Question

Why is the acidity order different in (a) and (b)?

Answer 1

In aromatic acid, electron withdrawing groups increase the acidity whereas electron releasing groups decrease the acidity.

Explanation:

Any substituent that makes the anion less stable should decrease acidity. Electron releasing group (-CH3, -OH, -OCH3, -NH2) decreases the acidity of carboxylic acid by intensifying the negative charge(by inductive effect) and destabilizing the carboxylate anion

On the other hand, Electron-withdrawing substituents (-Cl, -CN, -NO2) increase the acidity of carboxylic acid by dispersing the negative charge by inductive effect and stabilizing the carboxylate anion.

In aromatic acid too, electron withdrawing groups increase the acidity whereas electron releasing groups decrease the acidity.

• If they are attached to meta position, they show an electron-withdrawing acid-strengthening inductive effect.
• If they are attached to para position, they show an electron releasing acid-weakening resonance effect.