9.7 : Electrophilic Addition to Alkynes: Hydrohalogenation

Hydrohalogenation is another class of electrophilic addition reactions. It proceeds with the addition of a hydrogen halide, such as HCl or HBr, across a π bond.

The presence of two π bonds allows for the addition of two equivalents of hydrogen halide. Here, the first addition of HCl produces vinyl chloride, and the second yields a geminal dichloride.

Both additions follow Markovnikov's regioselectivity, with the chlorine getting added to the more substituted carbon.

One possible mechanism begins with a proton transfer, where the π electrons attack the hydrogen atom of HCl and displace the chloride ion to form a stable secondary vinylic cation.

Next, the chloride ion attacks the vinyl cation to form a vinyl chloride.

The addition of a second equivalent of HCl proceeds with the protonation of vinyl chloride, resulting in two possible carbocations.

Here, the secondary carbocation is favored over the primary, despite the positive charge being centered on the carbon attached to an electron-withdrawing halogen group. This is because the positive charge on the secondary carbocation is delocalized through resonance, thereby stabilizing the intermediate and supporting it's formation.

The final step involves the attack of the chloride ion to form a geminal dichloride.

Another competing mechanism is a termolecular process. It involves simultaneous interaction between three molecules, the alkyne and two equivalents of hydrogen halide. 

However, instead of a vinylic carbocation, the reaction proceeds via a low-energy transition state that bears a partial positive charge at the more substituted carbon. 

The trans addition of a hydrogen from one hydrogen halide and the addition of a halogen from the other hydrogen halide gives a haloalkene that reacts with the displaced hydrogen halide to form a geminal dihalide.

Lastly, like with alkenes, when a hydrohalogenation of terminal alkynes is carried out in the presence of peroxides and HBr, it proceeds in an anti-Markovnikov manner. Here, the bromine gets added to the less substituted carbon, producing a mixture of E and Z alkenes.