Cycloalkanes 1,2-dimethylcyclopropane

Because of their cyclic structures, cycloalkanes have two sides, a top side and a bottom side, leading to the possibility of isomerism in substituted cycloalkanes. For example, there are two different 1,2-dimethylcyclopropane isomers, one with the two methyls on the same side of the ring, and one with the methyls on opposite sides (Figure 3.9). Both isomers are stable compounds neither can be converted into the other without breaking and reforming chemical bonds. Make molecular models to prove this to yourself. 

The 1,2-dimethylcyclopropanes are special kinds of stereoisomers called cis-trans isomers. The prefixes cis- (Latin, on the same side ) and trans-(Latin, across ) are used to distinguish between them. Cis-trans isomerism is a common occurrence in substituted cycloalkanes. 

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. 

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