Calorimetric determination of the standard molar enthalpies of formation of o-vanillin and trimethoprim

Abstract

o-Vanillin and trimethoprim are two main raw materials for the synthesis of Valen Schiff bases which have been proved to possess established biological activities and may be used as the antibacterial agent candidates. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of a substance are some of the most fundamental thermodynamic properties, and closely related to other physical, biological, physiological, and chemical properties. However, up to now, no reports about those thermodynamic properties of the title compounds were found in literatures. In this paper, the constant-volume energies of combustion of o-vanillin and trimethoprim at T = 298.15 K and p = 3.01 MPa were measured by a precision oxygen-bomb combustion calorimeter to be ΔcU [o-vanillin(s), 298.15 K] = −(24,971.89 ± 22.02) J g−1 and ΔcU [trimethoprim(s), 298.15 K] = −(26,366.77 ± 21.96) J g−1, respectively. According to the definition of combustion enthalpy, the standard (pΘ = 0.1 MPa) molar enthalpies of combustion of o-vanillin and trimethoprim were determined to be \( \Delta_{\text{c}} H_{\text{m}}^{\varTheta } \) [o-vanillin(s), 298.15 K] = −(3,800.55 ± 3.35) kJ mol−1 and \( \Delta_{\text{c}} H_{\text{m}}^{\varTheta } \) [trimethoprim(s), 298.15 K] = −(7,657.05 ± 6.38) kJ mol−1, respectively. Finally, the standard (pΘ = 0.1 MPa) molar enthalpies of formation of o-vanillin and trimethoprim were calculated to be \( \Delta_{\text{f}} H_{\text{m}}^{\varTheta } \) [o-vanillin(s), 298.15 K] = −(490.85 ± 3.51) kJ mol−1 and \( \Delta_{\text{f}} H_{\text{m}}^{\varTheta } \) [trimethoprim(s), 298.15 K] = −(424.56 ± 6.64) kJ mol−1 from a combination of the experimental values of enthalpies of combustion and some other auxiliary thermodynamic data through a designed thermochemical cycle based on a supposed chemical reaction.