Solid state structures of p-cresol revisited

Abstract

The two previously reported solid state forms of p-cresol are revisited and detailed structural analysis, thermal analysis, lattice energy calculations and variable temperature powder X-ray diffraction are presented. A possible mechanism for the transformation from form II to form I is proposed. Polymorphism, the ability of a compound to crystallise in different forms, is an interesting phenomenon in solid state chemistry.1 In the pharmaceutical industry, polymorphism is a key issue since the different forms of a drug can have different physicochemical properties such as melting point, solubility and bioavailability.2 Similarly, different forms of pigments have profound impact on the paint and coatings industries. In some cases a mechanism for the transformation of one form to another can be deduced from a consideration of the crystal structures of each. p-Cresol IJ4-methylphenol) is extracted from coal tar and is used in chemical synthesis of antioxidants, anisaldehyde, pharmaceuticals and dyes.3 Apart from its uses in the manufacturing industry, p-cresol has also been widely used as a crystallisation solvent. Two solid state structures of p-cresol were reported several years ago on separate occasions. Form I crystallises from a chloroform solution in the monoclinic space group P21/n (refcode CRESOL014 and CRESOL105 ), while the metastable form II crystallises from an acetone solution in the monoclinic space group C2/c (CRESOL026 ). Both forms are low-melting solids, with melting points of approximately 35 and 36 °C respectively.