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. although vastly useful in synthesis, the reaction often suffers from side reactions that arise from carbocation rearrangement, alkyl migration, and over-alkylation. similarly, in friedel-crafts acylation, a lewis acid assists in the generation of an acylium ion from an acid chloride (or occasionally acid anhydride). although the acylium ion is often assumed to be the active intermediate, there is evidence that the protonated acylium dication is the active electrophile that undergoes subsequent electrophilic aromatic substitution.