Gas phase enthalpies of formation of nitrobenzamides using combustion calorimetry and thermal analysis

• Formation enthalpies of the nitrobenzamides were derived from combustion calorimetry.
• Enthalpies of vaporisation and sublimation were calculated by thermogravimetry.
• From gas phase enthalpies of formation the stability of the isomers is studied.
• Stability of isomers is not driven by a steric hindrance between functional groups.

The standard molar energies of combustion of 2-nitrobenzamide, 3-nitrobenzamide and 4-nitrobenzamide were determined with an isoperibolic, static-bomb, combustion calorimeter. From the combustion results, the standard molar enthalpies of combustion and formation for these compounds in the condensed phase at T = 298.15 K were derived. Subsequently, to determine the enthalpies of sublimation, the vapour pressure data as a function of the temperature for the compounds under investigation were estimated using thermogravimetry by applying Langmuir’s equation, and the enthalpies of vaporisation were derived. Standard enthalpies of fusion were measured by differential scanning calorimetry then added to those of vaporisation to obtain reliable results for the enthalpy of sublimation. From the combustion and sublimation data, the gas phase enthalpies of formation were determined to be (−138.9 ± 3.5) kJ · mol−1, (−122.9 ± 2.9) kJ · mol−1 and (−108.5 ± 3.7) kJ · mol−1 for the ortho, meta and para isomers of nitrobenzamide, respectively. The meaning of these results with regard to the enthalpic stability of these molecular structures is discussed herein.

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