Benzene ring in phenol is expected to give the characteristic substitution or replacement reactions, i.e., halogenation, nitration, sulphonation, alkylation, etc. In these reactions, the attacking reagent is electrophile. The phenolic group activates the ortho and para positions in the ring due to conjugation. Disubstitution reactions are quite fast as the electron density in the ring has increased considerably. Phenols do not respond to nucleophilic substitution reactions as it is influenced by phenolic group.
Picric acid is a very strong acid due to the presence of:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Three NO2 groups
Picric acid is 2,4,6-trinitrophenol. Phenol itself is only weakly acidic (pKa 10.0). The three nitro groups at the ortho and para positions are strongly electron-withdrawing (−I and −R effects); they delocalise the negative charge of the phenoxide ion onto their oxygen atoms and stabilise it greatly. Loss of \( \ce{H^{+}} \) becomes very easy, and picric acid (pKa 0.71) is as strong as many mineral acids.
Why the other options are incorrect
Option (1) COOH group — Incorrect. Picric acid has no carboxyl group; its acidic group is the phenolic –OH.
Option (3) H+ group — Incorrect. There is no “\( \ce{H^{+}} \) group”; \( \ce{H^{+}} \) is the ion released, not the cause of the acid strength.
Option (4) OH group — Incorrect. The –OH gives only weak acidity, as in phenol; the strength comes from the nitro groups.
NCERT (2026-27 edition):
Acidity of Phenols – …
