Which of the following compounds is more acidic than phenol?

The correct answer is Option (4) → o-Nitrophenol

• A phenol is acidic because it loses \( \ce{H^+} \) to give a phenoxide ion, whose negative charge is spread over the ring by resonance. A group that spreads this charge further makes the phenoxide ion more stable and the phenol more acidic (lower pK_a).

• The nitro group is electron withdrawing. At the ortho position it draws the negative charge of the phenoxide ion onto itself, so o-nitrophenol (pK_a 7.2) is a stronger acid than phenol (pK_a 10.0).

Why the other options are incorrect

Option (1) Ethanol — Incorrect. In the ethoxide ion the negative charge stays on the oxygen; there is no ring to spread it, and the ethyl group releases electrons. Ethanol (pK_a 15.9) is much weaker than phenol.

Option (2) o-Methoxyphenol — Incorrect.

• The –OCH₃ group releases electrons into the ring by resonance, as the –OR group does in anisole.

• It does not spread the negative charge of the phenoxide ion, so it does not make the phenol more acidic.

Option (3) o-Methylphenol — Incorrect. The methyl group is an electron releasing alkyl group, which does not favour the phenoxide ion. o-Cresol (pK_a 10.2) is less acidic than phenol.

NCERT (2026-27 edition):
(iii) Acidity of phenols – “In substituted phenols, the presence of electron withdrawing groups such as nitro group, enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalisation of negative charge in phenoxide ion when substituent is at ortho or para position.”