Stereoisomers

Diastereomers are stereoisomers that are not mirror images of each other, such as cis and trans isomers of 1,2-dimethylcyclohexane. Enantiomers are stereoisomers that are mirror images of each other and occur only in chiral molecules. Both differ in spatial arrangement, but enantiomers are non-superposable mirror pairs while diastereomers are not mirror images at all.

Cis-trans isomers like cis-2-butene and trans-2-butene are diastereomers with π bonds that prevent rotation around the double bond. Other diastereomers, such as cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane, lack π bonds and differ in spatial arrangement at some stereocenters. Both types are non-superposable but restricted rotation distinguishes alkene diastereomers.

Enantiomers are mirror images of each other, and only chiral molecules possess non-superposable mirror images. A chiral molecule and its mirror image form an enantiomeric pair. Achiral molecules lack this mirror-image relationship, so they cannot form enantiomers. Chirality is the fundamental requirement for enantiomeric pairs to exist.

You can draw an enantiomer by taking the mirror image of the chiral molecule from any position—behind, beside, or below it. Alternatively, exchange the positions of two substituents at each stereocenter. Both methods produce the same enantiomeric structure. For example, exchanging methyl and hydrogen groups at methamphetamine's stereocenter yields its enantiomer.

Diastereomers without π bonds, like cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane, have different spatial configurations at some stereocenters. These molecules cannot be rotated or flipped to overlap perfectly because their three-dimensional arrangements differ. Unlike alkenes with restricted rotation, cyclic diastereomers remain distinct due to fixed ring geometry and differing stereochemistry.

The π bond in alkenes like cis-2-butene and trans-2-butene prevents rotation around the double bond. This restriction locks the substituent groups in fixed positions, making interconversion between cis and trans forms impossible under normal conditions. The rigid π bond geometry ensures these diastereomers remain as distinct, stable compounds.

An enantiomeric pair, also called a pair of enantiomers, consists of a chiral molecule and its non-superposable mirror image. These molecules have identical connectivity but opposite spatial arrangements at all stereocenters. Enantiomers exhibit mirror symmetry and are fundamental to understanding chiral chemistry and molecular properties of enantiomers and optical activity.