4.3
Stereoisomers
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.
On the basis of mirror symmetry, stereoisomers of an organic molecule can be further classified into diastereomers and enantiomers. Diastereomers are stereoisomers that are not mirror images of each other. Substituted alkenes, such as the cis and trans isomers of 2-butene, are diastereomers, as these molecules exhibit different spatial orientations of their constituent atoms, are not mirror images of each other,and do not interconvert. Here, the interconversion is suppressed due to restricted rotation around the π bond. Another class of diastereomers — those without π bonds — are molecules that have non-superposable spatial orientations and exhibit different configurations of their substituent groups at some, but not all, stereocenters. For example, cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane are diastereomers, as these molecules are not superposable and have a different configuration of the methyl and hydrogen groups at only one out of their two stereocenters.
Enantiomers are stereoisomers that are mirror images of each other. As only chiral molecules can have non-superposable mirror images, enantiomers are chiral molecules. For example, the chiral molecule 2-butanol and its mirror image are enantiomers, as these molecules exhibit non-superposable spatial orientations of their constituent atoms and 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 by taking its mirror image from any position orexchanging the positions of two of the substituents at each stereocenter of the molecule.
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.
- Stereoisomers – Molecules with same structural formula but different spatial arrangement of atoms
- Diastereomers – Stereoisomers that aren't mirror images of each other (e.g., cis and trans isomers)
- Enantiomers – Stereoisomers that are non-superposable mirror images of each other
- Stereocenters – Atoms where the interchange of two groups gives different stereoisomers
- Superposable – Property of two objects where they can be placed directly over each other
- Define Stereoisomers – Understand different classes of isomers (e.g., stereoisomers)
- Contrast Diastereomers vs Enantiomers – Elucidate key differences (e.g., mirror image property)
- Explore Stereocenter Examples – Describes the role of stereocenters in determining stereoisomerism (e.g., 2-butene)
- Explain Superposability Concept – Discussion of how superposability defines chirality and enantiomeric relation
- Apply in Organic Chemistry Context – Examination of how stereoisomerism concepts apply in organic compounds' structure.
- What is the difference between diastereomers and enantiomers?
- How does the concept of superposability apply to stereoisomers?
- What is the role of stereocenters in determining stereoisomerism?
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