Diastereomers: Definition and Explanation
Diastereomers are a type of stereoisomer where the isomers have the same molecular formula and connectivity of atoms, but differ in the spatial arrangement of their atoms. Unlike enantiomers, which are non-superimposable mirror images of each other, diastereomers are not mirror images and do not exhibit identical physical properties. They can differ in properties such as melting points, boiling points, and reactivity. Diastereomers typically arise when a molecule has two or more chiral centers.
Threo and Erythro Diastereomers
Threo and erythro are terms used to describe specific diastereomers that arise from the stereoisomerism of compounds with two adjacent chiral centers, especially in the context of compounds with two identical substituents on adjacent carbon atoms (i.e., diastereomers in the case of compounds with two chiral centers and a symmetrical or asymmetrical structure).
1. Erythro Diastereomers
The term erythro is used when the two identical substituents (such as two hydroxyl groups or two methyl groups) are positioned on the same side of the plane of the molecule (i.e., the substituents are on the same face of the molecule). This results in the same side configuration around the chiral centers.
For example, consider the compound 2,3-butanediol:
- Erythro-2,3-butanediol has both hydroxyl groups (-OH) on the same side of the molecule (either both in the cis arrangement around the carbon-carbon bond).
In this case, both hydroxyl groups would be on the same side of the molecule's plane, leading to the erythro diastereomer.
2. Threo Diastereomers
The term threo is used when the two identical substituents are positioned on opposite sides of the plane of the molecule (i.e., the substituents are on opposite faces of the molecule). This results in the opposite side configuration around the chiral centers.
For the same compound 2,3-butanediol:
- Threo-2,3-butanediol has the hydroxyl groups on opposite sides of the molecule (i.e., in the trans arrangement around the carbon-carbon bond).
In this case, one hydroxyl group would be positioned above the plane, while the other would be positioned below, forming the threo diastereomer.
Example: 2,3-Butanediol
Let's consider the two diastereomers of 2,3-butanediol:
Erythro-2,3-butanediol: Both hydroxyl groups are on the same side (cis configuration). This is the diastereomer where the two identical groups (hydroxyl groups) are positioned in the same direction.
Threo-2,3-butanediol: The hydroxyl groups are on opposite sides (trans configuration). This is the diastereomer where the two identical groups are positioned in opposite directions.
Conclusion
Diastereomers are stereoisomers that are not mirror images of each other. The terms erythro and threo are used to describe specific diastereomers in molecules that contain two chiral centers with identical substituents. In the case of 2,3-butanediol, erythro indicates that the substituents are on the same side of the molecule, while threo means they are on opposite sides. These terms are important for distinguishing between different stereoisomers and understanding their spatial arrangement in 3D structures.
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