Bromohydrins

Bromohydrins are vicinal halohydrins in which a bromine atom and a hydroxyl group are attached to adjacent carbons of an alkyl chain. They are commonly prepared by electrophilic addition of bromine to alkenes in the presence of water, using Br2 in aqueous media.

Mechanistically, bromine adds to the carbon–carbon double bond to form a three-membered bromonium ion. Water then attacks the more substituted carbon of the bromonium ion, opening the ring and yielding a vicinal bromohydrin. This path proceeds with anti addition, so the bromine and hydroxyl groups occupy opposite faces of the original double bond.

In asymmetrical alkenes, regiochemistry favors hydroxyl installation on the more substituted carbon, with bromine on the less substituted carbon. For symmetrical alkenes, regioselectivity is not an issue. The reaction is generally anti stereoselective: if the starting alkene has defined geometry, the resulting bromohydrin displays a corresponding anti relationship between the Br and OH groups.

Bromohydrins are versatile intermediates in organic synthesis. They can be converted to epoxides by base-induced intramolecular substitution, releasing bromide and forming an oxirane ring. They also serve as precursors to diols or other functionalized motifs through substitution at the bromine atom or further transformations.

Typical conditions involve Br2 in water or aqueous organic solvents. Alternatives include N-bromosuccinimide (NBS) in aqueous media. Bromohydrins are useful for introducing both halogen and hydroxyl functionalities in a single step and for enabling subsequent ring-forming and functional-group transformations.