Friedel-Crafts Acylation

The same aluminum chloride catalyst used in Friedel-Crafts alkylation is also used for Friedel-Crafts acylation. In this context, aluminum chloride assists in the formation of an acyl cation from acid halide to serve as an electrophile for aromatic substitution. Unlike alkyl cation, acyl cation has the advantage of not rearranging. Electrophilic aromatic substitution begins with the addition of the electrophile into the aromatic pi system of the ring. A conjugated, carbocation intermediate is formed, a resonance combination of three forms, concentrating positive charge at three locations, the two ortho positions and one para. Finally, the proton departs and substitution at the carbon is complete with aromaticity restored. Acylation is frequently followed by Clemmensen reduction in synthesis, which will transform the acylbenzene into an alkylbenzene.

Consider the case where electrophilic aromatic substitution is attempted on an aromatic ring that already contains an original acyl substituent. If electrophilic aromatic substitution is attempted upon rings that already contain substituents, the location of the next substitution depends on the characteristics of the original substituent. Whether the new substitution occurs ortho, para or meta to the original substituent depends on whether the original substituent either stabilizes or destabilizes a concentration of positive charge upon its carbon at the carbocation intermediate stage. If the substituent already present is electron donating, it will stabilize the carbocation by donating negative charge. New substitutions will occur ortho or para to such electron donating substituents already present on the ring. Electron withdrawing substituents destabilize a carbocation, so the new substitution will most likely be meta. Acyl group is electron withdrawing due to the electronegativity of the oxygen atom, which draws electron density toward itself and leaves the carbon electron poor. Acyl is a ring deactivating, meta director for further electrophilic aromatic substitution.