Nuclear magnetic resonance studies. XIV. Conformational equilibria of α-halocyclohexanones

Abstract

The conformational equilibria of α-fluoro-, α-chloro-, and α-bromo-cyclohexanone have been examined by proton spectroscopy in cyclohexane, carbon tetrachloride, benzene, chloroform, p-dioxane, and acetonitrile solutions over a range of concentrations. The cis- and trans-4-t-butyl derivatives were used as reference compounds for the two chair conformations. Equilibrium constants were estimated from the α-proton spectral data in two ways and compared with the results obtained by a variety of methods. Consistently good agreement was found between our results and those available from dipole moment measurements. This study is the most extensive investigation of the α-halocyclohexanone equilibria to date. Calculations using simple classical models for steric, electrostatic, and induced dipolar interactions were found to predict the observed trends. The solvent effects on the axial and equatorial α-proton shieldings in the "fixed" systems are discussed in terms of solute–solute and (or) solute–solvent interactions, and evidence for reaction field contributions is presented.