Why is acetic acid (pKa ≈ 4.76) much MORE acidic than ethanol (pKa ≈ 16)?
AThe carboxylate ion (RCOO⁻) is stabilised by RESONANCE — the negative charge is delocalised over both oxygens; alkoxide (RO⁻) has localised charge on one O
BAcetic acid is heavier
CAcetic acid is polar; ethanol is not
DAcetic acid has more carbons
Answer & Solution
Correct answer: A. The carboxylate ion (RCOO⁻) is stabilised by RESONANCE — the negative charge is delocalised over both oxygens; alkoxide (RO⁻) has localised charge on one O
**Carboxylate resonance**: RCOO⁻ has two equivalent resonance structures with negative charge on each O → charge fully delocalised → very stable. **Alkoxide** has charge stuck on one O. The resonance stabilisation of carboxylate makes RCOOH a ~10¹¹ times stronger acid than ROH.
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