Polymorphism in 2,4,6-trinitrotoluene crystallized from solution

An examination has been made of the role of solvent type in the definition of the polymorphic nature of 2,4,6-trinitrotoluene precipitated from soluti on. A combination of calorimetric and structural techniques including in si tu crystallization studies using synchrotron radiation has shown that the v ariations in polymorphic form following precipitation from solution do not arise specifically from any stereospecific guidance that the nature of the solvent might impose on the structural form. Rather the differences are lin ked to the variations in solubility and hence supersaturation which might b uild up prior to nucleation and growth and to the isolation of the metastab le orthorhombic phase from the solvent. The final conclusion is that the ch anges fit well with Ostwald's Law of Stages with the orthorhombic form alwa ys precipitating initially followed by its conversion to the stable monocli nic form. The previously observed tendency for some solvents to yield one o r the other form then becomes attributable to kinetics in solution rather t han structural control. It can be associated with the solubility of the mat erial in the solvent used and the role of this factor in a solvent-mediated phase transformation. On this basis rules can be formulated for the isolat ion of the metastable forms of this and similarly related polymorphic syste ms.