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$_2$) attached to a carboxylic acid has conjugation with the carbonyl carbon.
The expected effect is that resonance should delocalize electron density into the carboxylate ion and destabilize it, lowering acidity.
However, the actual effect is opposite: the sp$2$ carbon of the vinyl group withdraws electron density inductively.
This increases the positive character of the carbonyl carbon and stabilizes the carboxylate ion by dispersing negative charge.
So, the electron withdrawing effect dominates over resonance donation — acidity increases.