Acylium tetrachloroaluminate

The first reagent combination, carboxylic acid chloride/A1C13, reacts via the A1C13 complex A of the acid chloride or via the acylium tetrachloroaluminate B formed from it by /)-elimination. A carboxylic acid anhydride and A1C13 react via analogous electrophiles, namely via the A1C13 complex D of the anhydride or via the acylium salt E formed therefore by a /T elimination. The protonated anhydride F and the protonated carboxylic acid C are the reactive electrophiles of the Friedel-Crafts acylations catalyzed by Bronsted acids. 

The mechanism of Friedel-Crafts acylation (shown next) resembles that fw alkylation, except that the carbonyl group helps to stabilize the cationic intermediate. The acyl halide forms a complex with aluminum chloride loss of the tetrachloroaluminate ion ( AICI4) gives a resonance-stabilized acylium ion. The acylium ion is a strong electrophile. It reacts with benzene or an activated benzene derivative to form an acylbenzene. 

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