Crystal structure and Hirshfeld surface analysis of a third polymorph of 2,6-di-meth-oxy-benzoic acid.

A third crystalline form of the title compound, CHO, crystallizing in the centrosymmetric monoclinic space group 2/, has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent mol-ecule with a synplanar conformation of the OH group. The sterically bulky -meth-oxy substituents force the carb-oxy group to be twisted away from the plane of the benzene ring by 74.10 (6)°. The carb-oxy group exhibits the acidic H atom disordered over two sites between two O atoms. A similar situation has been found for the second tetra-gonal polymorph reported [Portalone (2011 ▸). . E, o3394-o3395], in which mol-ecules with the OH group in a synplanar conformation form dimeric units strong O-H⋯O hydrogen bonds. In contrast, in the first ortho-rhom-bic form reported [Swaminathan (1976 ▸). . B, 1897-1900; Bryan & White (1982 ▸). . B, 1014-1016; Portalone (2009 ▸). . E, o327-o328], the mol-ecular components do not form conventional dimeric units, as an anti-planar conformation adopted by the OH group favors the association of mol-ecules in chains stabilized by linear O-H⋯O hydrogen bonds.