Aromatic acids can be divided into two kinds, in one kind of which the carboxyl (- COOH) directly connected to benzene ring or other aromatic ring, in the other kind of which the carboxyl is attached to the side chain of the aromatic ring. For example, benzoic acid belongs to the former and phenylacetic acid belongs to the latter. Carboxyl on aromatic ring is very similar to, but slightly more acidic than that of aliphatic acid, which can be transformed into ester, amide, anhydride and halide, etc. Aromatic acids are generally solid and difficult to dissolve in cold water.