Rotational isomerism. Part XIV. Medium effects on the conformational equilibria of halogenocyclohexanes

Abstract

The calculation of the solvent dependence of the conformational equilibrium in mono- and di-chloro- and -bromo-cyclohexanes is given and compared with experiment. The calculations are based on an electrostatic theory of solvation and the model uses a fixed cyclohexane geometry and constant C–X bond dipole moment. The model gives generally good agreement with the observed dipole moments of all the chloro- and bromo-cyclohexanes. There is no solvent dependence in cyclohexyl halides but a large solvent dependence in trans-1,2-, cis-1,3-, and trans-1,4-dichloro- and -dibromo-cyclohexanes. The model provides a quantitative explanation of all the observed effects. This agreement allows the prediction of the vapour state energy differences in these systems. The diaxial conformer of trans-1,2-dichloro-(and dibromo-) cyclohexane is the preferred conformer in the vapour phase (ΔE^v= 0·7 and 1·3 kcal mol^–1 respectively) and this can be rationalised in terms of halogen–halogen repulsions. However, the diaxial conformer of trans-1,4-dichloro-(or dibromo-) cyclohexane is also the preferred conformer in the vapour phase (ΔE^v 0·8 and 0·7 kcal mol^–1 respectively), and this has no simple rationalisation. The energy differences for benzene as solvent are anomalous in these, as in all other related equilibria, and a method of allowing for this anomaly is given.