Enthalpies of combustion of 4-nitropyridine N-oxide and pyridine-3-carboxylic acid N-oxide: the dissociation enthalpies of the N—O bonds in pyridine N-oxide derivatives

Abstract

The standard ( p° = 0.1 MPa) enthalpies of combustion at the temperature T= 298.15 K were measured by static-bomb calorimetry for crystalline 4-nitropyridine N-oxide and for pyridine-3-carboxylic acid N-oxide. The standard enthalpies of sublimation at T= 298.15 K were measured for 4-nitropyridine N-oxide by microcalorimetry and for pyridine-3-carboxylic acid N-oxide by a mass-loss effusion technique. From the standard molar enthalpies of formation of the gaseous compounds, the molar dissociation enthalpies of the (N—O) bonds were derived: D(N—O)/kJ·mol −1) in 4-O 2N- c-C 5H 4NO, 258.5 ± 5.3; in 3-HO 2C- c-C 5H 4NO, 252.8 ± 2.7. The standard molar enthalpies of sublimation of 2-H 3C- c-C 5H 4NO and 3-H 3C- c-C 5H 4NO were measured by microcalorimetry: (78.2 ± 2.2) kJ·mol −1and (82.2 ± 2.4) kJ·mol −, respectively, permitting the comparison of D(N—O) values in several pyridine N-oxide derivatives.