CONFORMATIONAL ANALYSIS .19. CONFORMATIONAL ENTHALPY AND ENTROPY OF HYDROXYL GROUP IN VARIOUS SOLVENTS . CONFORMATIONAL ENERGY OF METHOXYL

cis- and trans-4-t-butylcyclohexanol have been equilibrated by Raney nickel in several solvents over a range of temperatures, and the following thermodynamic parameters (cis ↔ trans) have been determined. In cyclohexane, 0.05 M, ∆H° = -0.58 kcal/mol, ∆S° = 0.06 cal/deg mol, ∆G°25 = -0.60 kcal/mol; 0.10 M, ∆H° = -0.61, ∆S° = —0.02, ∆G°25 = -0.61 (units as above); 0.20M, ∆H° = -0.63, ∆S° = -0.06, ∆G°25 FORMULA OMMITED The conformational free energy of the hydroxyl group determined by the nmr method in cyclohexane at high dilution (0.015 M), — ∆G°25 = 0.61 kcal/mol, agrees with the thermodynamic value, but in certain other solvents, even at high dilution, there was neither agreement nor consistency, which confirms earlier doubts regarding the general validity of the nmr method using a t-butyl holding group. The conformational energy values for methoxyl, determined by nmr in various solvents, range from 0.34 to 0.63 kcal/mol. Cryoscopic measurements of the degree of association of the cyclohexanols used in this study at various concentrations and two temperatures are reported. The dependence of the conformational thermodynamic parameters on solvent is rationalized in detail. Additional data are reported for the equilibration of 2-methoxy-6-methyloxane. © 1969, American Chemical Society. All rights reserved.