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.
