In Friedel-Crafts alkylation, a Lewis acid - usually a simple metal halide salt - promotes heterolytic cleavage of a carbon-halogen bond in an alkyl halide and generates a carbocation, which undergoes electrophilic aromatic substitution.
Important variants of the Friedel-Crafts reaction include chloromethylation (with formaldehyde and HCl), formylation (with HCl and CO or CN−), and acylation with a nitrile as the acyl source.