Q.25 The order of acidity of the following acids is
Given Compounds
| Compound | Substituent |
|---|---|
| (1) | –OCH3 (Methoxy) |
| (2) | –CH3 (Methyl) |
| (3) | –NO2 (Nitro) |
| (4) | –H (Benzoic acid) |
Key Concept: Acidity of Benzoic Acids
- Acidity depends on the stability of the conjugate base.
- Electron-withdrawing groups increase acidity.
- Electron-donating groups decrease acidity.
- Para substituents affect acidity through inductive and resonance effects.
Substituent Effect Analysis
(3) p-Nitrobenzoic Acid
The nitro group shows strong −I and −M effects, withdrawing electron density
and stabilizing the benzoate ion the most. Hence, it is the most acidic.
(4) Benzoic Acid
No substituent effect is present. Its acidity lies between electron-withdrawing
and electron-donating substituted benzoic acids.
(2) p-Methylbenzoic Acid
The methyl group shows a +I effect, donating electron density and slightly
destabilizing the conjugate base, reducing acidity.
(1) p-Methoxybenzoic Acid
The methoxy group has a strong +M effect which donates electron density by resonance,
destabilizing the conjugate base the most. Hence, it is the least acidic.
Correct Order of Acidity
3 > 4 > 2 > 1
Final Answer
The correct order of acidity is 3 > 4 > 2 > 1.
Correct option: (D)
Conclusion
The acidity of benzoic acids is governed by substituent effects.
Strong electron-withdrawing groups like nitro increase acidity,
while electron-donating groups like methoxy decrease it.
Hence, p-nitrobenzoic acid is the strongest and p-methoxybenzoic acid is the weakest.
