Experimental and computational thermochemical study of 1,3,5-trimethyl-, 1,5,5-trimethyl-, and 1,3,5,5-tetramethyl-barbituric acids
Graphical abstract
Introduction
Over the past years, we have been involved in the study of the thermochemistry of barbituric acid or (2,4,6(1H,3H,5H)-pyrimidinetrione) and its derivatives, with the aim to develop the understanding of the structural effects on their thermodynamic stabilities that are reflected in the gas-phase enthalpies of formation. On the other hand, reliable values of the enthalpies of formation in the gas phase for the family of barbiturates can be used for the estimation of the thermochemical properties that are not easy to study experimentally for related compounds having similar groups, and also to contribute to the study of the influence of steric, electrostatic, and stereoelectronic interactions produced by substituents on the thermochemical stability of these molecules. We have recently published thermochemical studies of the parent compound barbituric acid [1] and its 5,5-dimethyl [2], 1,3-dimethyl [3] and 5,5-diethyl (barbital) [4], [5] derivatives, and of a sulfur-containing barbituric acid, 2-thiobarbituric acid [6]. We have also reported thermophysical studies of some methyl and ethyl derivatives of the barbituric acid [7], and the 2-thiobarbituric acid [8]. The aim of the present work is to study the energy-structure relationship of three new methyl derivatives: 1,3,5-trimethylbarbituric acid or (1,3,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione), 1,5,5-trimethylbarbituric acid or (1,5,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione), and 1,3,5,5-tetramethylbarbituric acidor (1,3,5,5-tetramethyl-2,4,6(1H,3H,5H)pyrimidinetrione), whose structures are presented in figure 1.
Section snippets
Materials and purity control
The preparation and purification of the three barbituric acid derivatives were described in details in our previous publication [7]. For the present work, 1,3,5-trimethylbarbituric acid was in addition sublimated at 80 °C/0.1 Torr giving a fusion temperature of (90.0 to 90.9) °C.
Combustion calorimetry
An isoperibol bomb calorimeter was used for the measurement of energy of combustion of the barbituric acid derivates. The detailed procedure has been described previously [11]. We used small polyethylene pieces as the
Experimental results
Results for typical combustion experiments on 1,3,5-trimethylbarbituric acid, 1,5,5-trimethylbarbituric acid, and 1,3,5,5-tetramethylbarbituric acid, are given in TABLE 3, TABLE 4, TABLE 5, respectively, and correspond to the reactions (4) for the trimethylated derivatives, and (5) for the tetramethylated reaction:
Experimental results from the vapour pressures measurements on three barbituric acid
Acknowledgment
The support of the Spanish Ministerio de Economía y Competitividad under Project CTQ2010-16402 is gratefully acknowledged.
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