Saturday, January 26, 2008

IIT JEE Revision Benzene - Halogenation


Benzene will react with a mixture of Cl-2 and FeCl-3 (catalyst).

The output is a combination of benzene with Cl, Cl displacing one hydrogen atom from benzene(Chlorobenzene).

AlCl3 can also be used as catalyst.

Bromine also combines with benzene with AlCl3 as catalyst and forms bromobenzene.
At a slightly elevated temperature, chlorination of benzene in presence of a catalyst gives rise to a mixture of orthochlorobenzene and para chlorobenzene.

On prolonged chlorination, benzene forms hexachlorobenzene.

Reaction mechanism

Reaction is carried out in the presence of ferric salts i.e, FeCl3 in case of chlorine and FeBr3 in case of bromine.

The metal catalyst is a Lewis acid (Fe is electron deficient) and hence polarises the halogen molecule.

Cl2 + FeCl3 --> FeCl4ˉ + Cl+

Cl+ attacks the benzene molecule.

It has been suggested by some that free chloronium ion (Cl+) may not have actual existence. The electrophile is supposed to be made available by a complex between FeCl3 and Cl2

The electrophile attack creates a carbocation.

The carbocation loses a proton(H+) to FeCl4ˉ and the formation of aryl halide takes place.

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