Which one of the following has the maximum acid strength ?

Options

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

Explanation

To determine which compound has the maximum acid strength among the given options, we need to analyze the structures and the effects of substituents on the acidity of carboxylic acids.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds in question are ortho-nitrobenzoic acid, meta-nitrobenzoic acid, para-nitrobenzoic acid, and para-nitrophenol.

2. **Understand Acid Strength**: The strength of an acid is influenced by the stability of its conjugate base. The more stable the conjugate base, the stronger the acid. Electron-withdrawing groups (EWGs) increase acidity by stabilizing the negative charge on the conjugate base.

3. **Draw the Structures**:
- **Ortho-nitrobenzoic acid**: -COOH group is adjacent to -NO2 group.
- **Meta-nitrobenzoic acid**: -COOH group is separated from -NO2 group by one carbon.
- **Para-nitrobenzoic acid**: -COOH group is opposite to -NO2 group.
- **Para-nitrophenol**: -OH group is opposite to -NO2 group.

4. **Analyze the Effects of the Nitro Group**:
- In ortho-nitrobenzoic acid, the nitro group can exert both -I (inductive) and -R (resonance) effects, but the proximity can lead to intramolecular hydrogen bonding, which stabilizes the acid and makes it less likely to donate a proton.
- In meta-nitrobenzoic acid, the nitro group only exerts a -I effect, which is less effective in stabilizing the conjugate base compared to resonance.
- In para-nitrobenzoic acid, the nitro group exerts both -I and -R effects effectively without the interference of intramolecular hydrogen bonding, allowing for easier proton donation.
- In para-nitrophenol, the -OH group is less acidic than -COOH, and the presence of the nitro group does not enhance acidity as effectively as in the benzoic acids.

5. **Compare the Acidity**:
- Ortho-nitrobenzoic acid has reduced acidity due to intramolecular hydrogen bonding.
- Meta-nitrobenzoic acid has lower acidity due to the lack of resonance stabilization.
- Para-nitrobenzoic acid has the highest acidity due to strong -I and -R effects without hindrance from intramolecular interactions.
- Para-nitrophenol is the least acidic due to the nature of the -OH group.

6. **Conclusion**: Based on the analysis, the compound with the maximum acid strength is **para-nitrobenzoic acid**.

### Final Answer:
**Para-nitrobenzoic acid (Option C) has the maximum acid strength.**

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