Unsaturated compounds, especially alkenes and alkynes, add halogens:
In oxychlorination, the combination of hydrogen chloride and oxygen serves as the equivalent of chlorine, as illustrated by this route to 1,2-dichloroethane:
The addition of halogens to alkenes proceeds via intermediate halonium ions. In special cases, such intermediates have been isolated.
Bromination is more selective than chlorination because the reaction is less exothermic. Illustrative of the bromination of an alkene is the route to the anesthetic halothane from trichloroethylene:
Iodination and bromination can be effected by the addition of iodine and bromine to alkenes. The reaction, which conveniently proceeds with the discharge of the color of , is the basis of the analytical method. The iodine number and bromine number are measures of the degree of unsaturation for fats and other organic compounds.