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Friedel-Crafts Alkylation

A carbocation is easily attacked by the π cloud of an aromatic compound leading to an alkylarene by SEAr.

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Friedel-Crafts alkylation: benzene reacts with a carbocation R+ to give an alkylbenzene and H⁺

The mechanism is as usual:

Alkylation mechanism: the ring attacks the carbocation, the benzenonium ion forms and loss of the proton gives the alkylbenzene and HX

The necessary carbocations can be generated by various methods:

Ways of generating carbocations: from alkyl halides with AlCl₃, from alcohols with BF₃ and from olefins with H⁺

However, Friedel-Crafts alkylations have serious drawbacks:

Drawbacks of Friedel-Crafts alkylation: carbocation rearrangement (1-bromopropane gives 2-phenylpropane) and polyalkylation, because the alkylbenzene is more reactive than benzene
Portrait of Charles Friedel

Charles Friedel (1832-1899): French chemist and mineralogist. He worked with Wurtz. In 1877, together with Crafts, he put forward a general synthetic method that allowed the attachment of alkyl or acyl chains to benzene.

Portrait of James Mason Crafts

James Mason Crafts (1839-1917): American chemist who studied at Harvard and in Germany and France. He was professor of chemistry at Cornell and president of MIT.