The acidic strength of saturated aliphatic carboxylic acids depends mainly upon the inductive effect of the substituent and its position w.r.t., the -COOH group. Whereas electron donating substituents tend to decrease, electron withdrawing substituents tend to increase the acid sttrength. the acidic strength of aromatic carboxylic acids, on the other hand, depends upon both the inductive and the resonance effect of the substituents
Q. Among the following, the strongest acid is

Options

A. `CH_(3)COOH`
B. `C_(6)H_(5)COOH`
C. `m-CH_(3)OC_(6)H_(4)COOH`
D. `p-CH_(3)OC_(6)H_(4)COOH`

Explanation

To determine the strongest acid among the given options: acetic acid, benzoic acid, meta-methoxybenzoic acid, and para-methoxybenzoic acid, we will analyze the acidic strength based on the inductive and resonance effects of the substituents.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Acetic acid (CH₃COOH)
- Benzoic acid (C₆H₅COOH)
- Meta-methoxybenzoic acid (C₆H₄(OCH₃)COOH)
- Para-methoxybenzoic acid (C₆H₄(OCH₃)COOH)

2. **Compare Acetic Acid and Benzoic Acid**:
- Acetic acid is an aliphatic carboxylic acid, while benzoic acid is an aromatic carboxylic acid.
- When acetic acid loses a proton (H⁺), it forms the acetate ion (CH₃C(=O)O⁻).
- When benzoic acid loses a proton, it forms the benzoate ion (C₆H₅C(=O)O⁻).
- The benzoate ion is stabilized by resonance due to the aromatic ring, which delocalizes the negative charge. Therefore, benzoic acid is stronger than acetic acid.

3. **Analyze Meta and Para-methoxybenzoic Acids**:
- The methoxy group (–OCH₃) is an electron-donating group (EDG) that has both +M (resonance) and -I (inductive) effects.
- In meta-methoxybenzoic acid, the methoxy group primarily exerts a -I effect, which stabilizes the conjugate base (COO⁻).
- In para-methoxybenzoic acid, the methoxy group exerts a +M effect, which increases the electron density on the carboxylate ion, leading to destabilization due to repulsion between the negative charge and the electron-donating group.

4. **Determine the Strongest Acid**:
- Between meta-methoxybenzoic acid and para-methoxybenzoic acid, the meta-methoxybenzoic acid is more acidic because the -I effect of the methoxy group stabilizes the conjugate base more effectively than the +M effect destabilizes it.
- Thus, the order of acidity is:
- Benzoic acid > Acetic acid > Meta-methoxybenzoic acid > Para-methoxybenzoic acid.

5. **Conclusion**:
- The strongest acid among the given options is **benzoic acid**.