Dimethylcyclohexanes, conformational

Dimethylcyclobutane, isomerization, 30 31 Dimethylcyclohexane conformational analysis of, 18 14 experimental equilibrium, 18 17 epimerization of, 25 136 vibrational spectra, 42 239 Dimethylcyclohexylbenzene, 42 432 Dimethylcyclopentanes aromatization, 30 54 isomerization, 30 34... 

Wedge and dash drawings fail to show conformation and it s important to remember that the rings of cis and trans 1 2 dimethylcyclohexane exist m a chair conformation This... 

Their heats of combustion (Table 3 2) reveal that trans 1 4 dimethylcyclohexane is 7 kJ/mol (17 kcal/mol) more stable than the cis stereoisomer It is unrealistic to believe that van der Waals strain between cis substituents is responsible because the methyl groups are too far away from each other To understand why trans 1 4 dimethylcyclo hexane is more stable than cis 1 4 dimethylcyclohexane we need to examine each stereoisomer m its most stable conformation... 

The most stable conformation of trans 1 4 dimethylcyclohexane has both methyl groups in equatorial orientations The two chair conformations of trans 1 4 dimethyl cyclohexane are not equivalent to each other One has two equatorial methyl groups the other two axial methyl groups... 

The most stable conformation of cis 1 3 dimethylcyclohexane has both methyl groups equatorial... 

Most stable conformation of as 1 3 dimethylcyclohexane (no axial methyl groups)... 

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. 

Incorporation of stereogenic centers into cyclic structures produces special stereochemical circumstances. Except in the case of cyclopropane, the lowest-eneigy conformation of the tings is not planar. Most cyclohexane derivatives adopt a chair conformation. For example, the two conformers of cis-l,2-dimethylcyclohexane are both chiral. However, the two conformers are enantiomeric so the conformational change leads to racemization. Because the barrier to this conformational change is low (lOkcal/mol), the two enantiomers arc rapidly interconverted. 

While individual conformers of cu-l,2-dimethylcyclohexane are chiral, the two conformers are eoaotionjeric. 

Certain dimethylcycloalkanes contain a plane of symmetry. For example, both chair conformers of crs-l,3-dimethylcyclohexane possess a plane of symmetry bisecting the molecule through C-2 and C-5. The trans isomer does not have any element of symm and is chiral. 

Like the 1,4-dimethyl derivatives, trani-1,2-dimethylcyclohexane has a lower heat of combustion (see Table 3.2) and is more stable than d5-l,2-dimethylcyclohexane. The cis ster eoisomer has two chair conformations of equal energy, each containing one axial and one equatorial methyl group. 

Obtain energies for diequatorial and diaxial conformers of cis-l,3-dimethylcyclohexane, trans-1,2-dimethyl-cyclohexane and trans-l,4-dimethylcyclohexane. 

Draw 1,1-dimethylcyclohexane in a chair conformation, indicating which methyl group in your drawing is axial and which is equatorial. 

Draw two different chair conformations of fmns-l,4-dimethylcyclohexane. and label all positions as axial or equatorial. 

Active Figure 4.15 Conformations of c/s-1,2-dimethylcyclohexane. The two chair conformations are equal in energy because each has one axial methyl group and one equatorial methyl group. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Figure 4.16 Conformations of rrans-1,2-dimethylcyclohexane. The conformation with both methyl groups equatorial is favored by 11.4 kJ/mol (2.7 kcal/mol) over the conformation with both methyl groups axial.

A The diaxial conformation of c/s-l,3-dimethylcyclohexane is approximately 23 kj/mol (5.4 keal/mol) less stable than the diequatorial conformation. Draw the two possible chair conformations, and suggest a reason for the large energy difference. 

Since compounds with alkyl equatorial substituents are generally more stable, trans-1,2 compounds, which can adopt the ee conformation, are thermodynamically more stable than their cis-1,2 isomers, which must exist in the ae conformation. For the 1,2-dimethylcyclohexanes, the difference in stability is about 2kcal moP (8 kJ mol" ). Similarly, trans-1,4 and cis-1,3 compounds are more stable than their stereoisomers. 

Figure 4.23 The two chair conformations of fra s-l,4-dimethylcyclohexane. (Note All other C-H bonds have been omitted for clarity.)...

Figure 5.20 trans- 1,2-Dimethylcyclohexane has no plane of symmetry and exists as a pair of enantiomers (a and b). [Notice that we have written the most stable conformations for (a) and (b). A ring flip of either (a) or (b) would cause both methyl groups to become axial.]... 

Figure 5.21 ds-l,2-Dimethylcyclohexane exists as two rapidly interconverting chair conformations (c) and (d). 

Although analysis of the consequences of ring flip in a monosubstituted cyclohexane is pretty straightforward, the presence of two or more substituents requires careful consideration to decide which conformer, if any, is the more favoured. Let us illustrate the approach using 1,4-dimethylcyclohexane. Now, two configurational isomers of this structure can exist, namely trans and... 

This type of reasoning may be applied to other dimethylcyclohexanes, as indicated in the figure. There is no easy way to predict the result it must be deduced in each case. One conformer is of much lower energy in the cases of trans-1,2-, cis-1,3-, and fran -l,4-dimethylcyclohexane both conformers have equal energy in the cases of cis-1,2-, trans-1,3-, and CM-1,4-dimethyIcyclohexane. 

Conformational mobihty, such as we get in cyclohexane rings, makes the analysis more difficult, and manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2-dimethylcyclohexane as an example. Again, we have met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need to consider both configuration and conformation. 

It is clear that this representation of c/5 -dimethyl-cyclohexane shows a plane of symmetry, and we can deduce it to be a meso compound. No such plane of symmetry is present in the representation of fran -dimethylcyclohexane. Why does this approach work Simply because the transformation of planar cyclohexane (with echpsed bonds) into a non-planar form (with staggered bonds) is a conformational change achieved by rotation about single bonds. The fact that cyclohexane is non-planar means we may have to invoke the conformational mobihty to get the three-dimensional picture. 

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