Diastereomer

A diastereomer is a stereoisomer that is not an enantiomer. Stereoisomers are molecules with the same molecular formula and sequence of bonded atoms (constitution) but differ in the three-dimensional orientations of their atoms in space. Unlike enantiomers, which are non-superimposable mirror images, diastereomers are not mirror images of each other. Diastereomers can have different physical and chemical properties, such as melting point, boiling point, solubility, and reactivity. They arise when a molecule contains two or more chiral centers, and the configuration at at least one, but not all, of the chiral centers differs.

Diastereomer meaning with examples

• The (2R,3R) and (2S,3R) isomers of tartaric acid are diastereomers. These molecules have two chiral centers and are not mirror images of each other. The difference in their spatial arrangement leads to varying physical properties, like their melting points and how they interact with polarized light. Understanding the different configurations is crucial in fields like drug design where the diastereomers can have significantly different effects.
• Cis- and trans-isomers of substituted cycloalkanes are common examples of diastereomers. In these isomers, the substituents on the ring can be on the same side (cis) or opposite sides (trans), giving different three-dimensional arrangements. For instance, in 1,2-dimethylcyclohexane, the cis and trans forms are diastereomers and exhibit distinct chemical and physical properties. The spatial orientation greatly affects their reactivity and interactions.
• Consider the four possible stereoisomers of 2-bromo-3-chlorobutane. Some are enantiomers, mirror images that cannot be superimposed, while others, that is, the (2R,3R) and (2R,3S) forms, are diastereomers. The latter are stereoisomers which are not mirror images and can differ in their chemical reactivity. Understanding the relationships is essential for chemical synthesis and reactions.
• In sugars, such as glucose, epimers are a specific type of diastereomer that differ in configuration at a single chiral center. For example, D-glucose and D-galactose are epimers. Though very similar structurally, this seemingly small change in configuration can have significant consequences, influencing their biological activity and their different metabolism. These nuances are crucial for biological applications.
• The erythro and threo forms of 2,3-dibromobutane exemplify diastereomers. These molecules have two chiral centers, which can give four stereoisomers: two enantiomeric pairs. The erythro and threo forms are not mirror images and are not superimposable, representing the diastereomeric pair. This distinct arrangement affects physical properties and reactivity. Thus, the detailed understanding of stereoisomers is vital for understanding the properties.