Overview

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.

Addition of HCl to an Alkyne

Mechanism I – Vinylic carbocation Intermediate

The mechanism begins with a proton transfer from HCl to the alkyne. Here, the π electrons attack the hydrogen atom of hydrogen chloride and displace the chloride ion. This gives a stable secondary vinylic carbocation, which is further attacked by the chloride ion to form a vinyl chloride.

The addition of a second equivalent of hydrogen chloride proceeds with the protonation of vinyl chloride, giving two possible carbocations. In the secondary carbocation the positive charge is delocalized through resonance. As a result, it is more stable and favored over the primary carbocation.

A second nucleophilic attack by the chloride ion gives a geminal dichloride.

Mechanism II – Concerted Termolecular Process

In a vinylic carbocation, the positive charge resides on an electronegative sp-hybridized carbon making it unstable. The second mechanism avoids the formation of this carbocation. Instead, it proceeds via a termolecular (three molecules) process where the alkyne interacts simultaneously with two equivalents of a hydrogen halide like HCl. This leads to a transition state with a partially broken C–C π bond and partially formed C–Cl and C–H σ bonds. The net result is a trans addition of hydrogen from one HCl and a chloride from the other HCl to form a chloroalkene. This further reacts with the displaced HCl to form a geminal dichloride as the final product.

Halogenation of Alkynes with Peroxides

When treated specifically with HBr in the presence of peroxides, terminal alkynes undergo anti-Markovnikov's addition. The Br gets added to the less substituted carbon forming a mixture of E and Z alkenes.

Procedure

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.