Diastereomers are compounds that can be defined as having similar sequence of bonded elements and same molecular formula
They are however non superimposable and non-mirror images.
Diastereomers together with enantiomers are generally stereoisomers, they categorise under the broader view of isomers, and shows a comparison of at least two species.
The constitution with respect to Stereoisomers are identical are molecules of identical constitution but nevertheless different. Differences between diastereomers can be expressed in scalar terms, that is by the difference in the distances between certain characteristic pairs of atoms.
Stereoisomers are not in relation as an object and its mirror image are diastereomers; they are stereoisomers that are not mirror images.
Let us consider the formula of 2,3-dichloropentane which contains two chiral centres. Now compare the two forms of 2,3-dichloropentane given below.
It becomes immediately clear that the two formulae have identical configurations about one chiral carbon C2 and mirror image configurations about the other chiral carbon (C3). The net result is that the two forms are neither identical nor mirror images of each other. Such stereoisomers of a substance that are not mirror images of each other are diastereomers.
Characteristics of Diastereomers
- they have different physical properties such as melting points, boiling points, densities, solubilities, refractive indices, dielectric constants and specific rotations. Enantiomers have similar physical properties except the opposite sign of specific rotation.
- Diastereomers other than geometrical isomers may or may not be optically active.
- they show similar, but not identical chemical properties. The rates of reactions of the two diastereomers with a given reagent provided the reagent is not rapidly active.
- On account of differences in their physical properties, they can be separate from one another through techniques like fractional crystallization, fractional distillation, chromatography etc. The difference from enantiomers which can’t be separated by these techniques
If there is more than one chiral center in a molecule, the risk of making stereoisomers not mirroring each other’s images. These stereoisomers are diastereomers, which are not mirror images. Typically, they can only be detected when the molecule has two or more chiral centres, and are often chiral, and distinct from each other. Remember that pairs of diastereomers exist, and each has two chiral centres
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