18.9 : Electrophilic Aromatic Substitution: Friedel–Crafts Alkylation of Benzene

Friedel–Crafts reactions were developed in 1877 by the French chemist Charles Friedel and the American chemist James Crafts. Friedel–Crafts alkylation refers to the replacement of an aromatic proton with an alkyl group via electrophilic aromatic substitution. A Lewis acid catalyst such as aluminum chloride reacts with an alkyl halide to form a carbocation. The resulting carbocation then reacts with the aromatic ring and undergoes a series of electron rearrangements before giving the final product. For instance, benzene reacts with 2-chlorobutane in the presence of aluminum chloride to form 2-butylbenzene.

The alkylation starts with a Lewis-acid-base reaction in which the alkyl halide reacts with aluminum chloride, resulting in the formation of an electrophilic carbocation.

The carbocation attacks the π electron cloud of the aromatic ring, forming a resonance-stabilized arenium ion. The deprotonation of the arenium ion restores aromaticity, giving 2-butylbenzene and regenerating the catalyst.

With secondary and tertiary halides, the carbocation is the reacting electrophile. In the case of primary alkyl halides, the free primary carbocation is unstable and difficult to generate. Instead, a complex of an alkyl halide with aluminum chloride acts as the electrophile.