The three benzenedicarboxylic acid isomers

Benzenedicarboxylic acid is the systematic name for a benzene ring carrying two carboxylic acid (–COOH) groups, with molecular formula C₆H₄(COOH)₂ or C₈H₆O₄ and molecular weight of 166.13 g/mol.

All three phthalic acid isomers are produced industrially by catalytic liquid-phase air oxidation of the corresponding xylene isomer, using a cobalt/manganese/bromide catalyst system dissolved in acetic acid solvent at elevated temperature (roughly 150–200°C) and pressure (~20–30 bar). Terephthalic acid comes from oxidizing para-xylene, isophthalic acid from meta-xylene, and phthalic acid from ortho-xylene, with each methyl group on the ring being oxidized stepwise to a carboxylic acid group.

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