Enantiomers are stereoisomers that cannot be superimposed on their mirror images. Although these molecules have the same connectivities but have different spatial orientations.
Louis Pasteur was the first to identify enantiomers, and their first definition was proposed by Lord Kelvin. According to him, enantiomers are non-superimposable mirror images. The term enantiomer comes from the Greek word enantio, which means ‘opposite’.
They have the same physical and chemical properties, but they differently rotate the plane-polarized light. Two enantiomers can rotate plane-polarized light equally but in opposite directions. Similarly, they have opposite configurations at all chiral centers.
Enantiomers must have four different atoms or functional groups attached to the central atom.
If identical atoms or groups are present then these two structures would be superimposable to each other as shown below:
Any molecule that is attached to the four different groups can exist as a pair of enantiomers. It has non-superimposable mirror images. Some common examples of enantiomers are given below:
Enantiomers of glucose
The enantiomer of lactic Acid
Nomenclature for enantiomers
The absolute configuration of enantiomers can be explained by using:
- D/L nomenclature system
- R/S nomenclature system
Although D/L is an old system of nomenclature but still most widely used for amino acids and carbohydrates. In order to determine whether a species has a D or L configuration, we have to draw Fischer projections. For carbohydrates other than glyceraldehyde the D/L nomenclature is based on the substituent at the stereocenter farthest from the carbon atoms.
For example, if the amino group attached to the stereocenter is on the left side, the amino acid is in the L series. Similarly, if the amino group is on the right side of the stereocenter then the amino acid would be in the D-series.
R/S system is currently the most preferred system of nomenclature for the enantiomers. According to this system, if a molecule rotates plane-polarized light in a clockwise direction it is of R isomer whereas, if a molecule rotates plane-polarized light in an anti-clockwise direction then it is of ‘S’ isomer.
Properties of Enantiomers
Enantiomers have similar physical properties like melting point, boiling point, refractive index, and density. Similarly, they have the same chemical properties. Some more properties of these compounds are listed below:
- They have identical solubilities.
- Enantiomeric substances have identical bond lengths and bond angles.
- Their torsional angles are of the same energy and magnitude.
- Enantiomers have the same connectivities.
- They have the same IR and NMR spectra.
- These molecules have different orientations of similar functionalities in space.
- They have different optical activities.
- Enantiomers have the same reaction rates.
- They have different properties when they react with chiral substances
- Similarly, they respond differently to the chiral catalyst.
Why do enantiomers have different biological properties?
Enantiomers have the same physical and chemical properties but they have different biological activities. Because they react differently with chiral compounds, such that enantiomers bind to specific receptors in the body and these receptors are chiral in nature. So when they bind to the receptors they show unique behaviors.
The biological properties of enantiomers are explained by the three-point binding model. According to this model, biological receptors for chiral compounds have fixed geometry. In fact, they have certain groups that interact with chiral compounds. These groups have binding sites that interact with chiral compounds sterically or electronically.
Many natural products having a stereocenter can exist as a single enantiomer. i.e, carbohydrates and amino acids. When a chiral compound is eaten, drunk, or smelt the two enantiomers have different biological effects as they are in a different chiral environment. However, different chirality has no effect on their physical properties but their biological properties.
For example, carvone is a naturally occurring terpene. It has two enantiomers; (R)-carvone and (S)-carvone. Both of the enantiomers have the same boiling point, IR spectra, and other physical properties, but they have different biological properties. (R)-carvone has the essence of spearmint oil whereas (S)-carvone has the essence of caraway. The reason for this is their different behavior toward the receptor sites in the nose.
Resolution of Enantiomers
The separation of a racemic mixture into its enantiomeric components is known as resolution. Since enantiomers have the same physical and chemical properties, that’s why they are not easy to separate. Moreover, they have identical behavior in an achiral environment and they behave as optically inactive substances. Anyhow, there are several methods used for their resolution. Some famous methods are given below:
- Biochemical process
- Mechanical separations
- Kinetic methods
- Enzymatic resolution
- Formation of diastereomers
- Selective absorption
- Column chromatography
- Gas chromatography
Enantiomeric purity is also known as enantiomeric excess. It tells us about the concentration of an enantiomer in a given sample. It is the excess of one enantiomer over another enantiomer. When a sample of chiral substance mainly contains one enantiomer but some other enantiomers are also there then this is known as enantiomeric excess. It is used for quantification purposes.
The formula of enantiomeric purity
Enantiomeric excess is represented by “ee”
ee = Percentage of Major enantiomer – Percentage of Minor enantiomer
What are the limitations of the D/L nomenclature system?
- It is only applicable when there exists an oxidized carbon atom.
- It is not applied to all chiral compounds.
- Similarly, it is not applicable to compounds that do not have a stereocenter.
What is meant by racemic mixture?
A racemic mixture is an optically inactive compound having an equal concentration of two enantiomers. It is the equimolar mixture of a pair of enantiomers.
Differentiate between chiral and enantiomer.
Chirality is the property of a molecule, whereas enantiomers describe the comparison of properties of two molecules.
Why do enantiomers have different biological properties?
Enantiomers have different biological properties because the biological environment is also chiral in nature.
It can be defined as compounds having the same molecular formula but different orientations of atoms in space.
How can we identify enantiomers?
We can identify enantiomers as:
- These molecules exist in pairs.
- These are mirror images of each other.
- These images are non-superimposable to each other.
What is meant by optical purity?
Optical purity is the ratio of the observed specific rotation to the specific rotation of the enantiomeric pure sample. It is the comparison of the optical rotation of a pure sample with the optical rotation of a pure enantiomer.
- 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.
- Mechanism of different activities of ofloxacin (pubmed.ncbi.nlm.nih.gov)