Diastereomeric Excess: Definition and Analysis

The diastereomeric excess defines the excess of the diastereomer in a mixture of diastereomers. It is important to understand the definition of diastereomeric for understanding the term diastereomeric excess. The diastereomeric are stereoisomers that are not mirror images of each other and are non-superimposable. However, the compounds have the same molecular formula but have different physical properties and reactivity. The diastereomers occur when two or more stereoisomers of a compound have a different configuration located at one or more stereocenters. Diastereomers not only have different physical properties but even consist of different chemical reactivity. The article provides a comprehensive definition along with diastereomeric excess calculation and ratio. 

Defining the Diastereomeric Ratio 

The diastereomeric ratio is the mass of the diastereomer in excess: the mass of the diastereomer in deficit. The diastereomeric excess is hence represented by de.

de = (m₁ – m₂ / m₁ + m₂). 100 

where  

m₁ – m₂  = mass of the diastereomer in excess

m₁ + m₂ = mass of the diastereomer in deficit 

When there is a 1:1 mixture of two diastereomers, the value of de = 0%, whereas the value of the diastereomeric pure compound, the value of de = 100%

Let’s understand the diastereomeric ratio with the help of an example: 

When the mass fractions of the diastereomers are present of 70% to 30%, there is a 40% diastereomer excess, leading to the production of a diastereomer mixture. For this example, the ratio for the mixture would be dr=7:3

How to Calculate Diastereomeric Excess?

The diastereomeric excess calculation helps with the calculation of measurement of purity used for chiral substances. It measures the degree to which a sample consists of one diastereomer in greater amounts as compared to the other. Diastereomeric excess helps in quantifying the stereoselectivity, which indicates that once a chiral auxiliary is removed, the enantiomeric excess of the product becomes equal to the diastereomeric excess. The following are the formulae used for the diastereomeric excess calculation:

de = [a] – [b] / [a] + [b] where [a] and [b] are two diastereomeric

Diastereomers Excess with Example

Diastereomers excess are stereoisomers different from mirror reflections of one another.

Diastereomers excess are divided into two categories:

• Configurational Diastereomers: A chiral molecule’s non-enantiomeric stereoisomers have a connection.
• Cis-Trans Diastereomers: These stereoisomers form when a molecule’s rotation is limited, most commonly at a carbon-carbon double bond.

Stereoisomers of a material that are not mirror reflections of each other are called diastereomers. Epimers are diastereomers with only one stereocenter separation between two or more stereocenters. Two examples of epimers are D-glucose and D-galactose – they are each other’s diastereomers and epimers.

C₄H₆O₆ is an excellent example of tartaric acid’s diastereomers and enantiomers. The structure of tartaric acid is fascinating to explore. Tartaric acid is the most prevalent kind (R, R). It can, however, be produced in the meso form artificially (R, S). Because (R, R) tartaric acid is the antithesis of (S, S) tartaric acid, they are enantiomeric.

Characteristics of Diastereomers Excess

• The physical properties and reactivity of diastereomers vary. They have different melting and boiling temperatures, as well as densities.
• They have two stereocenters or more.
• Other diastereomers may or may not be optically active, except for geometrical isomers.
• The chemical properties of diastereomers excess are similar but not identical. If the solution is not rapidly active, the rates at which the two diastereomers react with it.
• Because of changes in their physical properties, diastereomers can be separated using techniques like fractional crystallisation, fractional distillation and chromatography. On the other hand, enantiomers cannot be separated using these procedures.
• A molecule with a single asymmetric carbon atom or stereocenter has two mirror-image forms. A molecule containing two asymmetric carbons can have up to four possible configurations, none of which can be mirror reflections of each other. As a molecule’s number of asymmetric centres grows, the options expand.
• Tartaric acid has two asymmetric centres, yet two of them are equal, resulting in a meso complex. The remaining two isomers are D- and L- mirror images, i.e. enantiomers, even though this structure is not optically active. The diastereomer of the other forms is the meso form.

Difference between Diastereomer and Enantiomer

Conclusion 

The term enantiomeric excess has existed for a long time and describes the overall enantiomeric composition and calculating the optical purity. However, the diastereomeric excess is used for qualitative stereoselectivity, and it is the excess of the diastereomers present in a mixture. Nowadays, however, the term diastereomeric excess is also replaced by the term diastereomeric ratio, which helps in the characterisation of the mixture. It helps in finding out which out of the diastereomer is present in excess. The topic gives an understanding of enantiomers and diastereomers, its difference, and their purpose. In summary, the enantiomers are mirror images that are non-superimposable, whereas the diastereomers are non-mirror images which non-superimposable.