Thermal analysis of novel biphenylamide derivatives

Abstract

The physicochemical properties of a small library of 4-methyl-biphenylamide derivatives have been investigated by means of differential scanning calorimetry, thermogravimetric analysis and hot-stage microscopy. The obtained results show that positional isomerism has a significant influence on the thermal behaviour of the 4-methyl-biphenylamide derivatives; however, no clear relationship between functional group isomerism and thermal properties could be established. Ortho-substituted derivatives revealed two polymorphic forms, whilst the para-substituted derivatives exhibit three polymorphic forms. The ortho-substituted biphenylamides were more likely to generate metastable forms when cooled from the melt. Furthermore, the self-heating properties were revealed by the para-substituted 4-methyl-biphenylamide derivatives, in which the highly energetic crystallization processes raised the sample temperature by as much as 4 °C during cooling. Such a high-energy exothermic crystallization process suggests crystallization to be highly favourable, from a thermodynamic standpoint. Hence, the para-substituted derivatives are unlikely to generate amorphous forms. Pharmaceutical application of these compounds will depend on the solubility of their crystalline forms, but their manufacture may possess some challenges due to the number of monotropic polymorphic forms that may coexist.