Physicochemical characterization of the orthorhombic polymorph of paracetamol crystallized from solution.

This paper describes a method for the laboratory-scale crystallization of the orthorhombic polymorph (form II) of paracetamol (acetaminophen) from solution. Its structure has been determined by single-crystal X-ray crystallography at 298 K (to confirm the results of data published in 1974) and at 123 K (to improve the overall accuracy of the structure determination). Despite considerable effort by many investigators, the crystallization of form II from solution, using the method given in the 1974 structure report, has been elusive. The incentive for this effort is that form II, unlike commercial paracetamol (form I), undergoes plastic deformation and is suitable for direct compression. Consequently, the ability to produce form II in quantity has attracted much interest because of the potential commercial benefits to be gained by not using binders during the manufacture of tablets. However, until now, the only method that has been reported for the bulk preparation of form II has been to grow it as polycrystalline material from fused form I. This study also compares the solid-state properties of form II with those of form I, with particular emphasis on the crystallography (both X-ray and optical), crystal morphology, thermal behavior, and compaction properties.