Arrange the following compounds in increasing order of their acidic c…

The correct answer is Option (4) → (D), (C), (A), (B)

A phenol is acidic because the phenoxide ion left after losing \( \ce{H+} \) is stabilised by resonance. A substituent that stabilises the phenoxide ion increases the acidity, and one that destabilises it decreases the acidity:

• An electron-withdrawing group such as \( \ce{-NO2} \) spreads the negative charge further. At the para (or ortho) position it acts by both resonance and the inductive effect, because the charge can move right onto the nitro group. At the meta position it acts by the inductive effect only, so the effect is smaller.

• An electron-releasing group such as \( \ce{-OCH3} \) at the para position pushes electron density into the ring by resonance and makes the phenoxide ion less stable, so the acid is weaker than phenol.

Applying this to the four compounds (pK_a values in brackets; a lower pK_a means a stronger acid):

• (D) p-methoxyphenol: electron-releasing \( \ce{-OCH3} \), weaker than phenol (about 10.2)

• (C) phenol: reference (10.0)

• (A) m-nitrophenol: \( \ce{-NO2} \) acting by the inductive effect only (8.3)

• (B) p-nitrophenol: \( \ce{-NO2} \) acting by resonance and the inductive effect (7.1)

    \( \displaystyle p\text{-methoxyphenol} \lt \text{phenol} \lt m\text{-nitrophenol} \lt p\text{-nitrophenol} \)

Shortcut: The electron-releasing methoxy compound is the weakest acid and p-nitrophenol the strongest, so the order starts with (D) and ends with (B). Only Option (4) does this.

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