Solvent and gas-phase effects on the equilibrium between configurational isomers of some 4-t-butyicyclohexanes

Abstract

Through a combination of gas chromatographic head space analysis, gas chromatography retention times, and calorimetrically determined enthalpies of solution, the effect of various solvents, relative to the gas phase, on values of ΔG°, ΔH°, and ΔS° for the equilibrium between r-1,t-2-dibromo-c-4- and r-1,t-2-dibromo-t-4-t-butylcyclohexane has been determined. It is shown that the more polar diequatorial dibromo-isomer is stabilised relative to the diaxial isomer both in terms of Gibbs energy and enthalpy in polar media. Although values of ΔG°, ΔH°, and ΔS° for the equilibrium in nonpolar solvents approach the gas-phase values, there are still noticeable differences in these thermodynamic parameters between the gas phase and solvents such as hexane and cyclohexane. The effects of various media on ΔG° values for the equilibria between cis- and trans-4-t-butylcyclohexyl bromide and cis- and trans-4-t-butylcyclohexyl acetate have also been determined, but values of ΔG° for these two equilibria are largely independent of the reaction medium.