18.14: Electrophilic Aromatic Substitution: Friedel–Crafts Acylation of Benzene
The Friedel–Crafts acylation reactions involve the addition of an acyl group to an aromatic ring. These reactions proceed via electrophilic aromatic substitution by employing an acyl chloride and a Lewis acid catalyst such as aluminum chloride to form aryl ketone.
The mechanism involves the formation of a complex between the Lewis acid and the acyl chloride. An acylium ion is formed by the cleavage of the carbon-chlorine bond of the complex. The acylium ion has a positive charge on the carbon and is resonance stabilized. This acylium ion acts as an electrophile and reacts with the aromatic ring. The arenium ion is deprotonated, restoring the aromaticity of the ring by forming an aryl ketone. The aryl ketone forms a complex with the Lewis acid, which hydrolyzes to liberate the ketone. The resulting product can be reduced using a Clemmensen reduction in the presence of HCl and amalgamated zinc for converting a carbonyl group into an alkyl group.