18.9 : 芳香族求電子置換反応: ベンゼンのフリーデルクラフツアルキル化

Friedel–Crafts alkylation of aromatic compounds was discovered by Charles Friedel and James Crafts.

It is a typical electrophilic aromatic substitution in which the hydrogen of an aromatic ring is replaced with an alkyl group.

For example, 2-chlorobutane reacts with benzene in the presence of aluminum chloride, a Lewis acid catalyst, to form 2-butylbenzene.

The alkylation starts with a Lewis acid–base reaction in which the alkyl halide reacts with aluminum chloride. The complex then dissociates to form a carbocation.

As an electrophile, the carbocation reacts with the π electron cloud of benzene, forming a resonance-stabilized arenium ion.

Finally, deprotonation of the arenium ion restores aromaticity, giving 2-butylbenzene and regenerating the catalyst.

Notably, with secondary and tertiary halides, the carbocation is the reacting electrophile.

With primary alkyl halides, the free primary carbocation is relatively unstable.

Therefore, a complex of an alkyl halide with aluminum chloride serves as the electrophile.