Question

Vinyl group directly attached with carboxylic acid should decrease acidity due to resonance, but it increases acidity. Explain.

Hint:

In –CH=CH$_2$ group, inductive effect>resonance → higher acidity.

Answer 1

A vinyl group (–CH=CH$_2$), when bonded to a carboxylic acid, exhibits conjugation with the carbonyl carbon. This conjugation is expected to delocalize electron density into the carboxylate ion via resonance, thereby destabilizing it and reducing acidity. Contrary to this expectation, the sp$2$ hybridized carbon of the vinyl group exerts an inductive electron-withdrawing effect. This inductive effect enhances the positive character of the carbonyl carbon, stabilizing the carboxylate ion by dispersing the negative charge. Consequently, the electron-withdrawing inductive effect supersedes resonance donation, leading to an increase in acidity.