Which of the following compounds will show highest acidic strength?

The correct answer is Option (1) → 2,4,6-Trinitrophenol

Phenol is acidic because the phenoxide ion left after losing \( \ce{H+} \) is stabilised by delocalisation of the negative charge over the benzene ring. An electron-withdrawing nitro group at an ortho or para position spreads this charge further, onto its own oxygen atoms (−R and −I effects), so the anion becomes more stable and the phenol more acidic. Each extra nitro group adds to the effect:

    \( \displaystyle \text{2,4,6-trinitrophenol} \gt \text{2,4-dinitrophenol} \gt \text{2-nitrophenol} \gt \text{phenol} \)

    \( \displaystyle pK_a:\quad \approx 0.4 \lt 4.0 \lt 7.2 \lt 10.0 \)

With three nitro groups, at both ortho positions and the para position, 2,4,6-trinitrophenol (picric acid) is the strongest acid; it is even stronger than many carboxylic acids.

NCERT (2026-27 edition):
Nitration – …

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.”