THERMODYNAMIC STUDY ON 3 ISOMERS OF CHLOROBENZOIC ACID

Our isoperibol rocking-bomb calorimeter, previously described, has been modified to: (i) reduce and stabilize the heat exchange between the calorimetric system and its surroundings; (ii) change its thermometer; (iii) associate with it a computer program for the robotics of different steps of an experiment, data acquisition, and determination of the thermodynamic quantities. After testing these modifications and calibrating the calorimetric system, the enthalpies of formation in the condensed state and at 298.15 K of the three isomers of chlorobenzoic acid have been determined by combustion calorimetry. Associating these values with these of their enthalpies of sublimation previously measured, we determined their enthalpies of formation in the gaseous state. The experimental values of the thermodynamic functions -Delta(c)H(m)(o) (s, 298.15 K), -Delta(f)H(m)(o) (s, 298.15 K), Delta(sub)H(m)(o) (298.15 K), and -Delta(f)H(m)(o) (g, 298.15 K) are given for 2-chlorobenzoic acid, 3-chlorobenzoic acid, and 4-chlorobenzoic acid. From the experimental value of the enthalpy of atomization, it has been possible to determine an enthalpy value for the C-b-Cl bond. The experimental and theoretical values of the resonance energy of the three isomers are compatible. The relative stability of some monosubstituted derivatives of benzoic acid studied in our laboratory is also discussed.