H-X Addition to Alkynes

Alkynes undergo addition reactions much like alkenes do!

In the addition of H-X to alkynes, as you would expect the addition of a H+ proton to an alkyne proceeds to form the most stable carbocation.

In the case of terminal alkynes, the alkyne it halogenated at the least substituted carbon as in the addition of H-X to alkenes.

Mechanistic studies have indicated a second order dependence on H-X concentration and as a result the following mechanism has been proposed.  In many cases the addition occurs with trans-stereochemistry which is also consistent with this mechanism.

  • rate = -k [H-X]2[alkyne]

In cases where the trans-stereochemistry is not observed, a secondary vinyl carbocation is proposed.  

If excess H-X is used then the addition can occur twice.