Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.

Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.

A bromonium ion is more stable than the analogous carbocation, as it has more covalent bondsand all the atoms have filled octets.

In the second step, the nucleophile, a bromide ion, attacks one of the carbon atoms in the bridged bromonium ion. Due to the non-availability of bonding orbitals and steric crowding, the nucleophile approaches the antibonding orbitals, pointing opposite to the carbon–bromine bond. This accounts for the anti addition.

Thus, the addition of two bromine atoms takes place from the opposite faces of the double bond in cyclopentene to yield trans-1,2-dibromocyclopentane.

The configuration of the starting alkene decides the stereochemical outcome for halogenation reactions. For example, the addition across cis-2-butene generates a pair of enantiomers, while addition across trans-2-butene produces a meso compound. Therefore, the halogenation of alkenes is a diastereospecific reaction.