Chlorination and bromination are important classes of electrophilic aromatic substitutions, where benzene reacts with chlorine or bromine to form halogenated substitution products.
These reactions are catalyzed by Lewis acids like aluminum chloride or ferric chloride for chlorination and ferric bromide for bromination.
Recall that, during bromination of alkenes, bromine polarizes and becomes electrophilic. However, the bromination of benzene requires a much stronger electrophile—the Lewis acid catalyst facilitates this.
The mechanism of bromination begins with a Lewis acid-base reaction, in which bromine reacts with ferric bromide to form a complex, which is sufficiently electrophilic to react with benzene.
Next, the electrophile reacts with the π electrons of benzene, forming the arenium ion, which is resonance-stabilized.
Finally, deprotonation of the arenium ion forms bromobenzene, restoring aromaticity and regenerating the catalyst.
The chlorination of benzene in the presence of ferric chloride follows a similar mechanism.
