Enthalpies and entropies of fusion and of sublimation at the temperature 298.15 K of thiourea and seven N-alkylthioureas

Enthalpies and temperatures of fusion were measured by differential scanning calorimetry (d.s.c.) for thiourea and seven of its N-alkyl derivatives: 1-methyl-2-thiourea, 1-ethyl-2- thiourea, 1,3-dimethyl-2-thiourea, 1,3-diethyl-2-thiourea, 1,3-di-n-propyl-2-thiourea, 1,3- di-n-butyl-2-thiourea, and 1,1,3,3-tetramethyl-2-thiourea. Entropies of fusion were derived from the experimental results. Thermal stability was investigated through successive fusion cycles and a pre-fusion thermal treatment, as well as by F.t.i.r. measurements on samples before and after melting. The possible influence of the container was assessed by conducting the fusion measurements in aluminium, stainless steel, and graphite crucibles. The enthalpies of sublimation at T = 298.15 K were obtained by combining the fusion enthalpies with either vaporization data or directly from those of sublimation at higher temperatures. In both cases, appropriate Cpm = f(T) equations for solid, liquid, and gaseous compounds were used to convert experimental data to T = 298.15 K. The enthalpies and entropies of fusion, and enthalpies and entropies of sublimation as a function of the number of carbon atoms in the alkyl substituents nC increased along separate straight lines depending on the type of N-alkylthioureas (mono-, di-, tetrasubstituted). Comparison was made with the corresponding data for urea and N-alkylureas, which are lower and quite constant as a function of nC.