Mechanism of Friedel-Crafts Acylation

Friedel-Crafts acylation follows the electrophilic substitution mechanism. The electrophile is formed with the help of the aluminium chloride catalyst.Forming the electrophile. AlCl₃ accepts a lone pair from the chlorine of the acyl chloride and removes it as AlCl₄⁻. This leaves RCO⁺, an acylium ion. For ethanoyl chloride:CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻Step 1. A pair of electrons from the delocalised ring attacks the positively charged carbon atom of the acylium ion (drawn as CH₃-C⁺=O), forming a new C-C bond. The intermediate is drawn as a horseshoe with a + charge; the carbon bonded to both H and COCH₃ is outside the delocalised system.Step 2. The C-H bond breaks and its electrons return to the ring, restoring delocalisation. A proton (H⁺) is lost, giving phenylethanone.Regenerating the catalyst. The H⁺ reacts with the AlCl₄⁻ ion:H⁺ + AlCl₄⁻ → AlCl₃ + HClThis forms the HCl by-product and regenerates aluminium chloride, so AlCl₃ acts as a catalyst.Compare this with nitration: the pattern of steps is the same, only the electrophile (RCO⁺ instead of NO₂⁺) and the catalyst (AlCl₃ instead of H₂SO₄) change. When drawing it, make sure the first arrow ends on the carbon of RCO⁺, not the oxygen.