Preparation of alkenes and alkynes

PREPARATION OF ALKENES AND ALKYNES

Key Notes

Alkenes

Alkenes can be synthesized by the reduction of alkynes or by the elimina-tion of alkyl halides or alcohols. Vicinal dibromides can be debrominated by treatment with zinc dust in acetic acid or with sodium iodide in acetone.

Alkynes

Alkenes can be treated with bromine to give a vicinal dibromide. Treatment of the dibromide with a strong base such as sodium amide results in the loss of two molecules of hydrogen bromide (dehydrohalogenation) and the for-mation of an alkyne.

Alkenes

Alkenes can be obtained by the transformation of various functional groups such as the reduction of alkynes, the elimination of alkyl halides, or the elimination of alcohols.

Alkenes can also be synthesized from vicinal dibromides, that is, molecules which have bromine atoms on neighboring carbon atoms. This reaction is called a debromination reaction and is carried out by treating the dibromide with sodium iodide in acetone or with zinc dust in acetic acid (Fig. 1).

The dibromide itself is usually prepared from the same alkene and so the reaction is not particularly useful for the synthesis of alkenes. It is useful, however, in protection strategy. During a lengthy synthesis, it may be necessary to protect  a  double  bond  so  that  it  does  not  undergo  any  undesired  reactions. Bromine can be added to form the dibromide and removed later by debromina- tion in order to restore the functional group.

Alkynes

Alkynes can be synthesized from alkenes through a two-step process which involves the electrophilic addition of bromine to form a vicinal dibromide then dehydrohalogenation with strong base (Fig. 2). The second stage involves the loss of two molecules of hydrogen bromide and so two equivalents of base are required.