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

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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 (ΔEv= 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 (ΔEv 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.

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