Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene

Bromine alone is not electrophilic enough to react with benzene's stable aromatic π electrons. A Lewis acid catalyst like ferric bromide accepts electron density from bromine, polarizing the Br–Br bond and creating a much stronger electrophile capable of attacking the benzene ring and initiating the substitution reaction.

The arenium ion is the reactive intermediate formed when the electrophilic bromine attacks benzene's π electrons. This carbocation is resonance-stabilized, distributing positive charge across the aromatic ring. Deprotonation of the arenium ion restores aromaticity, forming bromobenzene and regenerating the Lewis acid catalyst.

Chlorination and bromination follow identical mechanisms. Both require Lewis acid catalysts—aluminum chloride or ferric chloride for chlorination, and ferric bromide for bromination. The catalyst polarizes the halogen molecule, creating an electrophile that attacks benzene's π electrons to form the arenium ion intermediate.

Chlorination uses aluminum chloride or ferric chloride as Lewis acid catalysts. Bromination uses ferric bromide. These catalysts facilitate formation of the reactive electrophile by accepting electron density from the halogen molecule, enabling the electrophilic aromatic substitution reaction to proceed efficiently.

Alkene bromination occurs without a catalyst because the π electrons are already sufficiently nucleophilic. Benzene's aromatic π electrons are less reactive, requiring a Lewis acid catalyst to generate a stronger electrophile. The catalyst creates a polarized halogen species capable of attacking the stable benzene ring.

The Lewis acid catalyst is regenerated during the reaction. It initially accepts electrons from the halogen to form the electrophile, then is released when the arenium ion intermediate loses a proton. This regeneration allows the catalyst to participate in multiple reaction cycles without being consumed.

The arenium ion is a non-aromatic carbocation intermediate. When a proton is removed from the carbon bearing the halogen substituent, the π electron system is restored with alternating double bonds around the ring. This regenerates the stable aromatic benzene ring structure in the halogenated product.