4.3
Stereoisomers
On the basis of mirror symmetry, stereoisomers of an organic molecule can be further classified into diastereomers and enantiomers. Diastereomers are…
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.
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Q1: What is the difference between diastereomers and enantiomers?
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.
Q2: How do cis-trans isomers differ from other types of diastereomers?
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.
Q3: Why are enantiomers always chiral molecules?
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.
Q4: How can you draw the enantiomer of a chiral molecule?
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.
Q5: What makes diastereomers without π bonds non-superposable?
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.
Q6: Why does restricted rotation prevent interconversion between cis and trans alkenes?
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.
Q7: What defines an enantiomeric pair?
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.