How does the direct attachment of groups like phenyl or vinyl to carboxylic acids impact their acidity, and why does this deviate from the expected resonance effect?

Direct attachment of phenyl or vinyl groups to carboxylic acids typically decreases acidity, contrary to the expected resonance effect. This deviation arises because the conjugate base’s resonance stabilization is compromised due to the lack of effective electron delocalization within the aromatic or conjugated system. The electron-dense aromatic or conjugated ring tends to withdraw electron density from the carboxylate ion, diminishing resonance effects. Consequently, the direct attachment of phenyl or vinyl groups disrupts the anticipated enhancement of acidity through resonance, resulting in a less acidic carboxylic acid compared to its simpler counterparts.