Experimental and Topological Determination of the Pressure-Temperature Phase Diagram of Racemic Etifoxine, a Pharmaceutical Ingredient with Anxiolytic Properties

Information about the solid-state properties of etifoxine has been lacking, even if the active pharmaceutical ingredient has been used for its anxiolytic properties for decennia. The crystal structure of the racemic compound possesses a monoclinic space group P21/n with cell parameters a = 8.489(2) Å, b = 17.674(2) Å, c = 20.883(3) Å, b = 98.860(10)° and a unit-cell volume of 3095.8(9) Å3 at 293 K. The unit cell contains 8 molecules, while 2 independent molecules with different conformations are present in the asymmetric unit. The density of the crystal is 1.291 g/cm3 and its melting point was found at 362.6 ±0.3 K with a melting enthalpy of 85.6 ±3.0 J g-1. Its thermal expansion in the liquid and the solid state and the change in volume on melting and between the vitreous state and the crystalline solid have been studied. The results confirm the tendency of small organic molecules to increase about 11% in volume on melting, while the volume difference between the glass and the crystal at the glass transition temperature is about half this value at 6%. These values can be used in the construction of phase diagrams in the case that the experimental data for a given system is incomplete.