THERMODYNAMIC PARAMETERS OF CONFORMATIONAL EQUILIBRIUM IN 1,2-DICHLOROETHANE - INFLUENCE OF MEDIUM, BENZENE AND COMPENSATION EFFECTS

The conformational equilibrium of 1,2-dichloroethane (DCE) in 19 diffe rent media (vapour phase, liquid, solutions in n-hexane, n-heptane, ca rbon tetrachloride, benzene, toluene, ethyl benzene (EB), carbon disul phide, diethyl ether, bromoform, chloroform, dichloromethane, pyridine , acetone, methanol-d(3), acetonitrile, glassy polystyrene (PS) matrix and 1:1 mixture of PS and EB) has been studied by IR absorption spect ra. The enthalpy differences (Delta H-0) between trans (t) and gauche (g) conformers were determined from the dependencies of In(I-t/I-g) up on T-1, where I-t and I-g are the integrated intensities of the bands belonging to trans and gauche conformers, respectively. The values of RT In(I-t/I-g) and (R In(I-t/I-g) + Delta H-0/T) obtained at 296 K wer e used as measures of the free enthalpy (Delta G(0)) and entropy (Delt a S-0) differences of the conformers respectively, when considering th eir changes with solvent. Good correlations between Delta G(0), Delta H-0 and the function of the dielectric permittivity of the medium (0.5 - (epsilon - 1)/(2 epsilon + 1))(1/2) were observed for all solvents except methanol-d(3) (effect of self-association) and the aromatic sol vents (benzene effect). Notwithstanding the similar structures and die lectric permittivities of EB and PS, the former medium behaves as a si gnificantly more polar solvent. The analysis of the Delta H-0 values o btained for DCE in PS and EB enabled confirmation of Eliel and Hofer's concept of the origin of the benzene effect; the effect is assigned t o the relatively high anisotropy of the polarizability tenser of aroma tic compounds. As a consequence, the electrostatic and dispersion inte ractions of a solute dipole with a neighbouring aromatic ring is cruci al to their mutual orientation. Significantly non-linear (close to qua dratic) dependencies In(I-t/I-g) = f(T-1) were observed for DCE in tol uene and EB in the range 180-370 K; the Delta H-0, value increased fro m - 170 cal mol(-1) at 358 K to 820 cal mol(-1) at 190 K for the tolue ne solution. These results are considered within the framework of the manifestations of the benzene effect. Significant changes in Delta S-0 , with the media have been found; they cover more than 2.0 cal mol(-1) K-1 when going from the vapour phase to polar solvents. Satisfactory correlation between Delta H-0 and Delta S-0 (compensation effect) has been observed, the points corresponding to all the media, including me thanol-d(4) and the aromatic solvents, being scattered along a straigh t line. The tangent of the slope (Delta Delta S-0/Delta Delta H-0) = ( 1.4 +/- 0.4) x 10(-3) K-1 is close to those determined earlier for 1,2 -bromofluoroethane, trans-1,2-dichlorocyclohexane and o-iodophenol.