Alkenes

ALKENES

Alkenes are obtained from alcohol or alkyl halides by elimination reactions. Alcohols undergo eliminations of water by heating with sulfuric or phosphoric acid, or by passing the alcohol vapor over alumina or silica catalysts at high temperatures. Alkyl halides undergo loss of halogen acid (dehydrohalogenation) by heating with a solution of potassium hydroxide in ethanol.

Alkenesare produced in enormous quantities in the petroleum industry by the pyrolysis (cracking) of alkenes at 400°-600°, in the presence of metallic oxide catalysts. Large alkane molecules undergo rupture of carbon-carbon bonds to form a mixture of smaller alkanes and alkenes. Catalytic dehydrogenation of alkanes is also an important industrial method for producing alkenes. The ease of dehydration ofalcohols varies with the structural class, according to the sequence; tertiary › secondary › primary. Tert-butyl alcohol is converted rapidly to isobutylene by 40-50 percent acid at 135-140°. Phosphoric acid has the advantage of causing less oxidative degradation than strong sulfuric acid but the rate of raction is slower and higer temperatures are required.

The specific examples given in thisexperiment illustrate the dehydration of secondary alcohols by two slightly different procedures: 65 per cent sulfuric acid in excess at 80°-120°° (2-pentene and methylpentenes) and concentrated sulfuric acid in catalytic amount at higher temperature (cyclohexene).

Carbonium Ion Rearrangements. The ease of dehydration of an alcohol is controlled by the formation of the intermediate carbonium ionresulting from rupture of the carbon-oxygen bond. The stability and ease of formation of carbonium ions follow the sequence tertiary › secondary › primary. Elimination of a proton from the atom adjacent to the electron. Deficient carbon atom leads to an alkene. If two different alkenes can be formed, usually there is selectivity in the mode of elimination and one of the isomers predominates,2-pentanol, for axample, yields almost entirely 2-pentene and very little 1-pentene.

In many instances the double bond is formed at a position removed from the carbon atom that was bonded to the hydroxyl group and occasionally the carbon skeleton itself is altered during the reaction. The formation of 2-pentene from 1-pentanol is the consequence of migration of hydrogen with its bonding electrons(hydride shift), thereby converting the primary 1-pentyl carbonium ion into the more stable secondary pentyl carbonium ion before alkene formation takes place.

The occurrence of a similar migration of a methyl group with its bonding electrons (methide shift) is observed in the dehydration of neopentyl alcohol,(CH3)3C-CH2-OH. The shift of a methyl group converts the primary neopentyl carboniumion into the more stable tertiary pentyl structure and leads to the formation of 2-methyl-2-butene.

Addition of a carbonium ion to an alkene furnishes a new carbonium ion and leads stepwise to alkene polymers of increasing molecular weight (dimers, trimmers and higher polymers). Usually small amounts of such dimers anf trimmers are formed in the preparation of alkenes from alcohols.Reactions. Alkenes are used in the laboratory and in industry as starting materials or intermediates for the synthesis of many important compounds. These transformations generally involve the characteristic addition reactions of the carbon-carbon double bond (ionic mechanism), with electrophilic reagents such as sulfuric acid or halogen acids, hypochlorous acid, chlorine or bromine, and oxidizing agents.With unsymmetrical alkenes such as propylene and isobutylene, the mode of addition is highly selective: the electron-deficient fragment of the reagent adds to the carbon atom bearing the larger number of hydrogen atoms (the Markovnikov rule). This orientation of sddition is attributed to electron-release by the alkyl group (s), which increases electron density at the less highly substituted...