Friedel-Crafts Acylation

Friedel-Crafts acylation joins an acyl group, RCO, to a benzene ring. It is an electrophilic substitution and is an important way of forming a new carbon-carbon bond to an aromatic ring.Reagents: benzene and an acyl chloride, RCOCl.
Catalyst: aluminium chloride, AlCl₃.
Conditions: heat under reflux in anhydrous (dry) conditions. Water must be kept out because it reacts with AlCl₃ and with the acyl chloride.The product is an aromatic ketone (a phenylketone), and hydrogen chloride is given off. With ethanoyl chloride:C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HClThe product C₆H₅COCH₃ is phenylethanone. With propanoyl chloride the product is C₆H₅COCH₂CH₃, 1-phenylpropan-1-one.The aluminium chloride helps to generate a strong electrophile from the acyl chloride and is regenerated at the end, which is why it is a catalyst (the mechanism is in the next lesson).Why it is useful: the ketone group can be converted into other groups, for example reduced to a secondary alcohol with NaBH₄, so Friedel-Crafts acylation is a key step in synthesising drugs, dyes and other aromatic compounds.Worked example: 3.9 g of benzene (M_r = 78) is 0.050 mol. The ratio is 1 : 1, so at most 0.050 mol of phenylethanone (M_r = 120) forms: 0.050 × 120 = 6.0 g.