Friedel–Crafts acylation: practice and mechanism
The acylium ion CH₃C≡O⁺ substitutes a ring hydrogen: an aryl ketone forms, para to an o/p director.
Example
AlCl₃ turns the acyl chloride into the acylium ion, which attacks the ring and gives an aryl ketone. Strongly deactivated rings (e.g. nitrobenzene) do not react. Unlike alkylation there is no multiple substitution, because the C=O group deactivates the ring.
Mechanism step by step
- 1
AlCl₃ pulls Cl⁻ off the acyl chloride: the acylium ion CH₃–C≡O⁺ (resonance-stabilised).
- 2
The arene attacks the acylium ion: σ complex.
- 3
Loss of H⁺ restores aromaticity: aryl ketone. AlCl₃ is needed in stoichiometric amounts because it binds to the ketone.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Meta is wrong with this o/p-directing group. Because the electrophile is large, Friedel–Crafts acylation goes preferably para.
Often confused with: Nitration (SEAr) · Aromatic bromination (SEAr)
Frequently asked questions
Why no multiple acylation?
The C=O group introduced deactivates the ring, so a second attack is much slower. In alkylation it is the other way round.
Why acylation instead of alkylation?
The acylium ion does not rearrange, and the ketone can be reduced to the alkyl group afterwards (Clemmensen, Wolff–Kishner).