A new polymorph of 2,6-dimethoxybenzoic acid

A new crystalline form of 2,6-dimethoxybenzoic acid, C(9)H(10)O(4), 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)degrees. 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) angstrom], 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 center dot center dot center dot O hydrogen bonds. However, in the new tetragonal polymorph, molecules form dimeric units via pairs of O-H center dot center dot center dot O hydrogen bonds between the carboxy groups.