18.10 : 芳香族求電子置換: ベンゼンのフリーデルクラフツアシル化

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.