HX Addition to Alkynes
Addition of HX to Alkynes - Mechanism
Because of their instability, vinyl cations are unlikely intermediates in the mechanism of the HX addition to alkynes. As an alternative, a mechanism with a cyclic intermediate stage has been proposed, similar to that in the halogenation of alkenes.
There is some experimental evidence of a cyclic intermediate as the complicated reaction kinetics show the participation of more than one HX molecule and hydrogen halide-acetylene complexes are known. In addition, the preference for anti addition can be explained by a cyclic intermediate.
The resonance structures in the illustrations show the distribution of the positive charge in the cyclic intermediate to the hydrogens and the alkyne carbon atoms. This mechanism also explains the regiochemistry of the reaction indicating a preference for Markovnikov addition.
In the addition of HX to a terminal alkyne, the partially positive charge of the cyclic intermediate is better stabilized at the alkyl-substituted carbon than at the terminal carbon and thus the partial hydrogen-carbon bond to the alkyl-substituted carbon is weaker and longer than that to the terminal carbon. Therefore, the halide ion more easily attacks the alkyl-substituted carbon atom and the Markovnikov product is mainly formed.