(above) phthalic acid. This image is 500 x 500 pixels; the original image is 4096 x 4096 pixels.
phthalic acid consists of a BENZENE ring with two carboxyl groups attached to adjacent carbon atoms (ortho position), i.e. it is 1,2 benzenedicarboxylic acid. The related isomers are isophthalic acid with the carboxyl groups attached to carbons 1 and 3 (meta position) and terephthalic acid in which the two carboxyl groups are at opposite ends of the benzene ring (para position). Phthalic acid can be heated to yield phthalic anhydride.