Arrange the following compounds in increasing order of their pKa valu…

The correct answer is Option (3) → (B), (A), (D), (C)

The lower the pK_a, the stronger the acid (pK_a = −log K_a), so increasing pK_a runs from the strongest to the weakest acid. Two factors decide this order:

• Phenols vs alcohols: the phenoxide ion is stabilised by resonance (the negative charge spreads into the ring), while the ethoxide ion is destabilised by the +I effect of the ethyl group. So phenols are far more acidic than ethanol.

• Substituents on the ring: an electron withdrawing –\( \ce{NO2} \) group stabilises the phenoxide ion and increases acidity, most strongly from the ortho and para positions where it acts by resonance (−R) as well as −I; from the meta position only the −I effect operates. An electron releasing –\( \ce{CH3} \) group destabilises the phenoxide ion, so a cresol is less acidic than phenol.

• (B) o-Nitrophenol: pK_a 7.2

• (A) m-Nitrophenol: pK_a 8.3

• (D) o-Cresol: pK_a 10.2

• (C) Ethanol: pK_a 15.9

Shortcut: Ethanol (C) has the highest pK_a, so it comes last; Options (1) and (3) end with (C). A nitrophenol is more acidic than a cresol, so (A) must come before (D): Option (3).

NCERT (2026-27 edition):
Acidity of Phenols – …