Recently I came across this statement in my textbook:
Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid contrary to the decrease expected due to resonance effect.
On researching online, I found this statement as an answer:
Although we expect a decrease in acidity due to the resonance effect of phenyl and vinyl groups, but because of the greater electronegativity of the the $\ce{sp^2}$ hybridized carbon which is directly attached to the carboxylic carbon.
My Thoughts
Please keep in mind that the carbonyl carbon(here carboxylic carbon) is also $\ce{sp^2}$ hybridized. So $\ce{sp^2}$ hybridized carbon is directly attached to the carboxylic carbon. Therefore there is no electronegativity difference between the two carbons and as a result of this only +R effect should work and direct attachment of groups such as phenyl or vinyl to the carboxylic acid should not increase the acidity of corresponding carboxylic acid rather they should decrease its acidity due destabilization (due to electron donating nature vinyl and phenyl group by means of resonance) of conjugate base so formed.
The question
Kindly explain it theoretically by giving plausible reasons as to why the statement in my textbook is true?
Note: avoid quoting pKa values