Disappearing and reappearing polymorphs. The benzocaine:picric acid system.

The low-melting polymorphic modification of the 1:1 complex of benzocaine (BC) and picric acid (PA) had earlier been reported to be an example of a "disappearing polymorph". The BC:PA system has been reinvestigated by thermomicroscopy, calorimetry, solid-state NMR, and X-ray crystallography. The phase diagram has been derived, and robust procedures for the crystallization of the two 1:1 complexes, a hydrate of the 1:1 complex, and a 2:1 complex have been devised. The structures of all four phases have been determined and compared using graph set analysis to characterize the hydrogen-bonding patterns. It is shown that the thorough microscopic investigation of the thermal behavior, combined with calorimetric methods, can lead to the development of strategies to crystallize metastable polymorphic forms which may be difficult to obtain once their stable congeners have been obtained.