Which of the following will have the highest pKa value?

The correct answer is Option (1) → Phenol

pK_a = −log K_a, so a higher pK_a means a weaker acid.

Phenol is acidic because the phenoxide ion is stabilised by resonance. Any group that helps spread the negative charge further makes the phenol a stronger acid. The nitro group is strongly electron withdrawing:

• At the ortho and para positions it withdraws electrons by both the −R (resonance) and −I effects; the negative charge can move onto the \( \ce{NO2} \) group, so acidity rises sharply.

• At the meta position only the −I effect works, so acidity rises less.

So phenol, with no electron withdrawing group, is the weakest acid and has the highest pK_a.

Why the other options are incorrect

• Option (2) p-Nitrophenol — Incorrect. The para \( \ce{NO2} \) group stabilises the phenoxide by −R and −I effects; it is the strongest acid here, with the lowest pK_a.

• Option (3) m-Nitrophenol — Incorrect. The meta \( \ce{NO2} \) group acts only by its −I effect, but that still makes it more acidic than phenol.

• Option (4) o-Nitrophenol — Incorrect. The ortho \( \ce{NO2} \) group acts by −R and −I effects; intramolecular hydrogen bonding makes it slightly weaker than the para isomer, but still much stronger than phenol.

NCERT (2026-27 edition):
Acidity of Phenols