POLYMORPHISM IN ANTHRANILIC ACID - A REEXAMINATION OF THE PHASE-TRANSITIONS

The three known polymorphs of anthranilic acid (2-aminobenzoic acid) have been examined by X-ray diffraction photography and infrared spectroscopy. The primary product of the phase transition occurring in heated crystals of polymorph I appears to be polymorph III, not polymorph II as commonly reported. The polycrystalline product III phase reverts to I on grinding. The III --> I transition is also observed when an authentic sample of III is subjected to vigorous grinding. The occurrence of a II --> I solid-state transition, the subject of disagreement in the literature, has been observed in samples of II subjected to vigorous grinding and sometimes spontaneously in single crystals of II. The product phase in the single-crystal II transition can be identified as I by its discrete X-ray reflections. The particular conditions required for initiating the II --> I transition in intact single crystals of II have not yet been determined.