18.5: 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.

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.