Thermodynamic study of the three isomers of iodobenzoic acid

Abstract

2-Iodo-, 3-iodo- and 4-iodobenzoic acids have been investigated by sublimation calorimetry, drop calorimetry and thermal analysis. The measurements of sublimation enthalpy of these three compounds were carried out at 363 K using a Tian-Calvet microcalorimeter equipped with a Knudsen effusion cell. The saturated vapour pressures of these acids at the sublimation temperature 363 K were also calculated. The derived values of standard molar enthalpies of sublimation at 298.15 K are 92.59 ± 0.20, 96.38 ± 0.29, and 99.32 ± 0.36 kJ mol −1 for 2-iodo-, 3-iodo- and 4-iodobenzoic acid, respectively. The enthalpies of fusion, the temperatures of triple point and the mole fraction purities of these three compounds were determined using the mean temperature version DTA apparatus developed by the Centre de Thermodynamique et de Microcalorimétrie of the CNRS in Marseille. The triple point temperatures and the molar enthalpies of fusion for the 2-iodo-, 3-iodo- and 4-iodobenzoic acid have been found to be 435.02 ± 0.02 K, 21.38 ± 0.19 kJ mol −1; 460.30 ± 0.01 K, 28.70 ± 0.21 kJ mol −1 and 543.70 ± 0.03 K, 35.24 ± 0.13 kJ mol −1, respectively. Using the contributions of benzoic acid and iodine group, determined from literature data, we have calculated the enthalpy of sublimation of 4-iodobenzoic acid and compared the calculated result with the experimental value. It has also been possible to demonstrate and explain the existence of an intramolecular hydrogen bond in 2-iodobenzoic acid molecule.