Conformations disubstituted cyclohexane

Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

In a monosubstituted cyclohexane, the substituent can be either in an equatorial or axial position. Equatorial positions are more spacious and in substituted cyclohexanes they are preferred. Cyclohexane rings flip between chair forms and establish an equilibrium. In the process of flipping, all equatorial positions become axial and all axial positions become equatorial. The equilibrium favors the chair in which substituents are equatorial. In monosubstituted cyclohexanes, the conformation in which the substituent is equatorial is favored. In disubstituted cyclohexanes where one group is axial and one equatorial, the equilibrium favors the chair form where the larger group occupies the more spacious equatorial position. 

Disubstituted cyclopropanes exemplify one of the simplest cases involving stabil ity differences between stereoisomers A three membered ring has no conformational mobility so the ring cannot therefore reduce the van der Waals strain between cis sub stituents on adjacent carbons without introducing other strain The situation is different m disubstituted derivatives of cyclohexane... 

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. 

Cyclobutane adopts a puckered conformation in which substituents then occupy axial-like or equatorial-like positions. 1,3-Disubstituted cyclobutanes show small energy preferences for the cis isomer since this places both substituents in equatorial-like positions. The energy differences and the barrier to inversion are both smaller than in cyclohexane. 

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. 

Such interconversions with monosubstituted cyclohexanes and also with disubstituted ones do not involve any rearrangement i.e., no chemical bonds are broken nor reformed, only their conformation changes and this has been confirmed by NMR studies e.g., methyl cyclohexane at -110°C gives separate signals for equatorially or axially oriented methyl groups. 

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...

Hence A (fra .s) = 4117.24kcal/mol for fra .s-l,4-di- -butylcyclohexane and A /(m) = 4113.72kcal/mol for the cis form. These two results are indicative of ring conformation since cA-l,4-di-t-butylcyclohexane is undoubtedly in a twist-boat form while the other is in chair conformation. The spectra of t-butylcyclohexane (in chair conformation) and of rrani-l,4-di- -butylcyclohexane are indeed very similar, except, of course, for carbon 4, which is the same as carbon 1 in the disubstituted molecule, whereas it is similar to the unsubstimted carbons in the monosubstituted cyclohexane. 

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... 

Next relative configurations (R,S) are possible for 1,2- or 1,3-disubstituted isomers. (The 1,4 isomer has a plane of symmetry.) The relative stereochemistry can be denoted as cis or trans, depending on whedter the substituents point toward the same side or opposite sides of die ring. Finally, die cyclohexane ring can undergo chair-chair interconversion leading to different conformational isomers. These possibilities are shown for metiiylcyclohexanol in (6.6). 

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. 

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