Polymorphic Phase Transformation in the 3-Bromo-trans-cinnamic Acid System

3-Bromo-trans-cinnamic acid (3-BrCA) exists as two crystalline polymorphic forms (designated as and phases). A polymorphic phase transformation from the phase to the phase has been investigated using ex situ powder X-ray diffraction, in situ high-temperature optical microscopy, and differential scanning calorimetry. The transformation occurs at an observable rate at temperatures above about 100°C. A reverse transformation on subsequently cooling the phase is not observed. Thermodynamic aspects of the polymorphic 3-BrCA system are discussed, together with kinetic aspects of the transformation from the phase to the phase. The structural properties of the phase (reported previously) and the phase (determined in this work from single-crystal X-ray diffraction data) are in accord with the // structural classification of trans-cinnamic acid derivatives.