Disubstituted Cycloalkanes Cis-Trans Isomerism

The presence of two substituents on different carbons of a cycloalkane allows for the possibility of cis-trans isomerism similar to the kind we saw for alkenes in Section 1.13B. These cis-trans isomers are also stereoisomers because they differ from each other only in the arrangement of their atoms in space. Consider 1,2-dimethylcyclopropane (Fig. 4.21) as an example. 

The planarity of the cyclopropane ring makes the cis-trans isomerism obvious. In the first structure the methyl groups are on the same side of the ring therefore, they are cis. In the second structure, they are on opposite sides of the ring they are trans. 

Cis and trans isomers such as these cannot be interconverted without breaking carbon-carbon bonds. They will have different physical properties (boiling points, melting points, and so on). As a result, they can be separated, placed in separate bottles, and kept indefinitely. 

Write structures for the cis and trans isomers of (a) 1,2-dichlorocyclopentane and (b) 1,3-dibromocyclobutane. (c) Are cis-trans isomers possible for 1,1-dibromocyclobutane  

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 

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Cis-trans isomerism occurs in each of the above disubstituted cycloalkanes. 

A disubstituted cycloalkane such as (a) exhibits cis-trans isomerism, whereas a disubstituted benzene (b) does not. Explain. 

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. 

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