9.7: Electrophilic Addition to Alkynes: Hydrohalogenation
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.