Concept:Acidic strength is determined by the electron-withdrawing capacity of the group attached to the acidic -OH, which stabilizes the resulting negative charge on the conjugate base.
Formula:$$ K_a \propto \text{Stability of anion} \propto \text{Electron withdrawing groups} $$
Solution:- In ethanoic acid, the carbonyl group (C=O) exerts a strong electron-withdrawing inductive and resonance effect, massively stabilizing the acetate ion.
- In phenol, the phenyl ring also withdraws electrons via resonance, stabilizing phenoxide, but it is less effective than the carbonyl group.
- Ethanol has an electron-donating alkyl group, destabilizing the ethoxide ion.
- Thus, the electron-withdrawing effect of ethanoic acid is greater than that of phenol.
Why other options are incorrect:Acidity is not increased by electron-releasing effects (rules out A and B). While C is partially true (phenol withdraws more than ethanol), D is the primary reason that bridges the complete trend provided in the question.
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