Describe the electrophilic substitution mechanism for nitration
Follow nitronium-ion attack, the temporary loss of aromaticity and proton loss that restores the delocalised ring.
Section 30 · Hydrocarbons
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 30. Follow the listed reagents, catalysts, temperatures and reaction scope for benzene and methylbenzene. Directing effects are limited to –NH₂, –OH, –R, –NO₂, –COOH and –COR. Written practice is self-checked
Before you begin
- Use the Topic 29 model of benzene as a planar aromatic ring with a delocalised π system, and recognise electrophiles, nucleophiles and organic functional groups.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which species acts as the electrophile in benzene nitration?
What you will learn
- Identify NO₂⁺ as the electrophile
- Describe attack by the aromatic π system and formation of an intermediate
- Explain proton loss and restoration of aromaticity
Generate and identify the electrophile
Concentrated HNO₃ and H₂SO₄ generate NO₂⁺. The nitronium ion is electron-deficient and acts as the electrophile in benzene nitration.
The π-electron system of benzene attacks the electrophile. This forms a new C–N bond and a positively charged intermediate; aromatic delocalisation is temporarily interrupted at the carbon that now bears both H and NO₂.
Lose H⁺ to restore the aromatic system
A base removes the hydrogen from that carbon. The electrons in the C–H bond help reform the delocalised π system, so aromaticity is restored and nitrobenzene forms.
When drawing curly arrows, start each arrow at an electron pair or bond and point it toward where that pair moves. Show attack by the ring π electrons and the electron flow that restores the ring during deprotonation.
Pause and explain
What restores the aromatic π system after NO₂⁺ adds?
Worked example
Outline the nitration mechanism in three stages.
Show the worked solution
- The acid mixture generates NO₂⁺.
- The benzene π system attacks NO₂⁺ to form a σ-complex intermediate.
- A base removes H⁺ and the ring π system reforms.
Answer NO₂⁺ is generated; the ring attacks to form a positively charged intermediate; loss of H⁺ restores the aromatic ring and gives nitrobenzene.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Why does benzene temporarily lose aromatic delocalisation during nitration?
Give me a hint
Describe the intermediate after the new C–N bond forms.
Compare my reasoning
- The ring π electrons form a bond to NO₂⁺.
- One carbon becomes tetrahedral-like with both H and NO₂ attached.
- The continuous delocalised ring is interrupted in the intermediate.
Attack forms a new C–N bond and a positively charged intermediate, interrupting continuous delocalisation until H⁺ is removed.
Look for these in your work
- Electron-pair role stated
- Intermediate described
- Temporary disruption explained
State the key electron movement during deprotonation of the nitration intermediate.
Give me a hint
Follow the C–H bond pair as the proton leaves.
Compare my reasoning
- A base removes H⁺.
- The C–H bond electron pair contributes to re-forming the π system.
- The delocalised aromatic ring is restored.
A base removes H⁺ and the C–H bond electrons reform the ring π system, restoring aromaticity.
Look for these in your work
- Base action stated
- Electron source identified
- Aromaticity restored
A mechanism drawing has a curly arrow from NO₂⁺ to the centre of the benzene ring. Explain how to correct the arrow.
Give me a hint
Curly arrows show electron pairs moving from electron-rich to electron-poor positions.
Compare my reasoning
- The benzene π system supplies the electron pair.
- The arrow should start from a ring π bond.
- It should point toward the electrophilic nitrogen of NO₂⁺.
Start the arrow at a benzene π bond and point it to the electrophilic nitrogen; the arrow represents electrons moving from the ring to NO₂⁺.
Look for these in your work
- Electron source corrected
- Electrophile site identified
- Arrow direction explained
Common mistakes
- Starting a curly arrow at a positive charge rather than an electron pair
- Claiming aromaticity remains completely unchanged during electrophile attack
- Forgetting proton loss after the intermediate forms
- Drawing a permanent saturated ring product
What can you explain now?
Which species acts as the electrophile in benzene nitration?
Compare with the explanation
NO₂⁺
The acid mixture generates the nitronium electrophile.
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Make it stick
How does the nitration mechanism recover aromaticity after an electrophile bonds to the ring?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.