Heat capacities and standard molar enthalpy of formation of pyrimethanil maleic salt and pyrimethanil fumaric salt

Abstract

Novel anilino-pyrimidine fungicides, pyrimethanil maleic salt, and pyrimethanil fumaric salt (C28H30N6O4) were synthesized by a chemical reaction of pyrimethanil with maleic acid/fumaric acid. The low-temperature heat capacities of the two compounds were measured with an adiabatic calorimeter from 80 to 350 K. The heat capacities of pyrimethanil fumaric salt are bigger than that of pyrimethanil maleic salt in the measurement temperature range. The thermodynamic function data relative to 298.15 K were calculated based on the heat capacity-fitted curves. The melting points, the molar enthalpies (ΔfusHm), and entropies (ΔfusSm) of fusion of pyrimethanil maleic salt and pyrimethanil fumaric salt were determined from their DSC curves. The values indicate that pyrimethanil fumaric salt was more thermostable than pyrimethanil maleic salt. The constant-volume energies of combustion (ΔcUmo) of pyrimethanil maleic salt and pyrimethanil fumaric salt were measured using an isoperibol oxygen bomb combustion calorimeter at T = (298.15 ± 0.001) K. From the Hess thermochemical cycle, the standard molar enthalpies of formation of the two compounds were derived and determined to be ΔfHmo (pyrimethanil maleic salt) = −459.3 ± 4.9 kJ mol−1 and ΔfHmo (pyrimethanil fumaric salt) = −557.2 ± 4.8 kJ mol−1, respectively. The results suggest that pyrimethanil fumaric salt is more chemically stable than pyrimethanil maleic salt.