Preparation of alkenes

The most industrially important alkenes -- ethene, propene, and buta-1,3-diene -- along with all alkenes up to four carbons, can be obtained in pure form from petroleum products; ethene specifically is produced from natural gas and crude oil by the cracking process already met in section 15.1.5. In the laboratory, alkenes are chiefly made by 1,2-elimination reactions, in which two atoms or groups are removed from two adjacent carbon atoms, converting the sp3-sp3 single bond between them into an sp2-sp2 double bond; when the atom removed from the adjacent (beta) carbon is a hydrogen, the reaction is called a beta-elimination. The most common such reaction is dehydrohalogenation of an alkyl halide: boiling the alkyl halide with a hot, concentrated ALCOHOLIC solution of a strong base such as KOH or NaOH removes H (from the beta-carbon) and X (from the alpha-carbon, the one that originally carried the halogen) together as water and the base's halide salt, leaving the alkene. When more than one beta-hydrogen is available (as in 2-chlorobutane, which can eliminate toward either C1 or C3), Saytzeff's rule predicts that the major product is the MORE substituted, more stable alkene -- 2-chlorobutane gives 80% but-2-ene against only 20% but-1-ene -- and correspondingly, the ease of dehydrohalogenation of the starting alkyl halide itself follows tertiary > secondary > primary. A second elimination route is dehydration of an alcohol: heating the alcohol with concentrated sulphuric acid removes -OH from the alpha-carbon and H from a beta-carbon as water, and just as with alkyl halides, the ease of dehydration follows tertiary > secondary > primary, so a tertiary alcohol dehydrates under far milder acid concentration and temperature (e.g. 20% H2SO4 at 363 K for 2-methylpropan-2-ol) than a primary one (75% H2SO4 at 413 K for butan-1-ol). A third elimination route is dehalogenation of a vicinal dihalide (two halogens on adjacent carbons): heating with zinc metal removes both halogens together as ZnX2, regenerating the alkene -- 1,2-dibromoethane gives ethene, and 1,2-dibromopropane gives propene. Alkenes can also be made by an ADDITION reaction rather than an elimination: partial ( …