R3.4.13 Nitration of benzene

Nitration of benzene: Reactants

Benzene reacts with concentrated nitric acid and concentrated sulfuric acid to form nitrobenzene and water.

Role of sulfuric acid in nitration

Acts as a catalyst and protonates nitric acid to help generate the nitronium ion.

Electrophile in nitration of benzene

Nitronium ion (NO₂⁺), formed by reaction of nitric acid with sulfuric acid.

Nitronium ion formation

Sulfuric acid protonates nitric acid

Protonated nitric acid loses water to form NO₂⁺.

Mechanism step 1: Electrophile attack

NO₂⁺ is attracted to the pi electrons of benzene

Bond forms with a carbon atom, creating a carbocation intermediate.

Mechanism step 2: Reforming benzene ring

A pair of electrons from a C-H bond restore the delocalized pi system, forming nitrobenzene.

Reaction temperature control

At 50°C, nitrobenzene forms.

Higher temperatures lead to di- or tri-substituted products.

Importance of temperature in nitration

50°C forms nitrobenzene; 65°C forms 1,3-dinitrobenzene

110°C forms 1,3,5-trinitrobenzene.

Overall nitration equation

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (in presence of H₂SO₄ catalyst).

Catalyst regeneration in nitration

H⁺ from the mechanism reacts with HSO₄⁻ to reform H₂SO₄.