How is benzene nitrated?

The nitration of benzene is an example of an electrophilic substitution reaction - where one of the hydrogens (or other group attached to the benzene ring) is removed and replaced by another group. There are 3 steps in the nitration of benzene:

1. First an electrophile needs to be formed. This is done be reacting nitric acid with sulfuric acid. Sulfuric acid is the stronger acid (it dissociates more in solution) and so nitric acid acts as the base in this reaction.

H₂SO₄ + HNO₃ ---> H₂NO₃⁺ + HSO₄^-

The H₂NO₃⁺ formed then quickly breaks down into water and a nitronium ion (NO₂⁺) - this is the electrophile that can then attack the benzene ring.

H₂NO₃⁺ ----> H₂O + NO₂ ⁺

2. The nitronium ion attacks the benzene ring, breaking the stable ring of delocalised electrons.

3. Now one of the carbons in the structure is attached to both the nitrate and the hydrogen it was first bound to. This structure is now unstable and so the benzene ring forms the delocalised ring again, by breaking the carbon and hydrogen bond, using the electrons in that bond to reform the delocalised ring. The hydrogen is released as a H⁺ and is picked up by the HSO₄^- (formed in step 1) forming the sulfuric acid (therefore the sulfuric acid acts as a catalyst in this reaction)