Halohydration

Halohydration is a chemical reaction in which a halogen and water add across a carbon–carbon double bond to form a halohydrin, a vicinal haloalcohol. Typical reagents are halogens in aqueous solution, such as chlorine or bromine in water (Cl2/H2O, Br2/H2O), or related HOX donors. The reaction proceeds under mild conditions and furnishes products where the hydroxyl group and the halogen occupy adjacent carbons.

Mechanism: The alkene reacts with X2 to form a bridged halonium ion (for example, a bromonium). Water then attacks the more substituted carbon of the three-membered ring, opening it to give the halohydrin after deprotonation. The overall process is anti addition: the halogen and the hydroxyl group attach to opposite faces of the former double bond. Regioselectivity arises because the halogen preferentially binds the less substituted carbon, whereas the nucleophilic water attacks the more substituted carbon, yielding R-CH(OH)-CH2-X for asymmetrical alkenes.

Applications: Halohydration is a common route to vicinal halohydrins, which can be converted to epoxides by base-induced intramolecular substitution, generating the epoxide with loss of the halide. Halohydrins are also intermediates in the synthesis of alcohols bearing a neighboring halogen and in other organic transformations where simultaneous introduction of a hydroxyl and a halogen is useful. Limitations include potential side reactions with certain substrates and stereochemical outcomes that may vary with substitution and ring strain.