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Alkynes can be prepared by the elimination of HX from alkyl halides in a similar manner as alkenes (Section 8.1). Treatment of a 1,2-dihaloalkane (called a vicinal dihalide) with an excess amount of a strong base such as KOH or NaNH₂ results in a twofold elimination of HX and formation of an alkyne. As with the elimination of HX to form an alkene, we’ll defer a full discussion of this topic and the relevant reaction mechanisms to Chapter 11.

The starting vicinal dihalides are themselves readily available by addition of Br₂ or Cl₂ to alkenes. Thus, the overall halogenation/dehydrohalogenation sequence makes it possible to go from an alkene to an alkyne. For example, diphenylethylene is converted into diphenylacetylene by reaction with Br₂ and subsequent base treatment.

The twofold dehydrohalogenation takes place through a vinylic halide intermediate, which suggests that vinylic halides themselves should give alkynes when treated with strong base. (A vinylic substituent is one that is attached to a double-bond.) This is indeed the case. For example: