Discovered in the late 19th century by Charles Friedel and James Crafts, the Friedel-Crafts reaction is a method for adding carbon groups to a
benzene ring in aromatic compounds.
Although the benzene ring is rather unreactive, its high electron density allows it to react as a nucleophile under certain conditions. In the Friedel-Crafts reaction, the electrophile added to the ring is either an alkyl carbocation, to give an alkylated product, or an acylium cation, to give an acylated product.
Friedel-Crafts alkylation
To prepare the carbocation which is to be added to the ring, the appropriate alkyl chloride (R-Cl) is treated with aluminum chloride, AlCl3, which removes the chlorine, giving AlCl4- and the carbocation R+.
The carbocation is then reacted with the aromatic compound, Ar, which attacks the positively charged carbon. The alkyl group replaces a hydrogen on the benzene ring and thus the ring is alkylated to give the compound Ar-R.
For example, to add a propyl group (CH3CH2CH2, or R) to benzene, or Ar: 1-chloropropane, R-Cl, is reacted with aluminum chloride to give R+, which then reacts with benzene to form propylbenzene, Ar-R.
Friedel-Crafts acylation
This process adds an acyl group to the benzene ring, to form an aromatic ketone. The carbon chain is reacted as the acid chloride, R-COCl. This is first treated with aluminum chloride, which again removes a chlorine from the carbon fragment. This time the product is an acylium ion, R-C≡O+, with the positive charge located on the oxygen.
The aromatic compound is then reacted with the acylium ion, attacking the carbon and losing hydrogen to form a bond with the C-R=O group. Thus the product is an aromatic ketone, C-RO-Ar.
For example, to add an ethanoyl group, H3C-CO, to benzene: ethanoyl chloride, H3C-COCl is reacted with aluminum chloride to form H3C-C≡O+, which is attacked by benzene to form acetophenone, H3C-CO-C6H5.
The Friedel-Crafts reaction is limited to benzenes in which the ring system is not too strongly deactivated. Benzene rings with lots of electron-withdrawing substituents, e.g. nitro or halogen groups, do not have sufficient electron density to act as nucleophiles in this way.