Electrophilic Addition to Alkynes: Hydrohalogenation

Hydrohalogenation of alkynes produces two products depending on the number of HCl equivalents. The first addition of HCl forms a vinyl chloride, while the second addition yields a geminal dichloride. Both additions follow Markovnikov's rule, with chlorine attaching to the more substituted carbon of the triple bond.

The mechanism begins with π electrons attacking the hydrogen of HCl, displacing chloride to form a secondary vinylic carbocation. This intermediate is stabilized by resonance delocalization of the positive charge. Chloride then attacks the carbocation to form vinyl chloride, and a second HCl addition follows the same pathway to produce the geminal dichloride.

The termolecular process involves simultaneous interaction of three molecules: the alkyne and two equivalents of HCl. Rather than forming a vinylic carbocation, the reaction proceeds through a low-energy transition state with partial bonds. Trans addition of hydrogen from one HCl and halogen from the other produces a haloalkene, which reacts with displaced HCl to form the geminal dihalide.

Although the secondary carbocation has the positive charge on a carbon bearing an electron-withdrawing chlorine, it is more stable than the primary carbocation because the positive charge is delocalized through resonance. This resonance stabilization outweighs the destabilizing effect of the adjacent halogen, making the secondary intermediate the preferred pathway.

Terminal alkynes treated with HBr in the presence of peroxides undergo anti-Markovnikov addition, where bromine adds to the less substituted carbon. This produces a mixture of E and Z alkenes, contrasting with the typical Markovnikov regioselectivity observed in standard hydrohalogenation reactions.

Markovnikov's rule dictates that the hydrogen adds to the less substituted carbon of the triple bond, while the halogen adds to the more substituted carbon. This regioselectivity applies to both the first and second additions of hydrogen halide, ensuring consistent product formation across sequential additions.

The vinylic carbocation mechanism proceeds through a discrete secondary carbocation intermediate stabilized by resonance. The termolecular mechanism avoids carbocation formation by involving simultaneous three-molecule interaction through a transition state, offering an alternative low-energy pathway for the same overall transformation and product formation.