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Lviv Polytechnic National University
Ivan Franko National University of Lviv
Lviv Polytechnic National University
Lviv Polytechnic National University
Frantsevich Institute for Problems of Materials Science

The purpose of this work is to investigate the thermodynamic properties of the solubility of methyl 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydro-pyrimidine-5-carboxylate, which exhibits a wide spectrum of biological activity in organic solvents of different polarity and their character of mixing. As you know, the processes of synthesis and purification of solid substances, allmost, occur with the help of individual solvents or their mixture, so optimization of these processes is impossible without determining the thermodynamic parameters of their dissolution.

Enthalpy (ΔsolH)  and entropy (ΔsolS) of dissolution were determined from the temperature dependence of the solubility of methyl ester of 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahidropirymidyn-5-carboxylic acid in: ethyl acetate (ΔsolH = 24,05±0,53 kJ/mol, ΔsolS = 27,0±1,7 J/mol K); benzene (ΔsolH = 32,76±0,42  kJ/mol, ΔsolS = 40,3±1,3 J/mol K) and their mixtures in the ratio of ethyl acetate: benzene in % by weight: 75:25 (ΔsolH) = 23.30 ± 0.62 kJ / mol, (ΔsolS) = 23.8 ± 2.0 J / mol • K; 50:50 (ΔsolH) = 26.85 ± 0.44 kJ / mol; (ΔsolS) = 31.9 ± 1.4 J / mol • K; 25:75 (ΔsolH) = 28.58 ± 0.81 kJ / mol; (ΔsolS) = 34.2 ± 2.7 J / mol • K.

Enthalpies and entropies of mixing of investigated compound were determined using enthalpies and entropies of fusion of methyl 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydro-pyrimidine-5-carboxylate at an average temperature of experiments to determine their solubility: ΔfusН487,7 = 32,9 ± 1,7 kJ / mol; ΔfusS487.7 = 67.5 ± 2.0 J / mol. Quantitys of the enthalpy and entropy of the melting of the test substance were calculated at 298K: ΔfusH298 = 23.4 ± 1.8 kJ / mol, ΔfusS298 = 32.9 ± 2.1 J / mol. After that, thermodynamic mixing parameters for the studied systems were calculated: ethyl acetate (ΔmixH) = 0.7 ± 2.0 kJ / mol, (ΔmixS) = -15.1 ± 4.3 J / mol • K; benzene (ΔmixH) = 9.4 ± 1.9 kJ / mol, (ΔmixS) = -2.6 ± 2.5 J / mol • K; 75:25 (ΔmixH) = -0,1 ± 1,9 kJ / mol, (ΔmixS) = -19,1 ± 2,9 J / mol • K; 50:50 (ΔmixH) = 3.4 ± 1.9 kJ / mol, (ΔmixS) = -11.0 ± 2.5 J / mol • K; 25:75 (ΔmixH) = 5.2 ± 2.0 kJ / mol, (ΔmixS) = -8.7 ± 3.4 J / mol • K.

Positive or close to 0 thermodynamic parameters of solubility of the compound under study at 298K indicate that the destruction of intermolecular bonds in individual substances requires higher energy expenditure than is allocated as a result of the formation of new intermolecular bonds in the systems under study.

As a result of the studies, thermodynamic properties of solubility for methyl 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate in organic solvents of different polarity and their mixtures were determined. The experimental and calculated data can be used to optimize the purification and separation processes, to predict the reactive behavior of the substance in the solution. It was also possible to establish the presence of the compensatory effect of the process of mixing the investigated esters with ethyl acetate, benzene and their mixtures.

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