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

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, P21/n),
Form II (orthorhombic, Pbca), Form III (monoclinic, P21/c), and Form IV (monoclinic, P21/c). Interestingly, a hydrate
(triclinic, P1̅) 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.