CONFORMATIONAL-ANALYSIS .21. CONFORMATIONAL ISOMERISM IN CIS-CYCLOHEXANE-1,3-DIOL

cis-Cyclohexane-1,3-diol (CHD) has been isolated from a cis/trans isom eric mixture by first synthesizing the cyclic acetal 2,4-dioxabicyclo[ 3.3.1]nonane (D). The H-1 NMR spectra of CHD, D, cis- and trans-4-tert -butylcyclohexanol and tert-butylcyclohexane are reported. H-1 NMR par ameters obtained have enabled calculation of the Gibb's free energy ch ange (DELTAG-degrees) for the diequatorial double-line arrow pointing left and right diaxial equilibrium of CHD in 14 solvents. Values for D ELTAG-degrees range from 0.1 kcal mol-1double dagger in CCl4 to 2.7 kc al mol-1 in aqueous solution in favour of the conformation in which bo th hydroxy groups are equatorial. Similar measurements on cyclohexanol show that the DELTAG-degrees value of the hydroxy group in inert solv ents CCl4 and CDCl3 has a significant concentration dependence. The va lue of 0.9 kcal mol-1 for DELTAG-degrees characteristic for protic sol vents in 1 mol dM-3 solutions decreases to the lower value of 0.7 kcal mol-1 characteristic of a free OH group in < 1 0(-2) mol dm-1 solutio ns. There was no concentration dependence of the DELTAG-degrees value (0.80 kcal mol-1) in acetone solution. These results when considered w ith the results for CHD show: (i) there is no intra molecular hydrogen bonding in CHD in water, alcohols or strong hydrogen bond acceptor so lvents e.g. acetone or DMSO. The differences in the observed DELTAG-de grees values in these solvents can be attributed to (a) the varying po larity of the solvent and (b) to increased preferential solvation of t he diequatorial isomer in HBD solvents. (ii) An intramolecular hydroge n 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.