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What are anomers?

Morgan Brisse, Sarah Erhart
  • Author
    Morgan Brisse

    Morgan Brisse has assisted in teaching university-level biology courses as a teaching assistant for one year. She has a BS in Biochemistry with honors from the University of Michigan, and will be defending her PhD thesis in Biochemistry, Molecular Biology and Biophysics from the University of Minnesota this spring.

  • Instructor
    Sarah Erhart

    Sarah has taught college physical, organic, and general chemistry and high school biology. She has a master's degree in chemistry.

Learn about anomers. Understand the definition of anomers, how they occur, their types, and explore examples of anomers in carbohydrates and see their structures. Updated: 03/05/2022

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Anomers Definition

Two molecules are isomers of each other when they have the same chemical formula but different structures. How is this possible? They can either have different arrangements of their atoms or have different relative 3D structures. Molecules that have the same chemical formula but differ from each other in the arrangements of their atoms are known as constitutional isomers. An example of a pair of isomers is the molecules butane and isobutane. The chemical formula for both is C4H10, but the arrangement of their atoms is different. We can reflect this difference in their atomic arrangements by writing butane as CH3-CH2-CH2-CH3 and isobutane as HC(CH3)3.


The structures of the constitutional isomers isobutane (top) and butane (bottom)

Constitutional isomers


On the other hand, isomers that have the same chemical formula and arrangements of their atoms and only differ from each other on their relative 3D structures are called stereoisomers. Different relative 3D structures are possible when at least four unique molecular groups (atoms or groups of atoms) are bound to a single atom known as a stereocenter. For most molecules, we can calculate the number of possible stereoisomers by using the formula {eq}X=2^n {/eq}, where x is the number of stereoisomers and n is the number of stereocenters. This means that each stereocenter has two possible 3D arrangements of the groups attached to it. We can also use the number of stereocenters to define the relationships among its stereoisomers. A molecule with one stereocenter will have two stereoisomers that are mirror images of each other; these stereoisomers are known as enantiomers. A molecule with more than one stereocenter will have pairs of stereoisomers that are enantiomers; these molecules will differ in orientation from each other at every stereocenter. Any other pair of stereoisomers that differs in orientation at some (but not all) stereocenters do not mirror each other and are known as epimers or diastereomers. It is especially common to see epimer pairs in biological molecules given their size, complexity, and the ability of carbon to bind to 4 unique molecular groups.


The two possible stereoisomers of propylene glycol (middle and right)

Stereoisomers propylene glycol


An anomer is a specific type of epimer that occurs in certain carbohydrate (sugar) molecules. Carbohydrates are cyclical carbon chains where each carbon is bound to a hydroxyl (OH) group. This means that every carbon atom in the carbohydrate ring is a stereocenter, and we typically refer to changes in stereochemistry between carbohydrate stereoisomers by numbering the carbon atoms (C1-C5 for a six-member ring and C1-C4 for a five-member ring). This adds a third criterion when naming a carbohydrate molecule: the size of its ring (glucose has a six-member ring while fructose has a five-member ring), its existence as a monomer or a polymer (glucose is a monomer while sucrose is a dimer), and distinction from other its epimers (glucose and galactose are epimers by differing in orientation at C5). Most carbohydrate epimers will have a different name, as is the case for glucose and galactose. But anomers (which differ in orientation at C1) are instead given alpha and beta forms of the same molecule (much like the R and S designations for enantiomers) instead of being given different names.


The alpha and beta anomers of D-glucose as shown by chair (left) and Fischer (right) projections. The anomeric carbon in both projections is C1.

Glucose anomers


How do Anomers Occur?

Why are anomers treated as being more like each other than non-anomeric epimers are? The answer lies in the biochemical nature of the C1 anomeric carbon. Anomeric carbons are found in reducing sugars, which refer to sugars that can be reduced to form an aldehyde or a ketone. The C1 carbon is an anomeric carbon if it is not attached to another anomeric carbon in another sugar molecule. This means that all monosaccharides are reducing sugars, as are polysaccharides that are connected to each other by one anomeric carbon. Polysaccharides that are connected to each other by all their anomeric carbons (such as sucrose) are not reducing sugars.

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  • 0:00 Introduction to Isomers
  • 1:36 Anomers
  • 2:23 Carbohydrate Examples
  • 3:37 Lesson Summary
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Frequently Asked Questions

What is an anomer?

An anomer is a specific type of epimer that occurs in carbohydrates (sugars). They only differ in 3D orientation between each other at the anomeric carbon (usually C1 in chair and Fischer projections).

What are the two types of anomers?

The two types of anomers are alpha and beta. These are determined by seeing the orientation of the anomeric carbon.

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