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.

(D), (A), (B), (C)

The correct answer is Option (3) → (D), (A), (B), (C)

Key Factors in Acidic Strength

• Electron Withdrawing Groups (EWG): Groups like the nitro group ($-NO_2$) are strongly electron-withdrawing through both the inductive effect ($-I$) and resonance effect ($-R$). They stabilize the negative charge on the phenoxide ion, making the parent phenol more acidic.
• Number of Groups: The more electron-withdrawing groups present on the ring, the greater the stabilization and the higher the acidity.
• Position of Groups: Nitro groups at the ortho and para positions stabilize the charge via resonance, while at the meta (3 or 5) position, they stabilize primarily via the inductive effect.

Analyzing the Compounds

1. (D) Phenol: This is the baseline. It has no electron-withdrawing groups to stabilize the phenoxide ion. (Least Acidic)
2. (A) 3-Nitrophenol: Has one nitro group at the meta position. It increases acidity via the $-I$ effect.
3. (B) 3,5-Dinitrophenol: Has two nitro groups at the meta positions. Two groups provide more stabilization than one, making it more acidic than (A).
4. (C) 2,4,6-Trinitrophenol (Picric Acid): Has three nitro groups, and importantly, they are at the ortho and para positions. This allows for massive stabilization through resonance. This compound is so acidic it is comparable to mineral acids. (Most Acidic)

Increasing Order of Acidic Strength

The order from weakest acid to strongest acid is:

Phenol < 3-Nitrophenol < 3,5-Dinitrophenol < 2,4,6-Trinitrophenol

(D) < (A) < (B) < (C)

The correct answer is: (D), (A), (B), (C)