Acid Strength Carboxylic Substituents

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p-Nitrobenzoic acid vs benzoic acid acidity order:

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1. **Acidity Principle**: Acidity increases with anything that stabilizes the conjugate base (carboxylate anion).
2. **Electronic Effects of $-NO_2$**: The nitro group is one of the most powerful electron-withdrawing groups.
3. **Mechanisms**: It exerts a strong **Inductive effect ($-I$)** and a strong **Resonance effect ($-R$ or $-M$)** from the para position.
4. **Stabilization**: These effects pull electron density away from the carboxylate group, effectively dispersing the negative charge and making the anion more stable.
5. **Comparison**: Consequently, **p-nitrobenzoic acid** is significantly more acidic (lower $pK_a$) than unsubstituted benzoic acid.

Acid Strength Carboxylic Substituents

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