Conformational analysis. Part 21. Conformational isomerism in cis-cyclohexane-1,3-diol
cis-Cyclohexane-1,3-diol (CHD) has been isolated from a cis/trans isomeric mixture by first synthesizing the cyclic acetal 2,4-dioxabicyclo[3.3.1]nonane (D). The 1H NMR spectra of CHD, D, cis-and trans-4-tert-butylcyclohexanol and tert-butylcyclohexane are reported. 1H NMR parameters obtained have enabled calculation of the Gibb's free energy change (ΔG°) for the diequatorial ⇔ diaxial equilibrium of CHD in 14 solvents. Values for ΔG° range from 0.1 kcal mol–1 in CCl4 to 2.7 kcal mol–1 in aqueous solution in favour of the conformation in which both hydroxy groups are equatorial. Similar measurements on cyclohexanol show that the ΔG° value of the hydroxy group in inert solvents CCl4 and CDCl3 has a significant concentration dependence. The value of 0.9 kcal mol–1 for ΔG° characteristic for protic solvents in 1 mol dm–3 solutions decreases to the lower value of 0.7 kcal mol–1 characteristic of a free OH group in <10–2 mol dm–3 solutions. There was no concentration dependence of the ΔG° value (0.80 kcal mol–1) in acetone solution.
These results when considered with the results for CHD show: (i) there is no intramolecular hydrogen bonding in CHD in water, alcohols or strong hydrogen bond acceptor solvents e.g. acetone or DMSO. The differences in the observed ΔG° values in these solvents can be attributed to (a) the varying polarity of the solvent and (b) to increased preferential solvation of the diequatorial isomer in HBD solvents. (ii) An intramolecular hydrogen bond of ca. 1.6 kcal mol–1 energy is formed in CHD in CCl4 solution. This hydrogen bond may be present in the more polar solvents, but its effect is much less noticeable.