A new polymorph of 2,6-dimethoxybenzoic acid

A new crystalline form of 2,6-dimethoxybenzoic acid, C9H10O4, crystallizing in a tetragonal unit cell has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent molecule with a synplanar conformation of the carboxy group. The sterically bulky o-methoxy substituents force the carboxy group to be twisted away from the plane of the benzene ring by 65.72 (15)°. The carboxy group is disordered over two sites about the C—C bond [as indicated by the almost equal C—O distances of 1.254 (3) and 1.250 (3) Å], the occupancies of the disordered carboxym H atoms being 0.53 (5) and 0.47 (5). In the known orthorhombic form reported by Swaminathan et al. [Acta Cryst. (1976), B32, 1897–1900], due to the antiplanar conformation adopted by the OH group, the molecular components are associated in the crystal in chains stabilized by linear O—H...O hydrogen bonds. However, in the new tetragonal polymorph, molecules form dimeric units via pairs of O—H...O hydrogen bonds between the carboxy groups.