Diastereomers- Stereoisomerism, Definition and Explanation of Diastereoisomers, Threo and Erythro Forms, Characteristics, Practice Problem and FAQs

As kids, we were excited to watch cartoons, isn't it? Every kid's favorite is ‘Pokemon’.

For 90s kids, hearing the word ‘Pokemon’ is exciting!

Why are we talking of a 90s cartoon now? Does it have any relevance here?

Eevee is one of the Pokemons which can transform itself into eight different types of pokemon.

Eevee can evolve into vaporeon, a water-type pokemon, or into flareon, a fire-type pokemon. So, both vaporeon and flareon are evolutions of Eevee, but they are of different types.

Similarly in chemistry, there are molecules which have the same molecular formula but are not mirror images and are non-superimposable. These molecules are called ‘diastereomers’

So, let’s forget pokemon for a while and concentrate on diastereomers in this concept page article.

Table of Content

• Introduction to Stereoisomerism
• Definition and Explanation of Diastereomers
• Threo and Erythro forms
• Characteristics of Diastereomers
• Practice Problems
• Frequently Asked Questions

Introduction to Stereoisomerism

Stereoisomers and enantiomers fall under the broader concept of isomerism, in which compounds have the same molecular formula but differ in their orientations in 3-D space.

Stereoisomers are isomers that are formed by changing the orientation of an atom or group attached to carbon in three dimensions without changing the bonding pattern and the process of formation of stereoisomers is known as stereoisomerism.

Let us consider an example cis- but-2-ene and trans-but-2-ene both the molecule have the same formula but the orientation of the -(CH3)- group is different though the bonding pattern is the same. Therefore, both together constitute stereoisomerism

Note: Cis and trans isomers is classified as geometrical isomers which is a type of stereoisomers.

Definition and Explanation of Diastereomers

Diastereoisomers are the types of stereoisomers which are not the mirror image of each other. In the other words, those stereoisomers which are both non-superimposable and non-mirror images of each other are known as diastereomers.

Let us consider two molecules with the molecular formula C5H10Cl2 in the Fischer projection as shown in the diagram given below.

It can be seen in the structure of the compound that in the 1st compound on the right the chlorine atom is present on the right side attached to both the 2nd and 3rd chiral carbon whereas, in the 2nd compound on the left, the chlorine atom is present on the right side attached to the 2nd chiral carbon and left side attached to the 3rd chiral carbon respectively.

Therefore, it can be concluded that the position of the 2nd chiral carbon is the same in both the compound but in the 3rd chiral centre the position of the chlorine atom changes and constitute the diastereomeric pair as it is not the mirror image of each other.

Diastereomeric pairs can also be identified on the basis of R-S configuration along the chiral carbon. R-S configuration can be determined on the basis of the Cahn Ingold Prelog(CIP) priority rule which helps to determine the priority of the group attached along the chiral centre present in the molecule. And once the priority of the group is decided then on the basis of rotation of the group (Higher priority to lower priority) the R-S configuration is determined. According to the rule, the lowest priority group should present the vertical line but if it is present along the horizontal line the configuration is reversed.

In the case of diastereomers, the configuration along at least one configuration should be the same as well as different along the chiral carbon present in two different molecules.

In the compound present on the left side, the configuration along the 2nd and 3rd chiral carbon is (S, S). Whereas, for the compound present on the right side the configuration along the 2nd and 3rd chiral carbon is (S, R) respectively. As the configuration changes along one chiral carbon and along the other chiral carbon configuration remains the same, both molecules represent the isomers

Let us consider one more example of the molecules given below in which the orientation of the group is different as both the methyl group in the 1st molecule is on the same side and is known as cis form. In the 2nd molecule, the methyl group is on the opposite side and is known as the trans-form. Both cis and trans-form are geometrical pairs which do not form mirror images of each other therefore it is also known as diastereomeric pair.

Note: Geometrical isomers are also diastereomers as the molecule is a non-mirror image of each other.

Threo and Erythro form

Stereoisomers that are not superimposable mirror images are referred as Diastereomers. Diastereoisomers are either geometrical isomers or optical isomers having more than one chiral centres. The stereo isomers having similar groups on the same side in a Fisher projection is known as erythro form. If the similar groups, instead are present on opposite sides in the Fischer projection, it is named threo form.

Hydroxylation of Trans-crotonic acid forms enantiomers of threo-2,3-dihydroxybutanoic acid, while hydroxylation of cis-crotonic acid gives erythro diastereoisomer..

Each threo isomer is a diastereomer of each erythro isomer. The terms erythro and threo are generally reserved for molecules with asymmetric ends. Let us consider the examples,

In the compound present on the left-hand side both the -(OH)- group are present on the same side along the 2nd and 3rd chiral carbon and is known as erythro form. Whereas, in the case of compound present on the right-hand side the -(OH)- group are present on the opposite side along the 2nd and 3rd chiral carbon and is known as the threo form. Both erythro and threo forms are diastereomers as there is a change in orientation along only one chiral carbon while in other chiral carbon configuration remains the same.

