Solubilities of p-nitroanilines in various classes of solvents. Specific solute-solvent interactions

The solubilities phi(B) in volume fractions of p-nitroaniline (PNA) and N,N-dimethyl p-nitroaniline (N,N-PNA) were determined near 25 degrees C in some 40 solvents belonging to different classes using a thermoturbidimetric method. The ratio between the solubilities of the two compounds can be corrected for the differences in the melting characteristics determined from DSC measurements. The solubilities at 25 degrees C of N,N-PNA have then to be multiplied by the factor 2.72. The corrected ratios phi PNA/phi*N,N-PNA give indications on the formation of H-bonds between the solvent molecules and the NH groups of PNA. This is observed in solvent classes with electron donor oxygens like sulfoxides, ketones, esters and cyclic ethers. However, the ratio phi PNA/phi*N,N-PNA decreases in an homologous series when the molar volume of the solvent increases, indicating that the formation of H-bonds by PNA depends also on the concentration of the active sites in the solvent and that not all the PNA molecules are involved in H-bonding. phi PNA/phi*N,N-PNA is also markedly larger than 2 in the lower alcohols demonstrating the formation of N-H-O hydrogen bonds at the end of the self-association chains. The fraction gamma of the PNA molecules escaping from H-bonding in the electron donor solvents and in alcohols is quantitatively estimated from the solubility using the equations of the theory of the mobile order and disorder of Huyskens and Ruelle. In nitriles a weak formation of N-H-H=C bonds is observed. In contrast H-bonds are not observed in nitroalkanes; Unexpectedly the ratio's phi PNA/phi*N,N-PNA are much smaller than 1 in chloro alkanes. The formation of EDA bonds between N,N-PNA and these solvents acting as Lewis acids is therefore probable. (C) 1998 Elsevier Science B.V.