Nitration of Benzene

Benzene reacts with nitric acid to form nitrobenzene, C₆H₅NO₂, by electrophilic substitution.Conditions: a mixture of concentrated nitric acid and concentrated sulfuric acid (the nitrating mixture), warmed to about 50 °C. The temperature is kept below about 55 °C, because at higher temperatures more than one nitro group substitutes, giving 1,3-dinitrobenzene.Generating the electrophile: sulfuric acid is the stronger acid and protonates nitric acid, which then loses water to form the nitronium ion, NO₂⁺:
HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻Mechanism:
• A pair of electrons from the delocalised ring forms a bond to the nitrogen atom of NO₂⁺, giving a positive intermediate.
• The C-H bond breaks and H⁺ is lost, restoring the delocalised ring.Regenerating the catalyst:
H⁺ + HSO₄⁻ → H₂SO₄
so sulfuric acid acts as a catalyst.Overall:
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂OUses: nitrobenzene is a pale yellow oil. It can be reduced to phenylamine, which is used to make dyes. Nitration of arenes is also used to make explosives such as TNT.Worked example: 15.6 g of benzene (M_r = 78.0) is 0.200 mol. Nitrobenzene has M_r = 123, so the theoretical yield is 0.200 × 123 = 24.6 g. If 19.7 g is obtained, the percentage yield is 19.7 ÷ 24.6 × 100 = 80.1%.