Hydrogen atom disorder in the crystal structure of o-phenylenediacetic acid

H2opda (1) (o-phenylenediacetic acid) was prepared in the form of single crystals. It exhibits molecular crystal structure. The main feature of the supramolecular structure of 1 is formation of hydrogen bonded dimers via a pair of O-H×××O hydrogen bonds. The hydrogen atoms in COOH groups are disordered in 0.67(2):0.33(2) ratio. The conformation of the free acid is characterized by cis arrangement of the carboxylate groups with respect to the plane of the aromatic ring. The powder diffraction pattern of the bulk sample corroborated the phase identity of the commercial material with that of the studied single crystal.