Alkyne hydrohalogenation
Abstract
Hydrohalogenation of alkynes is an electrophilic addition reaction in which a hydrogen halide (HX, where X = I, Br or Cl) adds across the carbon–carbon triple bond. Depending on the number of equivalents of hydrogen halide employed, the reaction can yield either a vinyl halide or a geminal dihalide.
Keywords
Alkyne, Vinyl halide, Geminal dihalide
Category
Alkyne
Mechanism
The mechanism proceeds in two steps. First, the triple bond is protonated, leading to the formation of the most stable, most substituted vinyl carbocation (Markovnikov’s rule) and releasing a halide ion. Then, the halide ion attacks the vinyl carbocation, giving the corresponding vinyl halide.
Because the resulting alkene remains reactive toward electrophilic addition, a second equivalent of HX can add across the double bond. This second addition also follows Markovnikov regioselectivity, producing a geminal dihalide in which both halogen atoms are attached to the same carbon atom.
