Conformational equilibria in halocyclohexanones, an NMR and solvation study

Abstract

The conformational equilibrium in 2-chloro cyclohexanone is measured in thirteen solvents from the 220 MHz 1H NMR spectrum using the C 2-H couplings and chemical shifts and the cis and trans 4-t-butyl-2-chlorocyclohexanones as reference compounds. ΔG ea varies from 1.04 kcal mole in n-pentane to −0.58 kcal mole in DMSO. The large concentration dependence of the NMR parameters in non-polar solvents noted previously is confirmed. Generalised reaction-field theory is used to calculate this solvent dependence, using a refined model of the geometry and dipole moments of the conformers. The cyclohexanone ring is considerably flatter than that of cyclohexane and this has an appreciable effect on the resultant dipole moments. Use of this geometry and CO and C-Cl bond moments which reproduce the observed dipole moments of the t-butyl compounds together with the generalised reaction field theory gives calculated solvation energies in good agreement with the observed data and hence allows the prediction of the vapour state energy difference. The model is applied to a variety of halo-ketones and gives both a reasonable explanation of the observed solvent dependencies and also the vapour state energy differences. The vapour state conformer energies are compared with the corresponding values for the halocyclohexanes and illustrate the large polar and steric effects due to the introduction of the CO group.