Friedel-Crafts Acylation
Friedel-Crafts Acylation: AlCl₃ activates the acyl chloride by coordinating to its chlorine, generating a highly electrophilic acylium ion; benzene's π…
What happens
AlCl₃ activates the acyl chloride by coordinating to its chlorine, generating a highly electrophilic acylium ion; benzene's π electrons attack this acylium ion, and loss of a proton restores aromaticity, giving the aryl ketone.
Reaction summary
C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (AlCl₃ catalyst)
Reagents and conditions
- Starting material: Benzene
- Reagents: Acetyl chloride; AlCl₃ (Lewis acid catalyst)
- Products: Acetophenone
Mechanism
- Acylium ion formation: AlCl₃ pulls chloride off the acyl chloride, generating a resonance-stabilized acylium cation.
- Electrophilic attack: The aromatic ring's π system attacks the acylium carbon, temporarily breaking aromaticity and forming a resonance-stabilized arenium (Wheland) intermediate.
- Loss of H⁺ restores aromaticity: A base removes the proton from the sp³ carbon, regenerating the aromatic ring and releasing the neutral ketone product.
Selectivity
- Regioselectivity: On a substituted ring, the existing substituent's directing effect (ortho/para vs. meta director) controls where the new acyl group goes.
Common mistakes
- Confusing this with Friedel-Crafts alkylation, acylation does NOT suffer from carbocation rearrangement (the acylium ion is resonance-stabilized) and does not overreact, since the product ketone deactivates the ring.
- Forgetting a full equivalent of AlCl₃ is consumed (it complexes with the ketone product), unlike its truly catalytic role in some other Lewis-acid reactions.
Memory trick
"Acylation, not alkylation, for a clean single substitution": the ketone product deactivates the ring, so the reaction stops after one acylation.