Enthalpies of formation of methyl benzenecarboxylates

The relative stabilities of the isomeric dimethyl benzenedicarboxylates have been studied by a thermochemical investigation of dimethyl phthalate, dimethyl isophthalate and dimethyl terephthalate. The enthalpies of formation in the condensed state for dimethyl phthalate and dimethyl isophthalate, determined from combustion calorimetry measurements, are -683.8±2.7 and -730.1±2.0 kJ mol-1, respectively. The enthalpies of sublimation for dimethyl isophthalate and dimethyl terephthalate, determined by the Knudsen effusion technique are 100.9±0.2 and 104.6±0.3 kJ mol-1, respectively. From the above results and the value of the enthalpy of formation in the condensed state of dimethyl terephthalate, previously determined in our laboratory, and the value of the enthalpy of vaporization of dimethyl phthalate taken from the literature, the enthalpies of formation in the gaseous state at 298.15 K for the ortho-, meta- and para-isomers of dimethyl benzenedicarboxylates are found to be -606.1, -629.2±2.0 and -628.0±1.0 kJ mol-1, respectively. The relative stabilities of the isomeric dimethyl benzenedicarboxylates, isomeric dicyanobenzenes and dinitrobenzenes are compared.