Trans-1,2-Dimethylcyclohexane, structure

One after the other, step through (or animate) the sequence of structures depicting ring inversion in cyclohexane, methylcyclohexane and trans-l,2-dimethylcyclohexane. Is the overall motion involved in the ring inversion similar in all three Describe any differences. 

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... 

Problem 9.14 (a) Draw the possible chair conformational structures for the following pairs of dimethylcyclohex-anes (i) cis- and tram-1,2- (ii) < is- and trims-1,3- (iii) cis- and trans-1,4-. (b) Compare the stabilities of the more stable conformers for each pair of geometric isomers, (r) Determine which of the isomers of dimethylcyclohexane are chiral. ... 

Cis-trans isomerism is also possible in molecules with rings. Draw the cis and trans isomers of 1,2-dimethylcyclohexane. In Exercise 41, you drew all of the noncyclic structural and geometric isomers of C4H7E. Now draw the cyclic structural and geometric isomers of C4H7E. 

The stereochemistry of cycloalkanes is discussed extensively in the chapter by Anderson, and it is important for an understanding of the analyses offered for these compounds to distinguish between isomers and conformers. Because of the low energetic barrier for equilibration between axial and equatorial conformers, only low-temperature HNMR or CNMR as well as IR will freeze the equilibration and allow the spectral analysis of the conformers. However, disubstituted cycloalkanes will exhibit, in addition to structural isomerism, also cis, trans isomerism. Both cis and trans isomers of 1,4-dimethylcyclohexane undergo, at room temperature, a conformer equilibration. In the case of the trans isomer the conformer with two equatorial CH3 groups is more stable, while for the cis isomer both conformers are equally stable and cannot be separated. 

Trans 1,4-Disubstituted Cyclohexanes If we consider dimethylcyclohexanes, the structures are somewhat more complex because the cyclohexane ring is not planar. Beginning with rr<2 r-l,4-dimethylcyclohexane, because it is easiest to visualize, we find there are two possible chair conformations (Fig. 4.22). In one conformation both methyl groups are axial in the other both are equatorial. The diequatorial conformation is, as we would expect it to be, the more stable conformation, and it represents the structure of at least 99% of the molecules at equilibrium. 

FIGURE 5.19 trans-, 3-Dimethylcyclohexane does not have a plane of symmetry and exists as a pair of enantiomers. The two structures (a and b) shown here are not superposable as they stand, and flipping the ring of either structure does not make it superposable on the other, (c) A simplified representation of (b). 

Explain with the aid of conformational structures why ds-1,3-dimethylcyclohexane is more stable than trans-1,3-dimethylcyclohexane, whereas the reverse order of stability is observed for the 1,2 and 1,4 isomers. (Constructing models will help you with this problem.)... 

Trans 1,4-Disubstituted Cyclohexanes If we consider dimethylcyclohexanes, the structures are somewhat more complex because the cyclohexane ring is not planar. Beginning with fra/i5 -l,4-dimethylcyclohexane, because it is easiest to visualize, we find... 

Every compound with a cyclohexane ring has two chair conformers thus, both the cis isomer and the trans isomer of a disubstituted cyclohexane have two chair conformers. Let s compare the structures of the two chair conformers of cw-l,4-dimethylcyclohexane to see if we can predict any difference in their stabilities. 

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