Difference Between Anomers and Epimers

Main Difference – Anomers vs Epimers

Isomerism explains the differences between closely related molecules. Isomerism is divided into two major groups as structural isomerism and stereoisomerism. Structural isomerism shows different structures for the same chemical formula. Stereoisomerism shows different spatial arrangements of molecules having the same molecular formula. Anomers and epimers are found under stereoisomers. The terms anomer and epimer are used to describe carbohydrate structures. These are used to identify differences between organic compounds. The main difference between anomers and epimers is that anomers differ from each other in their structure at their anomeric carbon whereas epimers differ from each other at any one of the chiral carbons present in their structure.

Key Areas Covered

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

Key Terms: Anomers, Carbohydrates, Chiral Carbon, Epimers, Isomerism, Stereoisomer, Structural Isomers

What are Anomers

Anomers are stereoisomers that occur due to the difference in the configuration at their anomeric carbon. Anomeric carbon is the carbon atom that has an aldehyde or a ketone group in the acyclic form of a sugar molecule. Sugar molecules are composed of an aldehyde or ketone group at one end and an alcohol group at the other end of the acyclic form of the sugar molecule. In order to become more stable, these groups of the two ends can react with each other forming a cyclic sugar molecule. In this cyclic form, the anomeric carbon has a –OH group attached to it. The position of this –OH group in one anomeric molecule is in the opposite direction to that of the other molecule.

Figure 1: Two Anomers of Glucose

The conversion of one anomeric form to the other anomeric form is called anomerization. This is a reversible process. However, both anomers are stable molecules with a cyclic structure. The two anomers are named alpha (α) anomer or beta (β) anomer. As shown in the above image, the –OH group attached to the anomeric carbon of alpha anomer is in the opposite direction to that of the beta anomer of glucose. The anomeric carbon is given in a green color.

What are Epimers

Epimers are a type of stereoisomers that are different from each other only at one chiral carbon. Epimers are a type of diastereomers. Although there are more than one chiral carbons, epimers differ from each other only at one carbon center. Epimers are not mirror images of each other.

Figure 2: Epimers

As shown in the above image, D-glucose and D-mannose are epimers of each other. The blue colored part indicates the location where isomerism has occurred. Here, the –OH group is directed to the left side in D-mannose where it is in the right side for D-glucose. Other carbon atoms are also chiral carbons in those molecules, but are identical to each other. Therefore, the carbon atom where the isomerism has occurred is called the epimeric carbon.

Similarities Between Anomers and Epimers

Anomers and epimers types are stereoisomers.
Both types are formed due to the difference at one carbon atom.
Both types are commonly found in sugar molecules.
Isomerism occurs due to the differences in the position of one –OH group attached to a carbon.

Definition

Anomers: Anomers are stereoisomers that occur due to the difference in the configuration at their anomeric carbon.

Epimers: Epimers are a type of stereoisomers that are different from each other only at one chiral carbon.

Carbon where Isomerism occurs

Anomers: Isomerism occurs at anomeric carbon of the anomers.

Epimers: Isomerism occurs at the epimeric carbon of the epimers.

Structure of Sugar Molecule

Anomers: Anomers are cyclic molecules.

Epimers: Epimers can be either acyclic or cyclic molecules.

Conclusion

Anomers and epimers are stereoisomers. One form of anomer can be converted to the opposite form of anomer. That is called anomerization. The process of forming an epimer is called epimerization. Although both anomers and epimers are molecules that differ from each other at one carbon center, they are distinct terms. The main difference between anomers and epimers is that anomers differ from each other in its structure at their anomeric carbon whereas epimers differ from each other at any one of the chiral carbons present in their structure.

References:

1. “Epimers.” OChemPal, Available here. Accessed 21 Aug. 2017.
2. “Anomer.” Wikipedia, Wikimedia Foundation, 19 July 2017, Available here. Accessed 21 Aug. 2017.

Image Courtesy:

1. “Glucose anomer miguelferig” By miguelferig – Own work (Public Domain) via Commons Wikimedia
2. “Epimers-Glucose Mannose” By Mlicuana – Own work (Public Domain) via Commons Wikimedia