18.10:
Electrophilic Aromatic Substitution: Friedel–Crafts Acylation of Benzene
Unlike Friedel–Crafts alkylation, Friedel–Crafts acylation involves substituting hydrogen of an aromatic ring with an acyl group.
In this electrophilic aromatic substitution, benzene reacts with an acyl chloride in the presence of aluminum chloride, a Lewis acid catalyst, to form an aryl ketone.
The mechanism starts with the reaction of an acyl chloride with aluminum chloride to form a complex, which then dissociates to form an acylium ion, stabilized by resonance.
The acylium ion, acting as an electrophile, reacts with benzene to form an arenium ion.
Subsequently, the arenium ion is deprotonated, restoring aromaticity and giving an aryl ketone.
The resulting ketone, acting as a Lewis base, reacts with aluminum chloride to form a complex, which upon hydrolysis releases the free aryl ketone.
Interestingly, the carbonyl group of the aryl ketone can be reduced to a methylene group using the Clemmensen reduction, with HCl and amalgamated Zn.
18.10:
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.