The calculation of –ΔG° for axial–equatorial equilibria in monosubstituted cyclohexanes

Abstract

The existence of an important electrical effect upon conformational equilibria in monosubstituted cyclohexanes is proposed. Values of –ΔG° from the literature have been correlated with the equation –ΔG°_x=ασ_I,X+h(in which the σ_I constants are a measure of the localised electrical effects of substituents), with excellent results. Values of –ΔG° can be calculated with an absolute average deviation ⩽ 0·4 kcal./mole. The equations preferred for the calculation of new values of –ΔG° are: for planar π-bonded substituents such as NO₂ and CO₂R, –ΔG°_X=–1·47σ_I,X+ 1·80; for all other types of substituent, –ΔG°_X=–2·74σ_I,X+ 1·68. The success of this method is believed to be due to the approximate constancy of the difference in van der Waals energies between axial and equatorial conformers; thus, this energy difference is equal to h in the above equation. This method is a rapid convenient approach to the a priori calculation of –ΔG° for conformational equilibria in monosubstituted cyclohexanes.