Decreasing order of acidity of p-methoxy benzoic acid (A), p-nitrobenzoic acid (B) and benzoic acid (C) is –

Options

A. B, C, A
B. A, B, C
C. C, A, B
D. None

Answer

To determine the decreasing order of acidity of p-methoxy benzoic acid (A), p-nitrobenzoic acid (B), and benzoic acid (C), we need to analyze the stability of the anions formed after the deprotonation of these acids. The stability of the anion is influenced by the substituents on the benzene ring.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **p-Methoxy benzoic acid (A)**: Contains a methoxy group (-OCH₃) at the para position.
- **p-Nitrobenzoic acid (B)**: Contains a nitro group (-NO₂) at the para position.
- **Benzoic acid (C)**: No substituents on the benzene ring.

2. **Understand Acidity**:
- Acidity is determined by the ability of an acid to donate a proton (H⁺). After losing a proton, the resulting anion's stability is crucial. A more stable anion corresponds to a stronger acid.

3. **Analyze the Effects of Substituents**:
- **Methoxy group (A)**: The methoxy group is an electron-donating group (through resonance). When the acid loses a proton, the negative charge on the carboxylate ion is destabilized because the methoxy group increases electron density on the ring. This makes the anion less stable and thus decreases acidity.
- **Nitro group (B)**: The nitro group is a strong electron-withdrawing group. It stabilizes the negative charge on the carboxylate ion by pulling electron density away from the ring. This makes the anion more stable and thus increases acidity.
- **Benzoic acid (C)**: Without any substituents, the stability of the anion is intermediate. It does not have the destabilizing effect of the methoxy group or the stabilizing effect of the nitro group.

4. **Determine the Order of Acidity**:
- Based on the analysis:
- p-Nitrobenzoic acid (B) is the strongest acid due to the stabilizing effect of the nitro group.
- Benzoic acid (C) is the next strongest, as it has no substituents affecting acidity.
- p-Methoxy benzoic acid (A) is the weakest acid due to the destabilizing effect of the methoxy group.

5. **Final Order**:
- The decreasing order of acidity is: **p-Nitrobenzoic acid (B) > Benzoic acid (C) > p-Methoxy benzoic acid (A)**.