Polymorphism and Phase Transformation Behavior of Solid Forms of 4‑Amino-3,5-dinitrobenzamide

Abstract

We report the preparation, analysis, and phase transformation behavior of polymorphs and the hydrate of 4-amino-3,5-dinitrobenzamide. The compound crystallizes in four different polymorphic forms, Form I (monoclinic, <i>P</i>2₁/<i>n</i>), Form II (orthorhombic, <i>Pbca</i>), Form III (monoclinic, <i>P</i>2₁/<i>c</i>), and Form IV (monoclinic, <i>P</i>2₁/<i>c</i>). Interestingly, a hydrate (triclinic, <i>P</i>1̅) of the compound is also discovered during the systematic identification of the polymorphs. Analysis of the polymorphs has been investigated using hot stage microscopy, differential scanning calorimetry, in situ variable-temperature powder X-ray diffraction, and single-crystal X-ray diffraction. On heating, all of the solid forms convert into Form I irreversibly, and on further heating, melting is observed. In situ single-crystal X-ray diffraction studies revealed that Form II transforms to Form I above 175 °C via single-crystal-to-single-crystal transformation. The hydrate, on heating, undergoes a double phase transition, first to Form III upon losing water in a single-crystal-to-single-crystal fashion and then to a more stable polymorph Form I on further heating. Thermal analysis leads to the conclusion that Form II appears to be the most stable phase at ambient conditions, whereas Form I is more stable at higher temperature