Preparation of Alkynes by Double Elimination

The two basic methods used to prepare alkynes are double elimination from 1,2-dihaloalkanes and alkylation of alkynyl anions. This section deals with the first method, which provides a synthetic route to alkynes from alkenes Section 13-5 addresses the second, which converts terminal alkynes into more complex, internal ones. 

As discussed in Section 11-6, alkenes can be prepared by E2 reactions of haloalkanes. Application of this principle to alkyne synthesis suggests that treatment of vicinal dihaloalkanes with two equivalents of strong base should result in double elimination to furnish a triple bond. 

Double Elimination from Dihaloalkanes to Give Alkynes X X 

Indeed, addition of 1,2-dibromohexane (prepared by addition of Br2 to 1-hexene. Section 12-5) to sodium amide in liquid ammonia followed by evaporation of solvent and aqueous work-up gives 1-hexyne. 

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