Vinyl and phenyl groups

sp² carbons are more electronegative than sp³ ones: their greater s character makes the electrons more strongly attracted by the nucleus. Hence a neighbouring vinyl or phenyl group increases the acidity (pK_a = 4.31–4.35), because it slightly stabilizes the negative charge of the carboxylate, compared with the corresponding alkyl groups (pK_a = 4.74–5.05).

When the phenyl ring is bonded directly to the carboxylate group (benzoate), the stabilization can be more effective and the acidity is even higher (pK_a = 4.19).