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A-Level Chemistry May/June 2025 Q6(e): The NO2 group in 2-nitrophenol is electron withdrawing. Suggest the relative acidities…
A-Level Chemistry · Paper 9701/42 · May/June 2025 · Question 6(e) · [4 marks]
The NO2 group in 2-nitrophenol is electron withdrawing. Suggest the relative acidities of ethanol, 2-nitrophenol, phenol and water. Explain your answer.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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Relative Acidity
Most acidic > 2-nitrophenol > phenol > water > ethanol > Least acidic
Explanation
Acidity is a measure of the ease with which a substance can donate a proton (H⁺). This is determined by the stability of the conjugate base (anion) formed. A more stable anion means the O-H bond is weaker, and the proton is lost more easily, resulting in a stronger acid.
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2-Nitrophenol and Phenol: In both 2-nitrophenol and phenol, a lone pair of electrons from the oxygen atom of the -OH group is delocalised into the π system of the benzene ring. This delocalisation spreads the negative charge of the resulting anion (phenoxide ion), stabilising it. This makes phenols more acidic than water and ethanol.
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The electron-withdrawing nitro group (-NO₂) in 2-nitrophenol further delocalises the negative charge through the ring, greatly increasing the stability of the 2-nitrophenoxide ion compared to the phenoxide ion. This makes 2-nitrophenol the strongest acid in the list.
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Ethanol: The ethyl group (-C₂H₅) in ethanol has a positive inductive effect, meaning it is electron-donating. It pushes electron density onto the oxygen atom, which intensifies the negative charge on the resulting ethoxide ion. This destabilises the anion, strengthens the O-H bond, and makes ethanol the weakest acid in the list.
How the marks are awarded
- M1 — Stating the correct order of acidity: 2-nitrophenol > phenol > water > ethanol.
- M2 — Explaining that acidity depends on the stability of the anion formed, which weakens the O-H bond and allows the proton (H⁺) to be lost more easily.
- M3 — Correctly explaining that for phenols, a lone pair from the oxygen atom is delocalised into the π system of the benzene ring, stabilising the anion.
- M4 — Correctly explaining that for ethanol, the alkyl group has a positive inductive effect (is electron-donating), which destabilises the anion and makes it a weaker acid.
Common mistakes
- Getting the order of acidity incorrect, particularly placing water as more acidic than phenol.
- Confusing the electron-donating inductive effect of the alkyl group in ethanol with an electron-withdrawing effect.
- Vaguely stating that the 'ring pulls electrons' without mentioning the delocalisation of the oxygen's lone pair into the π system.
- Failing to explain that the stability of the conjugate base is the reason for the difference in acidity.
Examiner tip: Always explain relative acidity by comparing the stability of the conjugate bases, linking this stability to specific electronic effects like delocalisation or induction.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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