Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid because

Direct attachment of group such as phenyl or vinyl to the carboxylic acid, increase the acidity of corresponding carboxylic acid, contrary to the decrease expected due to resonance effect shown below This is because of greater electronegativity of sp2 hybridised carbon to which carboxyl carbon is attached. Preparation of Carboxylic Acid Derivatives