18.9: Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene
Chlorination and bromination are important classes of electrophilic aromatic substitutions, where benzene reacts with chlorine or bromine in the presence of a Lewis acid catalyst to give halogenated substitution products. A Lewis acid such as aluminium chloride or ferric chloride catalyzes the chlorination, and ferric bromide catalyzes the bromination reactions. During the bromination of alkenes, bromine polarizes and becomes electrophilic. However, in the bromination of benzene, the bromine molecule reacts with ferric bromide by donating a pair of electrons to the Lewis acid, which creates a more polar Br–Br bond and forming a more reactive electrophile.
The benzene attacks this electrophile to generate the arenium ion, which is resonance stabilized.
A proton transfer from arenium ion forms bromobenzene, thereby restoring aromaticity and regenerating the catalyst.
The mechanism of chlorination of benzene proceeds in the same manner as bromination of benzene.