Explain why arenes favour substitution over addition
Use the restoration of aromatic stabilisation to explain why electrophilic substitution predominates over addition.
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
What is restored after an electrophile substitutes for H on benzene?
What you will learn
- Relate aromaticity to delocalised π electrons
- Explain why substitution restores aromatic stabilisation
- Contrast substitution products with addition products
Aromatic stabilisation comes from delocalisation
In benzene, π electrons are delocalised around the ring. This shared electron system gives an extra stabilisation compared with a structure that has only isolated double bonds.
During electrophilic substitution, the ring temporarily loses continuous delocalisation in the intermediate, but deprotonation restores it. The product retains the aromatic ring and recovers this stabilisation.
Addition would leave the ring less stable
An addition reaction across an aromatic ring would form new σ bonds while permanently disrupting the delocalised π system. The product would lose aromatic stabilisation.
Because substitution restores the aromatic system whereas addition does not, substitution predominates for the specified arene chemistry. The explanation depends on relative stabilisation, not on claiming addition is impossible under every conceivable condition.
Pause and explain
Why does a permanent addition product lose the stabilisation associated with benzene?
Worked example
Explain why nitration produces nitrobenzene by substitution rather than a stable addition product.
Show the worked solution
- Nitration temporarily disrupts delocalisation as NO₂⁺ attaches.
- Loss of H⁺ restores the ring's delocalised π system.
- A permanent addition product would not recover aromatic stabilisation.
Answer Substitution predominates because proton loss restores the delocalised aromatic system, while addition would leave that stabilisation lost.
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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.
Compare the aromaticity of the substitution intermediate and the final product.
Give me a hint
Track the ring before attack, after attack and after proton loss.
Compare my reasoning
- Before attack, electrons are delocalised.
- Electrophile attack interrupts the delocalised system temporarily.
- Deprotonation restores it in the final substitution product.
The intermediate temporarily lacks the full aromatic delocalisation, but the final product regains it after H⁺ is removed.
Look for these in your work
- Initial state described
- Intermediate qualified
- Final state described
Give one energy/stability reason that substitution is preferred over addition in arenes.
Give me a hint
Mention which product keeps the stabilised aromatic π system.
Compare my reasoning
- Substitution removes a ring hydrogen and restores delocalisation.
- Addition leaves the π system disrupted.
- The aromatic product is therefore more stabilised.
Substitution is favoured because it restores the stabilised delocalised π system; addition would lose that aromatic stabilisation in the product.
Look for these in your work
- Substitution effect stated
- Addition effect stated
- Stability link explained
A learner says the ring stays aromatic throughout electrophilic substitution. Refine the statement.
Give me a hint
Separate the short-lived intermediate from the final product.
Compare my reasoning
- Electrophile attack temporarily interrupts aromatic delocalisation.
- A positively charged intermediate forms.
- Loss of H⁺ restores the aromatic system in the product.
Aromaticity is temporarily disrupted during attack but restored when the intermediate loses H⁺; it is not unchanged throughout the mechanism.
Look for these in your work
- Temporary change included
- Intermediate included
- Restoration included
Common mistakes
- Claiming aromaticity is never disrupted at any point
- Saying an addition product is impossible in every condition
- Calling the sigma complex the final product
- Explaining the preference without mentioning delocalisation or stabilisation
What can you explain now?
What is restored after an electrophile substitutes for H on benzene?
Compare with the explanation
The delocalised aromatic π system
Deprotonation restores the ring's delocalisation.
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Make it stick
How does aromatic stabilisation explain the preference for substitution rather than addition?
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.