Explain why carboxylic acids are significantly more acidic than alcohols, despite both functional groups containing an O–H bond.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Carboxylic acids are more acidic because the conjugate base, the carboxylate ion, can delocalise the negative charge over two oxygen atoms by resonance, stabilising the anion.
In contrast, the alkoxide ion from an alcohol carries the full negative charge on a single oxygen atom, making it less stable and higher in energy.
A more stable conjugate base shifts the equilibrium toward dissociation, giving a higher Ka (lower pKa) for carboxylic acids.
Additionally, the adjacent carbonyl group exerts an inductive electron‑withdrawing effect, reducing electron density on the O–H bond and further weakening it, which facilitates proton release.
In contrast, the alkoxide ion from an alcohol carries the full negative charge on a single oxygen atom, making it less stable and higher in energy.
A more stable conjugate base shifts the equilibrium toward dissociation, giving a higher Ka (lower pKa) for carboxylic acids.
Additionally, the adjacent carbonyl group exerts an inductive electron‑withdrawing effect, reducing electron density on the O–H bond and further weakening it, which facilitates proton release.
Examiner tips
- Use the word ‘delocalise’ and mention resonance; state the alkoxide has a single negative charge; link stability to Ka/pKa; include the inductive effect of the carbonyl.
- Keep each point brief and to the point, matching the four mark scheme items.
Mark scheme (4 marks)
- In carboxylic acids, the negative charge on the conjugate base (carboxylate ion) is delocalised / spread over both oxygen atoms via resonance, stabilising the anion.
- The alkoxide ion formed from an alcohol bears the full negative charge on a single oxygen atom, so it is less stable / higher in energy than the carboxylate ion.
- A more stable conjugate base shifts the equilibrium further towards dissociation / products, resulting in a higher Ka / lower pKa for carboxylic acids.
- The inductive / electron-withdrawing effect of the carbonyl C=O also reduces electron density on the O–H bond of the carboxylic acid, further weakening it and facilitating proton release.
Key terms in this question
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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