Enantiomers and diastereomers are types of stereoisomers, meaning they have the same molecular formula and bond connectivity but differ in the 3D arrangement of atoms in space.
Enantiomers
- Definition: Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other, much like your left and right hands. This means that no matter how you rotate them, you can't align one enantiomer exactly on top of the other.
- Chirality: For enantiomers to exist, the molecule must be chiral, meaning it lacks a plane of symmetry. This usually involves a carbon atom attached to four different groups (known as a chiral center or asymmetric carbon).
- Properties: Enantiomers share many physical properties (melting points, boiling points) but differ in how they rotate plane-polarized light. One enantiomer will rotate light in one direction (clockwise, labeled as "+") while the other will rotate it in the opposite direction (counterclockwise, labeled as "-").
- Example: A common example of enantiomers is (R)- and (S)-lactic acid. Each version of lactic acid is a mirror image of the other and cannot be superimposed.
2. Diastereomers
- Definition: Diastereomers are stereoisomers that are not mirror images of each other and are not superimposable. They differ in some, but not all, aspects of spatial arrangement, which means they are not related as mirror images.
- Chirality: Diastereomers may or may not have chiral centers, but at least two stereocenters are usually required to create a diastereomeric relationship.
- Properties: Unlike enantiomers, diastereomers often have different physical properties (like boiling point, melting point, and solubility) due to the difference in spatial arrangement.
- Example: Threo- and erythro-2,3-dichlorobutane are diastereomers of each other. They have the same molecular formula and connectivity but differ in the arrangement of chlorine atoms in space, which makes them non-mirror images.
Key Differences Summary
| Property | Enantiomers | Diastereomers |
|---|---|---|
| Mirror Image | Yes, non-superimposable mirror images | No, not mirror images |
| Chirality | Requires chirality | May or may not involve chirality |
| Physical Properties | Mostly identical (except optical rotation) | Different properties (melting point, etc.) |
| Examples | (R)- and (S)-lactic acid | Threo- and erythro-2,3-dichlorobutane |
Understanding enantiomers and diastereomers is important in fields like pharmacology, where the orientation of atoms in space can affect how a drug interacts with biological systems.
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