Thermodynamic properties of paracetamol impurities 4-nitrophenol and 4′-chloroacetanilide and the impact of such impurities on the crystallisation of paracetamol from solution

The impact of structurally-related additives and impurities on active pharmaceutical ingredients is an essential yet poorly understood area. This work describes the characterisation of temperature-dependent solid-liquid properties of 4-nitrophenol and 4′chloroacetanilide in four different alcohols and their effect as impurities on the crystallisation of paracetamol. The solubility of 4-nitrophenol appeared to be significantly higher than paracetamol whereas the solubility of 4′chloroacetanilide was lower than paracetamol. The solubility difference between the impurities could be rationalised based on their molecular structure and hydrogen bonding interactions. The solubility data was modelled using empirical and thermodynamic models. Recrystallisation of paracetamol from solutions containing the highly soluble 4-nitrophenol impurity resulted in small uniformly sized high purity paracetamol crystals whereas the presence of the poorly soluble 4′chloroacetanilide impurity induced the formation of large needle shaped crystals of paracetamol. These differences in crystallisation are a consequence of the solubility difference and the different functional groups of paracetamol and its impurities. Overall this study serves as fundamental information for the development of crystallisation approaches for the purification of paracetamol from its main impurities.