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A-Level Chemistry May/June 2025 Q6(b): Phenol can be nitrated with dilute nitric acid. Explain why the nitration of phenol occ…
A-Level Chemistry · Paper 9701/42 · May/June 2025 · Question 6(b) · [2 marks]
Phenol can be nitrated with dilute nitric acid. Explain why the nitration of phenol occurs under milder conditions than the nitration of benzene.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
In phenol, a lone pair of electrons in the p-orbital of the oxygen atom in the hydroxyl (-OH) group overlaps with and is delocalised into the π system of the benzene ring.
This delocalisation increases the electron density of the ring, making it more activated towards electrophilic attack. Consequently, the ring can more strongly attract and polarise the electrophile, NO₂⁺, allowing the reaction to proceed under milder conditions compared to benzene.
How the marks are awarded
- M1 — The answer correctly identifies that a lone pair of electrons in a p-orbital on the oxygen atom is delocalised into the benzene ring's π system. This covers the source (O lone pair), action (delocalisation/overlap), and destination (ring/π system).
- M2 — This mark is awarded for explaining the consequence of the delocalisation: the electron density of the ring is increased, which makes it better at attracting and polarising the electrophile (NO₂⁺).
Common mistakes
- Stating that the -OH group is 'activating' without explaining why in terms of electron delocalisation.
- Incorrectly stating that the electronegative oxygen atom withdraws electron density from the ring, making it less reactive (confusing the inductive effect with the dominant mesomeric/resonance effect).
- Giving an incomplete explanation, such as 'the lone pair is donated to the ring', without mentioning the p-orbital overlap with the π system.
- Only explaining why benzene is unreactive (e.g., 'it requires a catalyst') without comparing it to the enhanced reactivity of phenol.
Examiner tip: When comparing the reactivity of substituted arenes, always explain the effect of the substituent group on the electron density of the ring's π system.
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