In the compound present on the left-hand side, both the -(NH2)- group are present on the same sides along the 2nd and 3rd chiral carbon and is known as erythro form. Whereas, in the case of compound present on the right-hand side the -(NH2)- group are present on the opposite sides along the 2nd and 3rd chiral carbon and is known as the threo form. Both erythro and threo forms are diastereomers as there is a change in orientation along only one chiral carbon while in other chiral carbon configuration remains the same.

Characteristics of diastereomers

• Diastereomers exhibit difference in their physical properties of melting and boiling points, densities, solubilities, polarity, that can be used for their separation..
• Other than geometrical isomers, diastereomers may or may not be optically active.
• Diastereomers have chemical properties that are similar but not identical. If the reagent is not rapidly active, the rates of reaction of the two diastereomers with it.
• Diastereomers can be separated from one another due to differences in their physical properties using techniques such as fractional crystallization, fractional distillation, chromatography, and so on.

Practice problems

Q1. Select the correct relationship between D-tartaric acid and meso tartaric acid.

1. Diastereomers
2. Geometrical isomers
3. Enantiomers
4. Identical compound

Answer: (A)

Solution: D- tartaric acid is the optically active compound in which the -(OH)- group are present in the opposite direction attached to the chiral centre but in case of meso compound the -(OH)- group is present in the same side attached to the chiral centre and is optically inactive in nature. Meso can be obtained by changing the orientation of the valencies present along any one chiral centre present in the molecule. Therefore, D- tartaric acid and meso tartaric acid are diastereomers to each other.

Q 2.  The relationship between cis but-2-ene and trans-but-2-ene molecules are:

1. Geometrical isomers
2. Diastereomers
3. Enantiomers
4. Both A and B

Answer: (D)

Solution: Cis but-2-ene and trans-but-2-ene molecules are both geometrical isomers as well as enantiomers because it is not the mirror image of each other and both the compound have a different orientation in the space along the rotation restricted double bond present between carbon atoms.

Q3. Which of the following option is correct with respect to diastereomers?

1. Diastereomers have similar physical properties like- melting point, boiling point, solubility etc
2. Diastereomers can be separated from one another using techniques such as fractional crystallization, fractional distillation and chromatography
3. If there is only one chiral centre in the compound, then the mirror image of the compound will be diastereomer
4. Cis-trans isomers is classified as geometrical isomers but not diastereomers as there is no chiral centre present in the compounds.

Answer: (B)

Solution: Diastereomers have different physical properties. Different physical properties of diastereomers include melting and boiling points, densities, solubilities, refractive indices, dielectric constants, and specific rotations and can be separated by using techniques such as fractional crystallization, fractional distillation and chromatography. Cis-trans isomers are classified as both geometrical isomers as well as diastereomers as it is not a mirror image of each other.

If a compound contains only one chiral centre then the mirror image of the compound will result in the formation of a non-superimposable mirror image of each other known as enantiomeric pair and not diastereomers. Therefore, option(B) is correct.

Q4. Select the pair of compounds which does not represent the diastereomeric pair.

Both A and B are correct
Answer: (D)
The pair compounds given in options (A) and (B) are enantiomers as they are the non-superimposable mirror image of each other. The compound represented in option (C) is geometrical isomers as well as the diastereomers as it is not a mirror image of each other and both the compound have a different orientation in the space along the rotation restricted double bond present between carbon atoms. Therefore, option (D) is the incorrect option.

Frequently asked question-FAQs

Q1. What is the difference between enantiomers and diastereomers?
Answer: Optical isomerism is defined by the formation of non-superimposable mirror images. Non-superimposable mirror images form when chemical structures with the same molecular formula differ in the spatial arrangement of atoms. Enantiomers are mirror images that cannot be superimposed on one another. Whereas, diastereoisomers are the type of stereoisomers which are not the mirror image of each other. In the other words, those stereoisomers which are non-super impossible but not the mirror image of each other are known as diastereomers.

Q2. What are optically active compounds?
Answer: Enantiomers are asymmetric optically active compounds that can rotate plane polarised light when allowed to pass through it in the polarimeter tube either in a clockwise direction or anticlockwise direction and are not superimposable mirror images of each other.

Q3. What is Fischer's projection of a compound?
Answer: Fischer projection refers to the technique of presenting three-dimensional organic molecules in two-dimensional structures on a two-dimensional plane such as paper. Horizontal and vertical lines represent the bonds, and the intersection of two horizontal and vertical lines represents the central carbon atom. An example of the Fisher projection of meso-tartaric acid is given below.

Q4. What is the CIP priority rule?
Answer: The CIP rule specifies the criteria for prioritizing atoms/groups attached to a chiral centre. The following rules are used to assign atoms or groups in decreasing order of priority:

Rule 1: Priority of the group is directly proportional to the atomic number of directly attached atom

Rule 2: If directly attached atoms are the same then the priority of the group is decided by the atomic number of next atom and so on.

Rule 3: In the case of isotopes priority of the group is directly proportional to atomic mass

Rule 4: In the case of multiple bonds the bond is assumed to be converted into a single bond and the same atom with the number equal to the number of multiple bonds is added to it.

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