Phenol () is a much stronger acid than ethanol () but weaker than carboxylic acids. Which of the following substituents on the phenol ring would increase its acidity significantly, yielding a lower th

Question

Phenol ($pK_a \approx 10$) is a much stronger acid than ethanol ($pK_a \approx 16$) but weaker than carboxylic acids. Which of the following substituents on the phenol ring would increase its acidity significantly, yielding a $pK_a$ lower than that of standard unsubstituted phenol?

Prashant Kotian · Accepted Answer

Correct answer: A,D. The acidity of phenol depends heavily on the stability of the resulting phenoxide conjugate base. Electron-withdrawing groups (like ) stabilize the phenoxide anion through synergistic (inductive) and (resonance) effects, particularly when located at the ortho and para positions where the negative charge is delocalized. Alkyl () and alkoxy () groups are electron-donating and destabilize the anion, reducing acidity.

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