Arrange the following compounds in increasing order of their acidic strength:
(A) 3-nitrophenol
(B) 3,5-Dinitrophenol
(C) 2,4,6-Trinitrophenol
(D) Phenol
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (D), (A), (B), (C)
A phenol is acidic because it loses \( \ce{H+} \) to give the phenoxide ion, which is stabilised by resonance. Anything that stabilises the phenoxide ion more makes the phenol a stronger acid. A nitro group withdraws electrons and spreads the negative charge: from the meta position it acts only through its inductive (−I) effect, while from the ortho and para positions it also withdraws by resonance (−R), which is much stronger. Each extra nitro group adds to the effect.
(D) Phenol has no electron-withdrawing group: the weakest acid (pKa about 10).
(A) 3-Nitrophenol has one nitro group at a meta position, acting by −I only (pKa about 8.3).
(B) 3,5-Dinitrophenol has two meta nitro groups; the two −I effects add up, so it is stronger still (pKa about 6.7).
(C) 2,4,6-Trinitrophenol (picric acid) has three nitro groups, all at ortho and para positions, so the charge is delocalised onto all three by resonance. It is the strongest acid (pKa about 0.4), nearly as strong as a mineral acid.
Shortcut: Phenol must come first and picric acid last. Only Option (3) starts with (D) and ends with (C).
NCERT (2026-27 edition):
Acidity of Phenols – …
