hidrohalogenacija

Hidrohalogenacja, also known as hydrohalogenation, is the electrophilic addition of a hydrogen halide (HX, for example HCl, HBr, or HI) across a carbon–carbon multiple bond. In alkenes and alkynes this reaction formally converts unsaturated hydrocarbons into haloalkanes or vinyl halides. It is a classical method for introducing a halogen atom and a hydrogen atom in one step and is widely used in organic synthesis.

For alkenes, the reaction proceeds by protonation of the double bond by HX to give a carbocation intermediate, followed by nucleophilic attack by the halide. Regioselectivity is typically Markovnikov: the halogen attaches to the more substituted carbon. Carbocation rearrangements (hydride or alkyl shifts) can occur, altering the product distribution. The reaction generally proceeds with anti addition of HX, but stereochemical outcomes depend on the substrate.

Hidrohalogenacja of alkynes adds HX to form vinyl halides. With excess HX, a second equivalent adds to give dihalides. The regioselectivity mirrors Markovnikov rules for the first addition, so in terminal alkynes the halogen ends up on the internal carbon of the vinyl halide. Stereochemical outcomes for vinyl halides can vary with conditions, often favoring the more stable E isomer.

Under radical conditions, particularly with HBr in the presence of peroxides, anti-Markovnikov addition can occur for alkenes, giving the bromide on the less substituted carbon. This radical pathway is less common for HCl or HI. The choice of solvent and temperature affects the rate and selectivity.

Hidrohalogenacja provides a straightforward route to organohalides used in further transformations such as substitutions, eliminations, and cross-coupling. It is important to manage exothermicity and corrosivity of HX and to perform reactions in appropriate facilities.