Choose the correct order of increasing acidic strength of following c…

The correct answer is Option (2) → (C), (B), (A), (D)

Acidic strength depends on how stable the anion left after losing \( \ce{H+} \) is. Two factors decide this order:

• Alcohol versus phenol: in the phenoxide ion the negative charge is spread over the benzene ring by resonance; the alkoxide ion has no such delocalisation. So phenols are far more acidic than alcohols.

• Nitro groups: \( \ce{-NO2} \) is electron withdrawing and further stabilises the phenoxide ion, so a nitrophenol is more acidic than phenol, and two nitro groups act more strongly than one. At the meta position the effect is inductive only, but it still raises the acidity.

The given compounds, with their pK_a values (a lower pK_a means a stronger acid):

• (C) Propan-1-ol: an alcohol, pK_a about 16

• (B) Phenol: pK_a 10.0

• (A) 3-Nitrophenol: one nitro group, pK_a 8.3

• (D) 3,5-Dinitrophenol: two nitro groups, pK_a about 6.7

Shortcut: The alcohol is the weakest acid and the dinitrophenol the strongest, so the order starts with (C) and ends with (D): Options (2) and (3). Phenol, with no nitro group, must come before 3-nitrophenol, which leaves Option (2).

Note: It seems that there is a mistake in the original NTA paper. Option (1) is printed as (D), (B), (D), (C): it repeats (D) and leaves out (A), so it is not a valid order.

NCERT (2026-27 edition):
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. On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength. Cresols, for example, are less acidic than phenol.”