Which of the following has maximum acidic strength ?

Options

A. o-Nitrobenzoic acid
B. m-Nitrobenzoic acid
C. p-Nitrobenzoic acid
D. p-Nitrophenol.

Explanation

To determine which of the given compounds has the maximum acidic strength, we need to analyze the structures and the effects of substituents on the acidity of benzoic acid derivatives. Here’s a step-by-step solution:

### Step 1: Identify the Compounds
The compounds in question are:
1. Ortho-nitrobenzoic acid
2. Meta-nitrobenzoic acid
3. Para-nitrobenzoic acid
4. p-Nitrophenol

### Step 2: Understand the Role of the Nitro Group
The nitro group (NO2) is an electron-withdrawing group. Its presence on the benzene ring affects the acidity of the carboxylic acid group (COOH) by stabilizing the negative charge on the conjugate base (the carboxylate ion, COO⁻) after deprotonation.

### Step 3: Analyze the Position of the Nitro Group
- **Ortho-nitrobenzoic acid**: The nitro group is positioned adjacent to the carboxylic acid group. This proximity allows for strong resonance and inductive effects, stabilizing the conjugate base.
- **Meta-nitrobenzoic acid**: The nitro group is positioned one carbon away from the carboxylic acid group. The electron-withdrawing effect is weaker here compared to the ortho and para positions.
- **Para-nitrobenzoic acid**: The nitro group is opposite the carboxylic acid group. It can still exert a strong electron-withdrawing effect, stabilizing the conjugate base.
- **p-Nitrophenol**: This compound has a hydroxyl (OH) group, which can also influence acidity, but the presence of the nitro group will still play a significant role.

### Step 4: Compare the Acidity
- The acidity is generally enhanced when the electron-withdrawing group is in the ortho or para position due to the resonance stabilization of the conjugate base.
- The ortho position provides the strongest stabilization due to both resonance and inductive effects.
- The para position also provides stabilization but is slightly less effective than the ortho position.
- The meta position is the least effective in enhancing acidity.

### Conclusion
Based on the analysis, **ortho-nitrobenzoic acid** has the maximum acidic strength due to the strong electron-withdrawing effect of the nitro group in the ortho position, which stabilizes the conjugate base effectively.

### Answer
**Ortho-nitrobenzoic acid (Option 1) has the maximum acidic strength.**

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