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 different media (vapour phase, liquid, solutions in n-hexane, n-heptane, carbon tetrachloride, benzene, toluene, ethyl benzene (EB), carbon disulphide, diethyl ether, bromoform, chloroform, dichloromethane, pyridine, acetone, methanol-d3, acetonitrile, glassy polystyrene (PS) matrix and 1:1 mixture of PS and EB) has been studied by IR absorption spectra. The enthalpy differences (ΔH0) between trans (t) and gauche (g) conformers were determined from the dependencies of 1n(It/Ig) upon T-1, where I1 and Ig are the integrated intensities of the bands belonging to trans and gauche conformers, respectively. The values of RT ln(It/Ig) and (R ln(It/Ig) + Δ H0/T) obtained at 296 K were used as measures of the free enthalpy (ΔG0) and entropy (ΔSD) differences of the conformers respectively, when considering their changes with solvent. Good correlations between ΔG0, ΔH0 and the function of the dielectric permittivity of the medium (0.5 - (ε - 1)/(2ε + 1)) 1 2 were observed for all solvents except methanol-d3 (effect of self-association) and the aromatic solvents (benzene effect). Notwithstanding the similar structures and dielectric permittivities of EB and PS, the former medium behaves as a significantly more polar solvent. The analysis of the ΔH0 values obtained 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 to the relatively high anisotropy of the polarizability tensor of aromatic compounds. As a consequence, the electrostatic and dispersion interactions of a solute dipole with a neighbouring aromatic ring is crucial to their mutual orientation. Significantly non-linear (close to quadratic) dependencies ln(It/Ig) = f(T-1) were observed for DCE in toluene and EB in the range 180-370 K: the ΔH0 value increased from -170 cal mol-1 at 358 K to 820 cal mol-1 at 190 K for the toluene solution. These results are considered within the framework of the manifestations of the benzene effect. Significant changes in ΔS0 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 ΔH0 and ΔS0 (compensation effect) has been observed, the points corresponding to all the media, including methanol-d4 and the aromatic solvents, being scattered along a straight line. The tangent of the slope (ΔΔS0/ΔΔHD) = (1.4 ± 0.4) × 10-3 K-1 is close to those determined earlier for 1,2-bromofluoroethane, trans-1,2-dichlorocyclohexane and o-iodophenol. © 1995