Benzene undergoes nitration when heated with a mixture of concentrated nitric acid and concentrated sulfuric acid. Explain the mechanism by which nitrobenzene is formed, including the role of concentrated sulfuric acid and the type of reaction involved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nitrobenzene, C₆H₅NO₂, is an important industrial chemical used in the manufacture of dyes and pharmaceuticals. It is produced by reacting benzene with a nitrating mixture at approximately 55 °C.
Model answer (5 marks)
1. Concentrated sulphuric acid protonates nitric acid, forming the nitronium ion (NO₂⁺).
2. NO₂⁺ is the electrophile that attacks the π‑electron system of benzene.
3. The attack gives a σ‑complex (arenium ion) with a positive charge on the ring.
4. Loss of a proton from the σ‑complex restores aromaticity, giving nitrobenzene.
5. The overall process is an electrophilic substitution, not an addition reaction.
2. NO₂⁺ is the electrophile that attacks the π‑electron system of benzene.
3. The attack gives a σ‑complex (arenium ion) with a positive charge on the ring.
4. Loss of a proton from the σ‑complex restores aromaticity, giving nitrobenzene.
5. The overall process is an electrophilic substitution, not an addition reaction.
Examiner tips
- Show the stepwise mechanism: protonation, electrophile formation, attack, deprotonation. Use the term ‘nitronium ion’ and ‘electrophilic substitution’. Explain the role of sulphuric acid as a catalyst and proton donor.
Common mistakes
- Confusing the nitronium ion with NO₂· or NO₂⁻. Forgetting to mention that the reaction is not an addition. Not identifying sulphuric acid as a catalyst that generates the electrophile.
Mark scheme (5 marks)
- Concentrated sulfuric acid acts as a catalyst / protonates concentrated nitric acid to generate the nitronium ion (NO₂⁺)
- The nitronium ion (NO₂⁺) is the electrophile that attacks the benzene ring
- The electrophile is attracted to / attacks the delocalised π electron system of benzene, forming a positively charged intermediate
- A proton (H⁺) is lost from the intermediate / the ring re-aromatises by loss of H⁺
- The overall reaction is electrophilic substitution (not addition)
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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