Which of the following is the strongest acid ?

Options

A. Benzoic acid
B. 4-Nitrobenzoic acid
C. 4-Methoxy benzoic acid
D. 4-Methyl benzoic acid

Explanation

To determine which of the following compounds is the strongest acid among benzoic acid, 4-nitrobenzoic acid, 4-methoxybenzoic acid, and 4-methylbenzoic acid, we need to analyze the effects of substituents on the acidity of the carboxylic acid group (-COOH).

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Benzoic acid: C6H5COOH
- 4-Nitrobenzoic acid: C6H4(NO2)COOH
- 4-Methoxybenzoic acid: C6H4(OCH3)COOH
- 4-Methylbenzoic acid: C6H4(CH3)COOH

2. **Understanding Acidity**:
- 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) stabilize the negative charge on the conjugate base, thus increasing acidity.
- Conversely, electron-donating groups (EDGs) destabilize the conjugate base, decreasing acidity.

3. **Analyze Each Compound**:
- **Benzoic Acid**: No substituents that affect acidity significantly. It has a moderate acidic character.
- **4-Nitrobenzoic Acid**: The nitro group (NO2) is a strong electron-withdrawing group due to its -I (inductive) and -R (resonance) effects. This increases the acidity.
- **4-Methoxybenzoic Acid**: The methoxy group (OCH3) is an electron-donating group due to its +R effect, which decreases the acidity.
- **4-Methylbenzoic Acid**: The methyl group (CH3) is also an electron-donating group, which decreases the acidity.

4. **Conclusion**:
- Among the four compounds, only 4-nitrobenzoic acid has a strong electron-withdrawing group, making it the strongest acid. The other compounds either have no effect or decrease acidity.

### Final Answer:
**4-Nitrobenzoic acid is the strongest acid among the given options.**

---