Which of the following is the strongest acid?

The correct answer is Option (3) → p-Nitrophenol

• A phenol is acidic because it loses \( \ce{H^+} \) to give a phenoxide ion. A group that draws the negative charge of this ion further away from the oxygen makes the ion more stable and the phenol more acidic (lower pK_a).

• The nitro group is strongly electron withdrawing. At the para position it takes the negative charge of the phenoxide ion onto itself through the ring, so p-nitrophenol has the lowest pK_a of the four: 7.1.

Why the other options are incorrect

Option (1) o-Nitrophenol — Incorrect. The ortho nitro group also delocalises the charge and makes it far more acidic than phenol, but its pK_a (7.2) is a little higher than that of p-nitrophenol (7.1).

Option (2) p-Chlorophenol — Incorrect. Chlorine withdraws electrons far less strongly than a nitro group and cannot take the negative charge onto itself, so p-chlorophenol is much weaker than any nitrophenol.

Option (4) m-Nitrophenol — Incorrect. From the meta position the nitro group cannot take up the negative charge of the phenoxide ion, so its effect is smaller; pK_a 8.3.

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.”

“The greater the pK_a value, the weaker the acid.”

Table 7.3: pK_a Values of some Phenols and Ethanol