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 scannin g 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-diethy l-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 exp erimental results. Thermal stability alas investigated through successive f usion cycles and a pre-fusion thermal treatment, as well as by F.t.i.r. mea surements on samples before and after melting. The possible influence of th e 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 temperatu res. In both cases, appropriate C-p,C-m = f(T) equations far 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 subst ituents (n(C)) increased along separate straight lines depending on the typ e of N-alkylthioureas (mono-, di-, tetrasubstituted).