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Electrophilic Addition of Halogens

The π cloud of the C=C double bond can induce the heterolytic cleavage of the bromine molecule, giving rise to an intermediate with the structure of heterocyclopropane
(halonium ion).

YouTube video: reactivity of double bonds▶ Watch the video on YouTube
YouTube video: electrophilic addition to a double bond▶ Watch the video on YouTube

The cyclic intermediate is opened by the halide counterion, leading to a vicinal dihalo derivative of anti stereochemistry.

Animation of the addition of bromine to 2-butene
The reaction is only of practical use for Cl2 and Br2.
Addition of halogens: the double bond attacks X₂ to form the halonium ion, which the halide opens from the opposite side to give the anti vicinal dihalide

The opening of the halonium ion closely resembles that of a protonated epoxide. Click here to see the comparison.

The halonium ion, once it is produced, can be captured with all sorts of nucleophiles:

Br₂ with 1-methylcyclohexene in excess NaCl: the nucleophile opens the bromonium ion at the tertiary carbon (transition state similar to a tertiary carbocation), as in a protonated epoxide

If the nucleophile is water or alcohols, one gets halohydrins or haloethers, respectively.

This is an excellent way of functionalizing two carbons at once.

Formation of halohydrins and haloethers: water or an alcohol opens the bromonium ion and, after loss of HBr, the bromohydrin (R = H) or bromoether (R = alkyl) is obtained