Alkylation of terminal alkynes

ALKYLATION OF TERMINAL ALKYNES

Key Notes

Terminal alkynes

A terminal alkyne is defined as an alkyne having a hydrogen substituent.

The hydrogen of a terminal alkyne is weakly acidic and can be removed with a strong base such as sodium amide to produce an alkynide ion.

Alkylation

The alkynide ion can be treated with a primary alkyl halide to produce a disubstituted alkyne.

Terminal alkynes

A terminal alkyne is defined as an alkyne having a hydrogen substituent (Fig. 1). This hydrogen substituent is acidic and can be removed with strong base (e.g. sodium amide NaNH₂) to produce an alkynide (Fig. 2). This is an example of an acid–base reaction.

Alkylation

Once the alkynide has been formed, it can be treated with an alkyl halide to produce more complex alkynes (Fig. 3). This reaction is known as an alkylation as far as the alkynide is concerned, and is an example of nucleophilic substitution as far as the alkyl halide is concerned.

This reaction works best with primary alkyl halides. When secondary or ter-tiary alkyl halides are used, the alkynide reacts like a base and this results in elimination of hydrogen halide from the alkyl halide to produce an alkene.