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.
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.
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.