. Discuss the mechanism for aromatic bromination
The function of the Lewis acid can be seen in step 1. The ferric bromide reacts with bromine to produce a positive bromonium ion, Br+ (and FeBr4-). In step 2 this Br+ ion attacks the benzene ring to produce an arenium ion. Then finally in step 3 the arenium ion transfers a proton to FeBr4-. This results in the formation of bromobenzene and hydrogen bromide, the products of the reaction. At the same time this step regenerates the catalyst FeBr3.
The mechanism of the chlorination of benzene in the presence of ferric chloride is analogous to the one for bromination. Ferric chloride serves the same purpose in aromatic chlorination as ferric bromide does in aromatic bromination. It assists in the generation and transfer of a positive halonium ion. The rate of reaction is often of the form, Rate = K [Ar – H] [X2] [Lewis acid]
Hypo-halous acids (HO – X) in presence of strong acid also become a very powerful halogenating agent
Fluorine reacts so rapidly with benzene that aromatic fluorination requires special conditions and special types of apparatus. Even then, it is difficult to limit the reaction to monofluorination. Fluorobenzene can be made, however, by an indirect method.
Iodine, on the other hand, is so unreactive that a special technique has to be used to affect direct iodination; the reaction has to be carried out in the presence of an oxidizing agent ( to destroy strong reducing agent HI ) such as nitric acid.