Anomers and the Anomeric Carbons of Glucose and Fructose

Daniel Spaargaren, Laura Foist
  • Author
    Daniel Spaargaren

    Daniel Spaargaren has an associate’s degree in Exercise Science from Three Rivers Community College and bachelor’s degree in Exercise Science from Central Connecticut State University. He was a tutor at his college for math, chemistry, and physics. He also answered student’s chemistry questions online.

  • Instructor
    Laura Foist

    Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Understand what an anomeric carbon is by learning the anomeric carbon definition. Learn how to identify an anomeric carbon. Explore the anomeric carbon of glucose. Updated: 12/20/2021

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What is an Anomeric Carbon?

An anomer is an epimer of a cyclic sugar that has a different configuration at the anomeric carbon. An epimer is one of a pair of stereoisomers that differ only at one stereocenter. For each set of two anomers, there is an alpha anomer and a beta anomer. In the alpha anomer, the hydroxyl group (-OH) at the anomeric carbon is on the opposite side of the ring as the hydroxymethyl group (-CH2OH) at the chiral center that determines the D or L configuration. In the beta anomer, the hydroxyl group at the anomeric carbon is on the same side of the ring as the hydroxymethyl group at the chiral center that determines the D or L configuration. In a solution, the alpha and beta forms will freely interconvert in a process called anomerization.

The anomeric carbon is the hemiacetal or hemiketal carbon of the sugar. A hemiacetal carbon is bonded to hydrogen, an R group, a hydroxyl group, and an alkoxy group (-OR), and occurs from the addition of an alcohol to an aldehyde. A hemiketal carbon is bonded to two R groups, a hydroxy group, and an alkoxy group, and it occurs from the addition of an alcohol to a ketone. The hemiacetal or hemiketal carbon is formed during the cyclization reaction of a sugar from the open-chain form to the cyclic form.

How to Identify an Anomeric Carbon

Two projections commonly used to depict sugars are the Fischer projection and the Haworth projection. The Fischer projection is used to show a sugar in its open-chain form. It consists of a series of connected crosses, where the center of each cross is a stereocenter. The Haworth projection is used to show a sugar in its cyclic form. It consists of a drawn ring, where thick lines are coming out of the page and thin lines are going into the page, with substituents pointing up or down perpendicular to the ring. Groups projecting to the right in a Fischer projection will point down in a Haworth projection, and groups projecting to the left will point up.

Comparison of ribose in a Fischer projection and a Haworth projection

An anomeric carbon can be identified in the open-chain form of the sugar or the cyclic form. In the open-chain form, the anomeric carbon is the carbon of the carbonyl group, which is either an aldehyde or ketone, depending on the type of sugar. In the cyclic form, the anomeric carbon is the carbon that was previously part of the carbonyl group in the open-chain form but is now bonded to a hydroxyl group and the ring oxygen.

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Stereocenters and Sugars

A sugar is a monosaccharide or disaccharide, which includes molecules such as glucose, fructose, and lactose. Monosaccharides contain a carbonyl group (either an aldehyde or ketone) and a varying number of hydroxyl groups. Monosaccharides can have different numbers of carbons and can form different size rings depending on their molecular formulas. For example, the six-carbon monosaccharide glucose contains an aldehyde in its open-chain form and therefore produces a six-membered ring in its cyclic form. The six-carbon monosaccharide fructose contains a ketone in its open-chain form and therefore produces a five-membered ring in its cyclic form. Disaccharides are two monosaccharides bonded together.

A stereocenter is an atom in a molecule that has three or more different substituents that produce stereoisomers when these substituents are rearranged. Stereoisomers are molecules that have the same molecular formula and bond order but have different three-dimensional arrangements in space. When a sugar is in the open-chain form, the anomeric carbon is not a stereocenter because it has trigonal planar geometry and there is no way to rearrange the substituents to form stereoisomers. When a sugar is in the cyclic form, the anomeric carbon becomes a stereocenter because it can form a pair of diastereomers that occur from the alpha anomer and beta anomer forms.

Anomeric Effect

The anomeric effect is the preference of heteroatomic substituents of a carbon bonded to a heteroatom in a cyclohexane molecule to be in the axial position instead of the equatorial position, despite increased steric strain. The anomeric effect is also defined as the tendency of a molecule with the general structure X-C-C-Y to prefer the gauche conformation, where X and Y are heteroatoms. A heteroatom is any atom that is not carbon or hydrogen. Axial position and equatorial position occur in cyclohexanes in the chair conformation, which is the most stable conformation of cyclohexane. The axial position is when a bond is parallel to the axis of the ring, and the equatorial position is when a bond is parallel to the plane of the ring. The equatorial position is usually more stable than the axial position. The gauche conformation is when two atoms or groups have a dihedral angle between zero and 120 degrees. There are several possible explanations for why the anomeric effect occurs, but the most accepted reason is hyperconjugation, which is the interaction of the electrons in a sigma orbital with an adjacent orbital to give an extended molecular orbital. Other explanations are dipole stabilization and electrostatic repulsion.

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Frequently Asked Questions

How do you identify an anomeric carbon?

An anomeric carbon can be identified as the carbonyl carbon (of the aldehyde or ketone functional group) in the open-chain form of the sugar. It can also be identified as the carbon bonded to the ring oxygen and a hydroxyl group in the cyclic form.

What is an anomeric carbon?

An anomeric carbon is a hemiacetal or hemiketal carbon of a cyclic sugar. It is the carbon at which the alpha anomer or beta anomer is formed.

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