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 :

The correct option is (D), (A), (B), (C).

Explanation:
The acidic strength of substituted phenols depends on the stability of the phenoxide ion formed after losing a proton (H⁺). Electron-withdrawing groups (EWGs) stabilize the phenoxide ion by dispersing the negative charge, thereby increasing acidic strength. Conversely, electron-donating groups destabilize it and decrease acidic strength.

The nitro group (−NO₂) is a strong electron-withdrawing group that acts via two main mechanisms:
1. Inductive Effect (−I): Withdrawal of electron density through σ-bonds, which decreases with distance.
2. Resonance / Mesomeric Effect (−M): Withdrawal of electron density through the π-conjugated system. This effect operates only when the −NO₂ group is at the ortho or para positions relative to the hydroxyl (−OH) group. It does not operate at the meta position.

Let us analyze the given compounds based on these properties:

1. Phenol (D):
Phenol contains no substituents on the benzene ring. It lacks any electron-withdrawing groups to stabilize the negative charge on the phenoxide ion, making it the least acidic compound in the group.

2. 3-Nitrophenol (A):
Here, a single nitro group is attached at the meta position (position 3). Since resonance effects do not operate at the meta position, the −NO₂ group withdraws electron density solely through its inductive (−I) effect. This stabilizes the phenoxide ion relative to phenol, making it more acidic than phenol (D).

3. 3,5-Dinitrophenol (B):
This compound has two nitro groups, both located at meta positions (positions 3 and 5). Both groups withdraw electron density through their inductive (−I) effects. The combined inductive effect of two −NO₂ groups is stronger than that of a single group, leading to greater stability of the phenoxide ion compared to 3-nitrophenol (A). Thus, B is more acidic than A.

4. 2,4,6-Trinitrophenol (C):
Also known as picric acid, this compound has three nitro groups located at the ortho (positions 2 and 6) and para (position 4) positions. At these positions, all three nitro groups exert strong inductive (−I) and strong resonance/mesomeric (−M) effects. The combined, highly efficient electron withdrawal significantly stabilizes the phenoxide ion, making it extremely acidic (even more acidic than many carboxylic acids).

Comparing the overall electronic effects, the increasing order of acidic strength is:

$$\text{Phenol (D)}<\text{3-Nitrophenol (A)}<\text{3,5-Dinitrophenol (B)}<\text{2,4,6-Trinitrophenol (C)}$$

Therefore, the correct sequence is (D), (A), (B), (C).