Diastereomers are non-superimposable and non-mirror images of each other. These are the nonidentical stereoisomers. They have the same molecular formula but different orientations of similar atoms/groups in space. Similarly, they have different configurations at stereocenters. Diastereomers must have two or more stereocenters. They have different physical and chemical properties.
Diastereomers are not objects and mirror images of each other. They have different angles of rotations. They have quite different physical and chemical properties. Because of this diastereomers can be easily separated. Similarly, they have two or more stereocenters. Some of these stereocenters have similar configurations while others have different configurations.
Diastereomers have different specific rotations. One of the diastereomers may be chiral. It can rotate plane-polarized light. Others may be achiral and it can not rotate plane-polarized light at all.
Examples of diastereomers
Cis trans isomers are also diastereomers because they are non-mirror images of one another.
For example,
2-3, Dihydroxybutanoic acid(d-1)
Properties of diastereomers
- Diastereomers have different physical properties like refractive index, melting point, boiling points, etc.
- Similarly, they also have different chemical properties.
- They have different rates of reactions.
- Like enantiomers, they also rotate plane-polarized light in opposite directions.
- They have different solubilities.
- Diastereomers have multiple stereocenters. (two or more)
- They can be easily separated by chromatography or crystallization.
- They may be optically active or inactive.
- They have different bond lengths, bond angles, and torsional angles.
Types of diastereomers
Diastereomers can be classified into
- Threo diastereomers
- Erythro diastereomers
- Epimers
Threo diastereomers
A molecule with two or more stereocenters and having similar substituents on opposite sides of a carbon atom. In these configurations, similar groups are at different sides of the Fischer projection.
Fig D-2
Erythro diastereomers
A molecule with two or more stereocenters and having similar substituents on the same side of the carbon atom. In this configuration, similar groups are on the same side of the Fischer projection.
Fig D-3
Epimers
A molecule with two or more chiral centers and differs at only one chiral center. These are mostly carbohydrates that differ in only one chiral center. For example,
Fig D-4
Diastereomeric excess
Diastereomeric excess is analogous to enantiomeric excess. It is the excess of one of the diastereomers over another diastereomer. It is used for quantification purposes. When a sample contains a mixture of diastereomers then these may present in different concentrations. We can find out these concentrations of a diastereomer by using the formula:
formula:
Diastereomeric excess is represented by “de”
Similarly, diastereomeric excess can find out by using another simple formula:
de = % Major diastereomer – % Minor diastereomer
Resolution of Diastereomers
Diastereomers are stereoisomers that are nonidentical, and non-superimposable to each other. They have different physical and chemical properties. Because of this, they can easily be resolved by using different separating techniques like,
- Recrystallization
- Chromatography
- Distillation
Diastereomers vs. Enantiomers
| Category | Enantiomers | Diastereomers |
|---|---|---|
| Definitions | Enantiomers are non-superimposable mirror images of each other. | Diastereomers are non-superimposable, non mirror images of each other. |
| Physical Properties | Similar physical properties | Different physical properties |
| Chemical Properties | Same chemical properties | Different chemical properties |
| Optical Rotations | Opposite optical rotations | Same and different at different stereocenters. |
| Solubilities | They have identical solubilities. | They have different solubilities. |
| Bond length | Enantiomers have the same bond length. | Diastereomers have different bond lengths. |
| Reaction rates | They have the same reaction rates. | They have different reaction rates. |
| Chirality | Enantiomers are chiral compounds. | Diastereomers may be chiral or achiral. |
| Bond angles | They have identical bond angles. | They have different bond angles. |
| Molecular shape | Enantiomers have the same molecular shape and volume. | Enantiomers have different molecular shapes and volumes. |
| Stereocenters | One or more stereocenters. | Two or more stereocenters. |
| Optical activity | Optically inactive | Maybe optically active or inactive. |
| Configurations | Enantiomers have opposite configurations at all chirality centers. | Diastereomers have opposite at some chirality centers while others have similar configurations. |
| IR and NMR spectra | Same IR and NMR spectra. | Different IR and NMR spectra. |
| Free energy | Same free energy. | Different free energy |
| Separations | They can not be separated easily. | They can be separated easily. |
| Examples |
Concepts berg
Write two differences between enantiomers and diastereomers?
Enantiomers are mirror images whereas diastereomers are not mirrored images.
Similarly, enantiomers have the same physical and chemical properties whereas diastereomers have different physical and chemical properties.
Differentiate between meso compounds and diastereomers.
| Meso compounds | Diastereomers |
|---|---|
| Superimposable | Non-superimposable |
| Achiral | Chiral or achiral |
| Mirror images | Non-mirror images |
Why are diastereomers easy to separate?
Diastereomers are very easy to resolve because they have different physical and chemical properties.
What is meant by conformations?
Conformations are different three-dimensional arrangements of atoms due to free rotations of carbon-carbon (C-C) single bond.
How do you identify diastereomers?
Diastereropmers has two oir more steerocenters.
They are non-mirror images of each other.
They are non-superimposable to each other.
What are the different types of diastereomers?
There are different types of diastereomers
- Erythro diastereomers
- Threo diastereomers
- Epimers
What are diastereomers with an example
How do you draw a diastereomer?
What is Tautomerism for example?
What are anomers, enantiomers, and epimers?
Reference Books:
- Principle and Application of Stereochemistry by Micheal North.
- 7th Edition of March’s Advanced Organic Chemistry by Michael B. Smith.
- 7th Edition of Organic Chemistry by John McMurry.
References:
