes with homogeneous catalysts
C6H13C CC6H13 H2,[methyl benzoate] H13C6CH=CHC6H13 (Sodeoka,M.; Shibasaki,M.J. Org. Chem. 1985, 50, 1147-1149, 3246)
which shows the 7-tetra-decyne is reduced to the (Z)-alkene in quantitative yield, and this reaction is in the catalyst [methyl benzoate Cr(CO)3] and 70 atm, 120℃.
- Reduction with transition metal hydride reagents
The complex [RhH2(OC(=O)OH)(PPri3)2] gives dimethyl fumarate and (E)-stilbene on reduction of their respective alkyne precursors as shown in this example as: R H
RC CR = R= Ph, CO2Me
H R ( Yoshida, T.; Youngs, W.J.; Sakaeda, T.; Ueda, T.; Ibers, J. A. J. Am. Chem. Soc. 1983, 105, 6273-6278)
<4> Electrophilic addition of alkyne
The alkyne can react with one electrophilic reagents by addition, then gives the alkene halide as the product.
HC CH Cl2 HClC=CClH
As the same the ethyne also can make an addition reaction with Br2. But there must be the proper quantities.
In additional, alkyne can add to HX (X= Cl, Br, and I), then generating the one-halide alkene. However, it is more difficult to react with electrophilic. Therefore we use the catalyst that is HgCl2 and HCl with the graphite to urge the reaction to progress favourably.
HC CH HgCl2 CH2=CHCl
<5> Polymerization of ethyne:
The Ni(CN)2 is the catalyst of the reaction of polymerization, at the 80-120℃, 1.5 MPa.
4 HC CH
( cycloctene)
(Chinese universities’ textbook, alkene, polymerization, 1999)
<6> Form the petroleum
In industry producing, people usually use the petroleum to generate the alkenes by cracking.
- Conclusion:
For making the alkene, there are hundreds of methods. In spite of the fact that some of the methods are very complex, whatever they are all according to the properties of alkenes to discover.
Generally speaking, there are two main hints for making the alkene that generates the carbon-carbon double bond from the original carbon frame and synthesis the carbon-carbon double bond as well as construct a new carbons frame. Usually the elimination reactions belong to the first hint of making alkene.
As above of the methods of making alkene, dehydration is the most common method. But the petroleum cracking is the most useful in industry. Further the ethene and propylene are the most important in industry. Nowadays, the standard of the ethene manufacture is used to scale the development of the petroleum chemistry industry for a country. Therefore people are making great efforts to investigate and attempt their new ideas which are the advanced. Then making the progress from the exploring.
