Explain why benzene undergoes electrophilic substitution with concentrated nitric acid in the presence of concentrated sulfuric acid, rather than electrophilic addition, and identify the electrophile responsible for the reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid to form nitrobenzene. The reaction preserves the ring structure of benzene.
Model answer (4 marks)
Benzene has a delocalised π‑electron system that gives it aromatic stability. In the mixture of concentrated nitric and sulfuric acids, sulfuric acid protonates nitric acid to give the nitronium ion (NO₂⁺), the electrophile. The NO₂⁺ attacks the π‑electron cloud of benzene, forming a σ‑complex (arenium ion) that temporarily disrupts aromaticity. In the second step a proton is lost, restoring the delocalised system. Electrophilic addition would break the ring’s aromaticity completely and would require a highly unfavourable loss of the π‑electron system, so substitution is energetically preferred.
Examiner tips
- State the electrophile (NO₂⁺) and how it is formed
- Explain the role of benzene’s delocalised π system
- Mention that addition would destroy aromaticity
- Show that substitution restores aromaticity after proton loss
Common mistakes
- Confusing the nitronium ion with a nitrous acid species
- Claiming that addition is possible without noting loss of aromaticity
- Omitting the role of proton loss to restore aromaticity
Mark scheme (4 marks)
- The electrophile is the nitronium ion (NO₂⁺), generated when sulfuric acid protonates nitric acid.
- Benzene has a delocalised pi electron system / ring of delocalised electrons above and below the plane that attracts electrophiles.
- Addition would disrupt / destroy the delocalised electron system / aromaticity, which is energetically unfavourable.
- Substitution allows the delocalised system / aromaticity to be restored / retained when the proton is lost in the second step, making it the preferred pathway.
Key terms in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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