hydrohalogenation

Hydrohalogenation is a chemical reaction in which a hydrogen halide (HX) adds across a carbon–carbon multiple bond, typically an alkene or alkyne, to form a haloalkane or vinyl/alkyl halide. It is a straightforward method for attaching a halogen and a hydrogen across unsaturated substrates and is widely used in organic synthesis.

In the hydrohalogenation of alkenes, the reaction proceeds by electrophilic addition. The proton from HX adds first to the double bond to generate the more stable carbocation, and the halide anion then attacks the carbocation to form the alkyl halide. Regioselectivity is dominated by Markovnikov’s rule: the hydrogen attaches to the carbon with more hydrogens, and the halogen attaches to the more substituted carbon. Rearrangements of carbocations, such as hydride or alkyl shifts, can occur and influence the final product. The outcome can be altered under special conditions, such as anti-Markovnikov addition of HBr in the presence of radical initiators (peroxides), where a radical mechanism predominates.

For alkynes, hydrohalogenation adds HX across the triple bond to yield vinyl halides; with excess HX, a second equivalent adds to form geminal dihalides. Substrate substitution, solvent, temperature, and the presence of radical initiators can affect rate and selectivity. Fluorination is less common due to the strong H–F bond and reactivity differences.

Hydrohalogenation is used to prepare a range of organohalides and serves as a step in complex synthetic sequences. Reactions must be managed to avoid side processes such as rearrangements, polymerization, or over-addition in sensitive substrates.