Add time:07/18/2019         Source:sciencedirect.com

Solubility-temperature dependence data for four 5,5-disubstituted barbituric acids in 0.001 M HCl are presented. The solubility data were fitted to the Valentiner equation by the method of multiple linear regression and by the sigma plot method. Curvature m van 't Hoff (reciprocal temperature) plots for 5,5-diethyl- (1) and 5,5-diisopropyl- (2) barbituric acids indicated that the heat capacity change for the solution process has a finite value. The van 't Hoff plot for 5-ethyl-5-phenylbarbituric acid (3) was in two distinct sections. These corresponded to an anhydrous polymorph (Form II) and a monohydrate (Form XIII). Simultaneous solution of the equations for the two sections gave the transition temperature (38.2°C) for the two forms. For 5,5-dimethylbarbituric acid (4) above 15°C, the solubility-temperature dependence was very similar to (1) and (2). Below 20°C, non-linearity in the plot suggested the possibility of a change in the crystal structure for (4). Calculation of apparent thermodynamic functions for solution of (1), (2) and (4) from the equations of best fit showed that the apparent enthalpy changes for solution were linearly related to the melting points of the compounds. The apparent entropy and heat capacity changes for solution were linearly related to the size of the 5-alkyl substituents. Differences in the apparent thermodynamic functions for the two forms of (3) are expected to result solely from physical differences in the two crystalline forms. Introduction of the water molecule giving Form XIII of (3) brought about marked changes in the apparent enthalpy and entropy changes for solution of the two forms. These changes nearly cancel out so that the apparent free energies for the solution process (and hence the saturated solubilities) are nearly the same for the two forms.

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