4.3 : Stereoizomery

Recall that stereoisomers are molecules that only differ in their overall molecular shape. Based on the spatial arrangement of the constituent atoms, stereoisomers can be classified into diastereomers and enantiomers.

Diastereomers are stereoisomers that are not mirror images of each other. For example, the cis and trans isomers of 1,2-dimethylcyclohexane are diastereomers, as these molecules have the same connectivity but exhibit different molecular shapes and are not mirror images of each other.

Diastereomers can be further sub-grouped into two different classes: those with π bonds involved and those without.

In the former, such as in the case of cis-2-butene and trans-2-butene, diastereomers exhibit different spatial arrangements of the substituent groups around the π bond. Here, the restricted rotation around the π bond prevents interconversion between cis-2-butene and trans-2-butene, allowing them to exist as distinct compounds.

In the latter, such as in the case of cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane, diastereomers are molecules that are not superposable on each other and exhibit a different spatial arrangement of their atoms at some, but not all, stereocenters.

Molecules in the other class of stereoisomers, known as enantiomers, are chiral molecules that are mirror images of each other. A chiral molecule and its mirror image are collectively referred to as an enantiomeric pair, or a pair of enantiomers.

The enantiomer of a chiral molecule can be drawn either by taking the mirror image of the molecule from any position or simply by exchanging the positions of two of the substituents at each stereocenter of the molecule.

Consider the chiral molecule methamphetamine, a potent central nervous system stimulant. The mirror images of methamphetamine obtained by placing a mirror behind the molecule, next to the molecule, or below the molecule are all the same molecule and equivalent to the configuration obtained by exchanging the methyl and hydrogen groups at the stereocenter.