Disubstituted cyclohexanes, conformation

If a disubstituted cyclohexane has two different substituents then the most stable conformation is the chair that has the larger substituent m an equatorial orientation This IS most apparent when one of the substituents is a bulky group such as tert butyl Thus the most stable conformation of cis 1 tert butyl 2 methylcyclohexane has an equatorial tert butyl group and an axial methyl group... 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

Monosubstituted cyclohexanes are more stable with their substituent in an equatorial position, but the situation in disubstituted cyclohexanes is more complex because the steric effects of both substituents must be taken into account. All steric interactions in both possible chair conformations must be analyzed before deciding which conformation is favored. 

This is also true for 1,4 disubstituted cyclohexanes, but for 1,3 compounds, the reverse is true, the trans must have a e and cis either aa or ee conformation. But if the substituents are alkyl groups, the diequatorial predominates the diaxial. The trans 1, 2-e e conformations are thermodynamically more stable than cis 1, 2 isomers which therefore occurs as a e form. 

Depending on the orientation of substituents, the disubstituted cyclohexanes can be either equatorial-axial or diequatorial. This is illustrated with 2-methyl cyclohexanol. It exists in two stereoisomeric forms and each of which has two conformations which are readily interconvertible. 

We now focus our attention on conformational isomerism in 1,2-disubstituted cyclohexanes. Our model system will be 1,2-difluorocyclohexane and we first examine the relative stabilities of the two trans 1,2-difluorocyclohexanes, i.e. axial-axial (aa) and equatorial-equatorial (ee). These two molecules are shown below along with a listing of the dominant stabilizing a—a interactions. 

Table 52. Relative energies of the conformations of 1,2-disubstituted cyclohexanes, CgH 10XY...

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... 

Several further molecules which may be regarded as 1,2-disubstituted cyclohexanes have been studied by electron diffraction. Principal conformational findings for several of these compounds are summarized in Table 8. 

Problem-Solving Strategy Drawing Chair Conformations 116 3-14 Conformations of Monosubstituted Cyclohexanes 117 3-15 Conformations of Disubstituted Cyclohexanes 120... 

Some students find it difficult to look at a chair conformation and tell whether a disubstituted cyclohexane is the cis isomer or the trans isomer. In the following drawing, the two methyl groups appear to be oriented in similar directions. They are actually trans but are often mistaken for cis. 

The chair conformer of cis-1,4-disubstituted cyclohexane has one substituent equatorial, the other axial. This will not necessarily be the case for other substitution patterns for example, the chair conformer of a cis-1,3-disubs tituted cyclohexane has either both substituents axial or both equatorial. Remember, the raV and trans prefixes merely indicate that both groups are on the same side of the cyclohexane ring. Whether the substituents are both axial/equatorial or one axial and the other equatorial depends on the substitution pattern. Each time you meet a molecule, draw the conformation or make a model to find out which bonds are axial and equatorial. 

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