The Friedel‐Crafts acylation reaction, another example of an electrophilic aromatic substitution reaction, is similar to the Friedel‐Crafts alkylation reaction except that the substance that reacts with benzene is an acyl halide,
instead of an alkyl halide, R &bond; X. An acetyl chloride reaction appears as:
The mechanism for the generation of the acylium ion, is
The remainder of the mechanism is identical to that of the alkylation of benzene. Because the acylium ion is resonance stabilized, no rearrangements occur.
1. The reaction of acetyl chloride with aluminum chloride forms an electrophile.
2. The electrophile attracts the π electron system of the benzene ring to form a nonaromatic carbocation.
3. The positive charge on the carbocation that is formed is delocalized throughout the molecule.
4. The loss of a proton from the atom to which the acetyl group has bonded restores the aromaticity.
5. The proton reacts with the AlCl 4 − to regenerate the AlCl 3 catalyst and form the product HCl.