Cis–Trans Isomerism in Cycloalkanes

In many respects, the chemistry of cycloalkanes is like that of open-chain alkanes: both are nonpolar and fairly inert. There are, however, some important differences. One difference is that cycloalkanes are less flexible than open-chain alkanes. In contrast with the relatively free rotation around single bonds in open-chain alkanes, there is much less freedom in cycloalkanes. Cyclopropane, for example, must be a rigid, planar molecule because three points (the carbon atoms) define a plane. No bond rotation take place around a cyclopropane carbon–carbon bond without breaking open the ring (Figure 1-1).

Figure 1-1 Bond rotation in ethane and cyclopropane. (a) Rotation occurs around the carbon–carbon bond in ethane, but (b) no rotation is possible around the carbon–carbon bonds in cyclopropane without breaking open the ring.

  Larger cycloalkanes have increasing rotational freedom, and very large rings (C₂₅ and up) are so floppy that they are nearly indistinguishable from open-chain alkanes. The common ring sizes (C₃–C₇), however, are severely restricted in their molecular motions. Because of their cyclic structures, cycloalkanes have two faces when viewed edge-on, a “top” face and a “bottom” face. As a result, isomerism is possible in substituted cycloalkanes. For example, there are two different 1,2-dimethylcyclopropane isomers, one with the two methyl groups on the same face of the ring and one with the methyl groups on opposite faces (Figure 1-2). Both isomers are stable compounds, and neither can be converted into the other without breaking and reforming chemical bonds.

Figure 1-2 There are two different 1,2-dimethylcyclopropane isomers, one with the methyl groups on the same face of the ring (cis) and the other with the methyl groups on opposite faces of the ring (trans). The two isomers do not interconvert.

Unlike the constitutional isomers butane and isobutane, which have their atoms connected in a different order, the two 1,2-dimethylcyclopropanes have the same order of connections but differ in the spatial orientation of the atoms. Such compounds, with atoms connected in the same order but differing in three-dimensional orientation, are called stereochemical isomers, or stereoisomers. More generally, the term stereochemistry is used to refer to the three-dimensional aspects of chemical structure and reactivity.

The 1,2-dimethylcyclopropanes are members of a subclass 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 and in many cyclic biological molecules.