Friedel-Crafts Acylation

In Friedel-Crafts acylation, benzene reacts with an acyl chloride in the presence of a halogen carrier such as AlCl₃, under anhydrous conditions. The product is an aromatic ketone, and a new carbon-carbon bond forms to the ring.Mechanism (electrophilic substitution)
1. The halogen carrier generates the electrophile, an acylium ion:
CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻2. The delocalised electrons of benzene attack the acylium ion, forming a positively charged intermediate.
3. The intermediate loses H⁺, restoring the delocalised ring.
4. The catalyst is regenerated: H⁺ + AlCl₄⁻ → AlCl₃ + HClOverall, with ethanoyl chloride:C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HClThe product is phenylethanone.Acylation has an advantage over alkylation. The C=O group withdraws electron density from the ring, making the product less reactive than benzene, so a second acyl group is not added and a single product forms. The ketone can then be converted into other compounds; for example, NaBH₄ reduces it to a secondary alcohol.Worked example: 7.80 g of benzene (0.100 mol) gives 9.00 g of phenylethanone, C₈H₈O (M_r 120).
n(phenylethanone) = 9.00 ÷ 120 = 0.0750 molpercentage yield = 0.0750 ÷ 0.100 × 100 = 75%