Electrophilic Addition Reactions of Alkynes

The alkynes behave in many ways as if they were doubly unsaturated alkenes. For example, bromine adds to ethyne in two stages - first to give trans-1,2-dibromoethene by antarafacial addition, and finally to give 1,1,2,2-tetrabromoethane:

Likewise, anhydrous hydrogen fluoride adds first to give fluoroethene and ultimately to give 1,1-difluoroethane:

However, there is an interesting contrast in reactivity. Alkynes are substantially less reactive than corresponding alkenes toward many electrophiles. This is perhaps surprising because the electrons of a triple bond, like those of a double bond, are highly exposed, which suggests that the reactivity (nucleophilicity) of a triple bond should be high. Evidently this is not the case. A simple but reasonable explanation is that the carbocation formed from the alkyne is less stable than that from the alkene because it cannot achieve the sp2 hybrid-orbital configuration expected to be the most stable arrangement for a carbocation: