The ascending order of acidity of -OH group in the following compounds is:

Step 1: Rank the compounds by their effect on -OH acidity

Acidity of -OH depends on stability of the conjugate base (anion after losing H+).

Step 2: Key principles

• Aliphatic alcohol (Bu-OH): Only inductive effect stabilizes alkoxide. No resonance delocalization. Least acidic (pKa ≈ 16-18).
• p-Methoxyphenol (C): $\ce{-OCH3}$ is EDG (+M effect) → destabilizes phenoxide → less acidic than phenol (pKa ≈ 10.2).
• Phenol (D): Phenoxide stabilized by resonance with ring (pKa = 10.0).
• p-Nitrophenol (B): One $\ce{-NO2}$ EWG at para → resonance stabilization of phenoxide → more acidic (pKa ≈ 7.1).
• 2,4-Dinitrophenol (E): Two $\ce{-NO2}$ groups → much greater stabilization → most acidic (pKa ≈ 4.0).

Step 3: Assemble order

Ascending acidity (least to most acidic):

(A) Bu-OH < (C) p-MeO-phenol < (D) phenol < (B) p-NO2-phenol < (E) 2,4-dinitrophenol

Answer: (A) < (C) < (D) < (B) < (E)

Key Points to Remember:

• Aliphatic alcohols are less acidic than phenols (no resonance in alkoxide).
• EDG on phenol ring (e.g., -OMe, -OH, -NH2) decrease acidity; EWG (-NO2, -CN, -COOH) increase acidity.
• Resonance effect of EWG is operative only at ortho and para positions (not meta).
• More EWG groups at ortho/para → progressively higher acidity.