Diastereomers | Explanation with Example

This blog will provide you detailed information about diastereomers (sometimes called diastereoisomers), their properties and detailed explanation of them with some examples. After this, you will be able to distinguish diastereomers. So, let’s start with their introduction.

Introduction of Diastereomers

Stereoisomers have same chemical formula, structural formula and same sequence of connectivity but differ in arrangement of atoms in space. They occur when two or more stereoisomers of a compound have different configuration. We can categories stereoisomers into three types. Enantiomers, Diastereomers and conformers. Diastereomers are a type of stereoisomers.

Definition

All optical isomers that are not mirror images are called diastereomers. They are non-identical stereoisomers. They are also called as Diastereoisomers. They are neither enantiomers nor conformers.

Explanation with Example

3-bromo-2-butanol

Diastereomers of 3-bromo-2-butanol
Diastereomers of 3-bromo-2-butanol

Explanation of diastereomers in 3-bromo-2-butanol:

As we can see, first two structures are enantiomers of each other and last two structures are enantiomers of each other. If we place a mirror between first and second structure, they will be mirror image of each other and superimposable. And same for third and fourth structure.

If we look at the direction of 1st structure, it is clockwise (+ve). If we look at the direction of 2nd structure, it is anticlockwise (-ve). And if we look at the direction of 3rd structure, it is anticlockwise (-ve). Have a look at the direction of 4th structure, it is clockwise (+ve). (This rotation is decided by D and L conventions)

The four possible combinations of these diastereomers are:

  • (SS) 2S,3S-3-Bromo-2-butanol
  • (RR) 2R,3R-3-Bromo-2-butanol
  • (SR) 2S,3R-3-Bromo-2-butanol
  • (RS) 2R,3S-3-Bromo-2-butanol These all four structures are diastereomers of 3-bromo-2-butanol.

If we compare:

1st with 4th, 1st with third, 2nd with fourth, second with third;

  • They are not a mirror image to each other.
  • They are non super-imposable to each other.
  • Hence, they are known as diastereomers.
NOTE:
     If we have more than two chiral centers for example three chiral centers with configuration RRR, then the configuration of any two of these three chiral centers will be changed (eg: RSS or SSR or SRS) not the configuration of all three have to be changed. Configuration of any one chiral carbon have to be same with parent compound. 
Let's have a look at another example to understand this.
Diastereomers of Aldopentose
Diastereomers of Aldopentose

Four diastereomers are possible for Aldopentose. All four diastereomers have different arrangement of molecules in space. They all are non mirror images of each other and they do not superimpose each other. First diastereomer of aldopentose contain 2R, 3R, 4R configuration. Second diastereomer of aldopentose contain 2S, 3R, 4R configuration. Third diastereomer of aldopentose contain 2R, 3S, 4R configuration. Fourth diastereomer of aldopentose contain 2, 3S, 4R configuration.

Diastereomers take place when two or more stereoisomers of a compound have different configuration at one or more of the equivalent stereocenters and are not mirror image of each other. They do not take place when all the stereoisomers have different configuration. When two diastereomers differ from each other at only one stereocenter called epimers. Each stereocenter gives rise to two different con figurations.

Some other Examples of diastereomers

Simple example of diastereomers

If we have a look at these compounds, they both are diastereomers of each other. Only one position is changed (from R to S). In first diastereomer, Bromine is attached with a dash bond which is going away from the observer while in second diastereomer, Bromine is attached with a solid wedge bond which is going towards observer. Position of hydroxyl group is same in both diastereomers (S,S).

Cis-trans Diastereomers

Have a look at first example, now you can easily understand that how position of Ch3 group is interchanged and the position of first hydrogen is same.

In next example shown above, you can see the attachment of one hydroxyl group in first diastereomer is with a wedge bond while in second it is connected with a dash bond.

Now you have a task; 
Why and how these above compounds are diastereomers? Show some potential in comments section. 

Properties of diastereomers

  • They are as non mirror images.
  • Diastereomers have different physical properties.
  • They have different chemical reactivity.
  • They have same structural and chemical formula.
  • Diastereomers show similar chemical properties.
  • They have different free energy.
  • Diastereomers have different solubility.
  • They have different arrangement of atoms in space.
  • Diastereomerism can also take place at a double bond.
  • Many conformational isomers are diastereomers as well.
  • They can be separated from one another through techniques like fractional crystallization, fractional distillation, chromatography etc.
  • They have different melting points, boiling points, densities, refractive indices, dielectric constants and specific rotations.

Conclusion

So, Diastereomers are not mirror images, non superimposable and optical active compounds with different configuration (R and S) at two or more chiral centers but not all. They have same structural, chemical and molecular formula. The base on which we distinguish them is the arrangement of their molecules in space. They have different melting points, boiling points and densities. They can be separated by fractional distillation and chromatographic techniques.

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