Here's how:
1. Dehydrating Agent:
* Nitric acid (HNO₃), the other reactant, is a weak electrophile. To increase its electrophilicity, it needs to be protonated.
* Sulphuric acid, being a strong acid, protonates nitric acid to form the nitronium ion (NO₂⁺), a strong electrophile.
* The reaction:
HNO₃ + H₂SO₄ ⇌ H₂NO₃⁺ + HSO₄⁻
H₂NO₃⁺ ⇌ NO₂⁺ + H₂O
2. Catalyst:
* The nitronium ion then attacks the benzene ring, leading to the formation of nitrobenzene.
* The reaction:
C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺
* Sulphuric acid also helps to remove the water produced in the reaction, driving the equilibrium towards the formation of nitrobenzene.
In summary, the key roles of sulphuric acid in nitrobenzene synthesis are:
* Protonating nitric acid to form the nitronium ion.
* Acting as a catalyst to facilitate the reaction.
* Removing water to shift the equilibrium towards product formation.
Without sulphuric acid, the reaction would proceed very slowly, if at all.
