Alkyl bromide photobromination: catalysis by hydrogen bromide and the elimination–readdition pathway

In the photobromination of alkyl bromides, hydrogen bromide is shown to act as a catalyst and a kinetic study implies that two molecules of the acid are involved in catalysis. The catalysis is specific in two ways: only HBr is effective and it operates only with alkyl bromides having a β-hydrogen available for substitution. HBr favours the formation of 1,2-dibromides. This catalytic pathway is superimposed on the classical, uncatalysed mechanism. Isotopic labelling experiments show that an elimination–readdition pathway may also account for part of the reaction (maximum 20%) but cannot explain the migration of bromine which is observed in the formation of β-dibromides.