Halides, alkyl, base induced alkenes

The synthesis of stereodefined acyclic alkenes via 3-elimination reactions—such as (1) dehydration of alcohols, (2) base-induced eliminations of alkyl halides or sulfonates (tosyl or mesyl esters), and (3) Hofmann eliminations of quaternary ammonium salts—often suffers from a lack of regio- and stereoselectivity, producing mixtures of isomeric alkenes. 

Purpose. This reaction illustrates the base-induced dehydrohalogenation of alkyl halides with strong base and is used extensively for the preparation of alkenes. The stereo- and regiochemical effects of the size of the base is investigated, and the product mixture is analyzed by the use of gas chromatography. 

Geometrical Considerations in the E2 Elimination Reaction. Base-induced elimination of alkyl halides (dehydrohalogenation) is a general reaction and is an excellent method for preparing alkenes.This process is often referred to as -elimination, since a hydrogen atom is always removed p to the halide (leaving group) ... 

Here too, a second alkylation can be made to take place yielding RC=CR or R C=CR. It should, however, be remembered that the above carbanions—particularly the acetylide anion (57)—are the anions of very weak acids, and are thus themselves strong bases, as well as powerful nucleophiles. They can thus induce elimination (p. 260) as well as displacement, and reaction with tertiary halides is often found to result in alkene formation to the exclusion of alkylation. 

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