Trans-1,2-Dihalocyclohexanes, conformational

In disubstituted compounds, the rule for alkyl groups is that the conformation is such that as many groups as possible adopt the equatorial position. How far it is possible depends on the configuration. In a c -l,2-disubstituted cyclohexane, one substituent must be axial and the other equatorial. In a tram-1,2 compound both may be equatorial or both axial. This is also true for 1,4-disubstituted cyclohexanes, but the reverse holds for 1,3 compounds the trans isomer must have the ae conformation and the cis isomer may be aa or ee. For alkyl groups, the ee conformation predominates over the aa but for other groups this is not necessarily so. For example, both fram-l,4-dibromocyclohexane and the corresponding di-chloro compound have the ee and aa conformations about equally populated228 and most irons-1,2-dihalocyclohexanes exist predominantly in the aa conformation.229 Note that in the... 

It has been pointed out6 that trans-1,4-dihalocyclohexanes are ideally suited for determination of changes in free energy, enthalpy, and entropy by electron diffraction, since the ratio between the conformers may be found quite accurately by comparing areas of the appropriate peaks in the experimental RD curve (see Fig. 17). Accurate thermodynamic parameters of gas phase conformational equilibria are rather scarce and these molecules may perhaps be able to provide some much-wanted high-quality data with the aid of the modern electron-diffraction technique. 

In the case of diaxial/diequatorial equilibria of trans -l,2-dihalocyclohexanes (with and without a 4-ter+butyl group), AG° [aa ee) also shows a pronounced dependence on the medium, varying from about 4 kJ/mol in apolar solvents to about —2 kJ/mol in polar solvents [90-94, 192-194]. This is mainly due to the very different dipole moments of the two conformers. In general, the more dipolar diequatorial isomer is favoured in polar solvents. 

Abraham RJ, Rosetti ZL (1973) Rotational ismnerism. XV. Solvent dependence of the conformational equilibriums in trans-1,2 and trans-l,4-dihalocyclohexanes. J Chem Soc Perkin II 582-587... 

The conformational relationship of the bromine atoms is clear in 20 and 21 because of the rigidity of the steroid structure. Individual cyclohexane rings usually have considerable conformational mobility, however, and trans-1,2-dihalocyclohexanes exist as a mixture of diaxial and diequatorial conformers (Figure 10.40). While the diequatorial conformer is expected to be ttie major conformer, the anti-coplanar elimination suggested in Figure 10.39 must occur from the minor diaxial conformer. In the case of cis-l,2-dihalocyclohexanes. 

Table II gives an example, the conformational equilibrium between the di-equatorial and di-axial forms of trans 1,2-dihalocyclohexanes, which are particularly favourable cases to examine as the observed energy differences could be measured accurately and unequivocally by direct integration of the low temperature NMR spectra [10].

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