Nuclear magnetic resonance experiments on acetals. Part XXXIII. Conformational equilibria in some 4,4-gem-substituted cyclohexanone dimethyl acetals

Abstract

Thermodynamic parameters for the conformational equilibria of seven 4-methyl-4-R-1,1-dimethoxycyclohexanones (R = Et, Prⁿ, Prⁱ, Buⁿ, and Bu^t) were obtained from temperature-adapted shift values. It was assumed that bulkier alkyl groups will take the equatorial position rather than the methyl group. ΔΔG° values are found to be larger than expected from the additivity principle, except for Bu^t where the contrary is found. For all derivatives ΔΔH°=ca. 485 cal mol^–1. Unfavourable entropic contributions are found for axial α-branched substituents.