Typical reaction: 1. The reaction of benzene with a mixture of concentrated nitric and sulphuric acids at 50^oC to give nitrobenzene.

The mechanism:

The formation of the electrophile, NO₂⁺, is a consequence of sulphuric acid being a stronger acid than nitric acid:

HNO₃  +  2H₂SO₄  ®  NO₂⁺  +  H₃O⁺  +  2HSO₄^-

Points:

• The use of the Kekule form of benzene is not required - you can use the delocalised form if wished.
• The loss of the proton from the intermediate must be to a base, a proton acceptor, which is the hydrogen sulphate ion.

Typical reaction: 2. The Friedel-Crafts reaction between chlorine and benzene in the presence of anhydrous aluminium chloride as catalyst. The reactants are heated under reflux.

The mechanism:

Points:

• As with nitration, the lost proton from the intermediate should be accepted by a base; in this case AlCl₄^-.
• It is difficult to stop the reaction at monosubstitution, and mixtures of alkylbenzenes are always obtained.
• Again, the delocalised form of benzene can be used.

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