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Diastereomers are compounds that contain the same molecular formula and the sequence of bonded elements but are not superimposable and non-mirror images. Enantiomers and diastereomers are commonly called stereoisomers which always involve the comparison of at least two species. Stereoisomers are molecules of identical structure but are quite different. Differences between diastereomers can be conveyed in scalar terms i.e. the differences between the distances of certain characteristic pairs of atoms.
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Key Terms: Diastereomers, Stereoisomers, Enantiomers, Molecules, Melting Point, Boiling Point, Threo Diastereomers, Isomers, Optical Rotation
What are Diastereomers?
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Stereoisomers are molecules of the identical constitution but different. Stereoisomers that are not connected to an object and its mirror image are termed diastereomers. For a better understanding, consider a 2-3 dichloropentane formula that contains two chiral centres. Compare the two forms of dichloropentane given below:
Diastereomers
It becomes clear that the two formulae have identical configurations of one chiral carbon C2 and mirror-image configurations of the other chiral carbon C3. The net result is that the two forms are neither identical nor mirror images of the other. Such stereoisomers that are neither identical nor mirror images are diastereomers.
3-chloro-2-butanol provides another example of a compound having two different chiral carbon atoms and can exist in four stereoisomeric forms as given below.
Diastereomers
From the image above it can be concluded that images 1 and 3, 1 and 4, 2 and 3, and 2 and 4 represent pairs of diastereomers.
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Characteristics of Diastereomers
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- They have different physical features such as densities, solubilities, refractive indices, melting point, boiling point, dielectric constants and specific rotations. Enantiomers have similar physical properties except for the opposite sign of specific rotation.
- Diastereomers other than geometric isomers may or may not be optically active.
- Diastereomers show similar but not identical chemical features. The rates of reaction of the two diastereomers with a given reagent provided the reagent is not promptly active.
- Diastereomers can be separated from one another through techniques like fractional crystallization, fractional distillation, chromatography etc, based on their physical properties.
Chemical Properties of Diastereomers
Different chemical properties are displayed by diastereomers, owing to their differences in structural arrangement and orientation of atoms in space. Stereo-selectivity is quite noteworthy when it comes to isomers.
Stereo-selectivity indicates the type of reaction where the formation of one stereoisomer is preferred over the other. When a diastereomer reacts with some specific reagents, the products formed are a pair of diastereomers. One diastereomer will be formed in preference over the other even here. Such a reaction will then be known as a diastereoselective reaction.
Erythro and Threo Diastereomers
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Diastereomers are either geometric isomers or compounds consisting of two or more chiral centres. A diastereomer is referred to as erythro if its Fischer projection shows similar groups on the same side of the molecule. It is called the if similar groups are on the opposite sides of the Fischer projection.
For example, hydroxylation of trans-crotonic acid provides two enantiomers of the three-2,3,-dihydroxybutanoic acid whereas the same reaction with cis-crotonic acid gives the retro enantiomers.
Every pair of threo is called a diastereomer of each pair of erythro isomers. The terms erythro and three are generally used only with molecules that do not have symmetric ends.
Properties of Enantiomers and Diastereomers
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Enantiomers have identical chemical and physical features, except for optical rotation. Compared to enantiomers diastereomers have different physical properties and sometimes different chemical properties too. The table below illustrates the similarities and differences in properties of enantiomers and diastereomers. The comparison of the features of enantiomers and diastereomers are
| Property | Enantiomers | Diastereomers |
|---|---|---|
| Melting Point | Identical Melting Point | Different Melting Point |
| Boiling Point | Identical Boiling Point | Different Boiling Point |
| Solubility | Same | Different |
| Optical Rotation | Same, but opposite sign different-different | Distinct values may have the same or opposite sign |
| Chemical Properties | Similar | May be different |
| Free Energy | Same | Different |
Things to Remember
- Diastereomers are compounds that contain the same molecular formula and the sequence of bonded elements but are not superimposable and non-mirror images.
- A diastereomer is referred to as erythro if the Fischer projection shows similar groups on same side of the molecule.
- It is referred to as threo if similar groups are on the opposite sides of the Fischer projection.
- Enantiomers have identical chemical and physical features, except optical rotation.
- Compared to enantiomers diastereomers have different physical properties and sometimes different chemical properties too.
- Diastereomers other than geometric isomers may or may not be optically active.
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Sample Questions
Ques. What is the difference between enantiomers and diastereomers? (3 Marks)
Ans. Enantiomers have identical chemical and physical features, except optical rotation. Compared to enantiomers diastereomers have different physical properties and sometimes different chemical properties too. Enantiomers have the same melting point, boiling point, solubility, chemical properties, and free energy. Diastereomers have different melting points, boiling points, solubility, chemical properties and free energy
Ques. What are diastereomers? Explain by an example. (3 Marks)
Ans. Diastereomers often include compounds which are ring structures. For instance, two compounds with a six-membered ring, each with two substituents, a chlorine atom, and an ethyl group. They are also not mirrored images of each other which defines them as diastereomers.
Ques. Are enantiomers optically active? (3 Marks)
Ans. Yes. All enantiomers are engaged with optical effects. Optical behaviour is also one of the concepts of enantiomers because enantiomers are molecule isomers which are non-super-imposable mirror images of each other and rotate light in opposite directions.
Ques. Are diastereomers always chiral? (3 Marks)
Ans. Diastereomers are frequently chiral and distinct from each other. Pair of diastereomers exist, and each one has two chiral centres. The chirality of one of them would be “R, S” in the original classic diastereomer, and the other would be “R, R.”
Ques. Which are two types of stereoisomers? (2 Marks)
Ans. Diastereomerism (including ‘cis-trans isomerism’) and Optical Isomerism (also known as ‘enantiomerism’ and ‘chirality’) are the two primary forms of stereoisomerism.
Ques. How do you identify diastereomers? (3 Marks)
Ans. If there is more than one chiral centre in a molecule, one has the risk of making stereoisomers not mirroring each other’s images. These stereoisomers are called diastereomers, which do not mirror images. Typically, diastereomers can only be identified when the molecule has two or more chiral centres.
Ques. Are D and L enantiomers? (3 Marks)
Ans. The enantiomers D and L refer to the molecule’s configurational stereochemistry. L isomers have the hydroxyl group placed on the left side of the carbonyl asymmetric carbon furthest, while D isomers have the hydroxyl group on the right side.
Ques. What are erythro diastereomers? (3 Marks)
Ans. Diastereomers are either geometric isomers or compounds consisting of two or more chiral centres. A diastereomer is called erythro if its Fischer projection shows similar groups on the same side of the molecule
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