THERMODYNAMIC STUDY OF THE 3 ISOMERS OF BROMOBENZOIC ACID

The enthalpies of sublimation and fusion and triple-point temperatures of 2-bromo-, 3-bromo- and 4-bromobenzoic acids have been determined p recisely by sublimation calorimetry, drop calorimetry and differential thermal analysis. The measurements of sublimation enthalpy of the thr ee acids were made at 333, 348 and 363 K, respectively, using a Tian-C alvet microcalorimeter equipped with Knudsen effusion cells. The deriv ed standard molar enthalpies of sublimation at 298.15 K are (95.94 +/- 0.41), (99.20 +/- 0.18), and (103.08 +/- 0.59) kJ . mol(-1) for the 2 -bromo- , 3-bromo- and 4-bromobenzoic acids, respectively. In addition , the saturated vapour pressure of these compounds was also calculated on the basis of the sublimation experiments. The enthalpy of fusion, the triple-point temperatures' and the mole fraction purities of the s amples of the investigated substances were measured using the mean tem perature version DTA apparatus developed by the CTM of the CNRS in Mar seille. The triple-point temperature and the molar enthalpies of fusio n of the 2-bromo-, 3-bromo and 4-bromobenzoic acids have been found to be (422.37 +/- 0.01) K, (24.54 +/- 0.07) kJ . mol(-1); (429.68 +/- 0. 01) K, (21.27 +/- 0.09) kJ . mol(-1) and (527.61 +/- 0.02) K, (28.70 /- 0.08) kJ . mol(-1) respectively. Using the contribution of benzene ring, carboxylic and bromine groups, determined from literature data, we have calculated the enthalpy of sublimation of 4-bromobenzoic acid and compared the value with our experimental result. It is also possib le to demonstrate and explain the existence of an intramolecular hydro gen bond in 2-bromobenzoic acid molecule.