Formation of Halohydrins (Addition of Hal₂ and H₂O to Alkenes)

Summary

The key steps in the reaction of an alkene with Br₂/H₂O are: (i) formation of the cyclic bromonium ion by syn addition of bromine to the alkene, followed by (ii) nucleophilic ring opening of the cyclic bromonium ion by an S_N2 reaction (shown here).

Key reaction:

Nucleophilic attack of water
on cyclic bromonium ion