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Enantiomers And Diastereomers

In Education
June 10, 2020
Enantiomers And Diastereomers

Enantiomers and diastereomers, commonly called stereoisomers, fall under the broader concept of isomerism, which always involves the comparison of at least two species. The enantiomers of a compound have the same type of chemical bonds with corresponding polarities, internuclear distances and bond angles. So they have identical physical properties in nuclear distances and bond angles. So they have identical physical properties except for the direction of rotation of the plane of polarised light. They have identical chemical properties too except the difference towards optically active reagents.

Stereoisomeric compounds that are not enantiomers are called diastereomers. Diastereomers are basically stereoisomers that are not enantiomers of each other. Although a molecule may have only one enantiomer, it may have several diastereomers. Diastereomers differ in energy content, and thus in every physical and chemical property however the differences may be so minute as to be nearly indistinguishable. Stereoisomers with low energy barriers to this conversion are called conformational isomers, whereas conversions with high energy barriers are defined as configuration isomers.

Properties of Enantiomers

Enantiomers have identical physical properties. So they cannot be distinguished based on their melting points, boiling points and densities. But enantiomers do differ in terms of optical activity. Enantiomers rotate plane polarized light with the same magnitude but in opposite directions. Because enantiomers have identified physical properties, separation of enantiomers using conventional methods such as simple distillation and recrystallization is not possible.

Enantiomers react the same way with achiral molecules but react differently with chiral molecules. This difference in reactivity of enantiomers toward chiral molecules is utilized in separating enantiomers. The process is called resolution. The enantiomeric mixture is reacted with a chiral molecule to form a pair of diastereomers. Because diastereomers have different physical properties they can easily be separated.

Characteristics of Diastereomers

The important characteristics of diastereomers are as follows.

  • Chemically similar but distinguishable, diastereomers have different melting points and boiling points and react at different levels. Diffraction A wave’s tendency to bend around the edges of barriers or gaps.
  • They show similar but non-identical, chemical properties. In the reactions of two diastereomers with a given reagent, the rates of reactions are generally different.
  • They have different physical properties such as melting point boiling points, densities, solubilities and refractive indices. Even specific rotations are different; this does not, of course, apply to diastereomers which are geometrical isomers since such diastereomers are not optically active.
  • On account of differences in physical properties, they can be rather easily through fractional crystallization, fractional distillation, chromatography etc.

However, two stereoisomers may not be both enantiomers and diastereomers of each other at once. The amounts of energy necessary to convert given stereoisomers into their isomeric forms may be used for their classification. 

Stereoisomerism can result from a variety of sources including the single chiral carbono, for example a chiral atom, which is a tetrahedral atom with four different substituents. It is not necessary for a molecule to have a chiral carbon in order to exist in enantiomeric forms but it is necessary that the molecule as a whole be chiral. Stereoisomerism is possible in molecules that have.