Difference Between Enantiomers and Epimers

Main Difference – Enantiomers vs Epimers

Stereoisomers are molecules that have the same molecular formula but different spatial arrangements. Enantiomers and epimers are optical isomers. Optical isomers are a subclass of stereoisomers. They are able to rotate the plane polarized light. In order to become stereoisomers, there should be at least one chiral carbon in molecules. In other words, a molecule with a chiral carbon can have stereoisomers due to different spatial arrangements of other groups attached to the chiral carbon. One molecule can have more than one chiral carbon. The main difference between enantiomers and epimers is that enantiomers are mirror images of each other whereas epimers are not mirror images of each other.

Key Areas Covered

1. What are Enantiomers
      – Definition, Basic Structure, and Examples
2. What are Epimers
      – Definition, Basic Structure, and Examples
3. What are the Similarities Between Enantiomers and Epimers
      – Outline of Common Features
4. What is the Difference Between Enantiomers and Epimers
      – Comparison of Key Differences

Key Terms: Chiral Carbon, Chirality, Diastereomers, Enantiomer, Epimers, Epimerization, Optical Isomers, Stereoisomers

What are Enantiomers

Enantiomers are optical isomers that are non-superimposable mirror images of each other. Therefore, these molecules are always found in pairs. Since they are non-superimposable, the two molecules are not identical. But the molecular formula of the two enantiomers is the same. They are different from each other according to the spatial arrangement of the molecules.

Enantiomers have the same physical and chemical properties except for the direction they rotate the plane polarized light. They rotate plane polarized light in opposite directions. Therefore, a mixture of enantiomers having equal amounts of two enantiomers will not show a net rotation in the plane polarized light. This type of mixture is called a racemic mixture.

Main Difference - Enantiomers vs Epimers

Figure 01: Enantiomers of 2-butanol. Here the + and – marks are used to indicate the opposite directions in which the plane polarized light is being rotated by each molecule.

Enantiomers have chiral carbon atoms. A chiral carbon is a carbon center that is bonded to four different atoms or groups. The presence of a chiral carbon (in a molecule) is called chirality. In order to become an enantiomer, two molecules should have different configurations at every chiral carbon. For example, if a molecule has two chiral carbons and another molecule has the same molecular formula with two chiral carbons, the two molecules should be different at both chiral carbons, not just at one chiral carbon.

What are Epimers

Epimers are stereoisomers that contain more than one chiral carbon but differ from each other in the configuration at only one chiral carbon. Therefore, they are not mirror images of each other. Epimers are a subclass of diastereomers. Therefore, epimers are also optical isomers. This means epimers can rotate plane polarized light.

Formation of epimers is called epimerization. This epimerization process forms epimers by changing a chiral carbon in a molecule which has several chiral carbons. Since the difference in two epimers is in one chiral carbon, this carbon is called epimeric carbon.

Difference Between Enantiomers and Epimers

Figure 2: D-glucose is an epimer of D-mannose.

As shown in the above image, D-glucose and D-mannose are epimers of each other. This is because they differ from each other at one carbon atom (epimeric carbon) while the rest of the configuration of molecules are identical.

Similarities Between Enantiomers and Epimers

Enantiomers and epimers are stereoisomers.
Both are types of optical isomers.
Both enantiomers and epimers show chirality.

Definition

Enantiomers: Enantiomers are optical isomers that are non-superimposable mirror images of each other.

Epimers: Epimers are stereoisomers that contain more than one chiral carbon but differ from each other in the configuration at only one chiral carbon.

Chiral Carbons

Enantiomers: Enantiomers are different from each other at every chiral carbon.

Epimers: Epimers are different from each other at only one (or few) chiral carbons, but not all.

Mirror Images

Enantiomers: Enantiomers are non-superimposable mirror images of each other.

Epimers: Epimers are not mirror images of each other.

Properties

Enantiomers: The physical and chemical properties of enantiomers are the same except for the rotation of plane polarized light.

Epimers: The physical and chemical properties of epimers are different from each other.

Conclusion

Enantiomers and epimers are stereoisomers. Enantiomers are pairs of molecules that are different from each other only according to the spatial arrangement of the atoms or groups around a chiral carbon in those molecules. Epimers are different from each other in their configurations at only one chiral carbon. The main difference between enantiomers and epimers is that enantiomers are mirror images of each other whereas epimers are not mirror images of each other.

References:

1. Hunt, Dr. Ian R. “Enantiomers.” Ch 7: Enantiomers, Available here. Accessed 18 Aug. 2017.
2. “Epimers.” OChemPal, Available here. Accessed 18 Aug. 2017.

Image Courtesy:

1. “Enantiomers of 2-Butanol by + and – labels” by HGTCChem . (CC BY 2.0) via Flickr
2. “Epimers-Glucose Mannose” By Mlicuana – Own work (Public Domain) via Commons Wikimedia