Characterization of two polymorphic forms of tolfenamic acid, N-(2-methyl-3-chlorophenyl)anthranilic acid: their crystal structures and relative stabilities

Abstract

The crystal structures of two polymorphic forms of tolfenamic acid have been determined by X-ray diffraction methods. The two forms contain the molecule as different conformers. In the yellow form the planes of the two benzene rings form an angle of 46°, in the white one the corresponding angle is 73°. This difference is also reflected in the geometry of the amino group, where the yellow form exhibits a larger degree of conjugation. An analysis of the crystal structures and their physical properties suggests that the yellow form is the stable low temperature modification.