Arrange the following according to their increasing acidity: A. 2,4,6…

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

An acid is stronger when its conjugate base is more stable. The phenoxide ion is stabilised by resonance (the alkoxide ion is not), so phenols are far more acidic than alcohols. On the ring, electron withdrawing NO₂ groups increase the acidity (more of them, more acidic; at ortho and para positions they also act by resonance), while an electron donating CH₃ group decreases it.

• (B) Propan-1-ol (pK_a about 16) — an alcohol; the propoxide ion has no resonance. The weakest acid.

• (E) 4-Methylphenol (pK_a 10.2) — a phenol, but the +I effect of \( \ce{CH3} \) makes it slightly weaker than phenol.

• (C) 3-Nitrophenol (pK_a 8.3) — one \( \ce{NO2} \), acting only by its −I effect from the meta position.

• (D) 3,5-Dinitrophenol (pK_a 6.7) — two \( \ce{NO2} \) groups withdrawing by −I.

• (A) 2,4,6-Trinitrophenol (picric acid, pK_a 0.4) — three \( \ce{NO2} \) groups at ortho and para positions withdraw electrons by −I and −R; the strongest acid.

Shortcut: The alcohol must come first; only Option (2) starts with B.

NCERT (2026-27 edition):
Acidity of Phenols – “… Arrange the following compounds in increasing order of their acid strength: Propan-1-ol, 2,4,6-trinitrophenol, 3-nitrophenol, 3,5-dinitrophenol, phenol, 4-methylphenol.
Solution: Propan-1-ol, 4-methylphenol, phenol, 3-nitrophenol, 3,5-dinitrophenol, 2,4,6-trinitrophenol.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: