Standard molar enthalpy of sublimation of crystalline 3-pyridinecarboxylic acid

✍ Scribed by Maria D.M.C. Ribeiro da Silva; ; Jorge M. Gonçalves; ; W.E. Acree; Jr.

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 49 KB
• Volume: 32
• Category: Article
• ISSN: 0021-9614
• DOI: 10.1006/jcht.2000.0675

✦ Synopsis

The effect of substitution of various groups into the pyridinic ring of pyridine N-oxides on the dissociation enthalpy of the N-O covalent bond D o m (N-O) has been investigated in several compounds, (1,2) including the three pyridinecarboxylic acid N-oxide isomers, 2-, 3-, and 4-CO 2 HPyNO. In order to do this, the enthalpies of combustion of the three pyridinecarboxylic acid N-oxide isomers, (1,2) of the 2-and 4-pyridinecarboxylic acids, 2-and 4-CO 2 HPy, (2) and of 3-pyridinecarboxylic acid, 3-CO 2 HPy, (3) were measured by static bomb calorimetry. The Calvet microcalorimetric technique was used for measuring the enthalpies of sublimation of all these compounds, (1,2,4) except for 4-pyridinecarboxylic acid, for which a Knudsen effusion method with a quartz balance was used. (2) A thermodynamic study of the three pyridinecarboxylic acid isomers has been recently published by Sabbah et al., (5) and it includes new measurements of the enthalpies of combustion and sublimation obtained, respectively, by rotating microbomb calorimetry and a calorimetric method using a Knudsen cell. (5) The differences between the values obtained for the enthalpies of formation in the gaseous state of the 2-and 4-pyridinecarboxylic acids derived by Sabbah et al. (5) from his experimental results and those previously obtained by Ribeiro da Silva et al. (2) are not significant. However, there is a big discrepancy between the corresponding values for 3-pyridinecarboxylic acid obtained by Sabbah et al., (5) f H o m (g) = -(239.3 ± 1.0) kJ • mol -1 , and that calculated by Pilcher et al., (4)