hydrohalogenoinnin

Hydrohalogenation, or hydrohalogenoinnin in Finnish, is the electrophilic addition of hydrogen halides HX to unsaturated hydrocarbons, such as alkenes and alkynes. The reaction forms organohalogen compounds and is one of the most straightforward methods to convert alkenes into alkyl halides and alkynes into vinyl and dihalides.

For alkenes, the mechanism proceeds by protonation of the carbon–carbon double bond by HX to generate a carbocation intermediate, followed by nucleophilic attack by the halide. The process is generally Markovnikov-selective, placing the halogen on the more substituted carbon. Carbocation rearrangements (hydride or alkyl shifts) can occur, altering the product. The reaction rate and selectivity depend on the halogen, with HCl and HBr commonly used; peroxide-initiated HBr additions can give anti-Markovnikov products in some cases by a radical pathway.

For alkynes, two equivalents of HX add across the triple bond to furnish dihalides; the first addition often yields a vinyl halide, and the second addition converts it to a geminal dihalide when excess HX is used. Terminal alkynes follow similar regiospecificity, and the final product depends on stoichiometry and reaction conditions.

Limitations include possible rearrangements for alkenes and the need to control over-addition, especially with reactive iodides or bromides. HF is particularly hazardous and less commonly used due to its strong H–F bond and handling hazards. Hydrohalogenation is routinely employed in organic synthesis to prepare alkyl and vinyl halides, serve as a step in multi-step syntheses, and modify substrate reactivity.