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QUESTION

# Can anyone explain to me how to identify epimers and anomers in carbohydrates (sugars)? Also, state examples.

Epimers and anomers are both optical isomers that differ in the configuration at a single carbon atom, but there is a difference in their definitions.

Epimers

Epimers are optical isomers that differ in the configuration of a single carbon atom

For example, D-galactose and D-mannose are epimers of D-glucose.

D-Galactose is an epimer of D-glucose because the two sugars differ only in the configuration at "C-4".

D-Mannose is an epimer of D-glucose because the two sugars differ only in the configuration at "C-2".

Anomers

When a molecule such as glucose converts to a cyclic form, it generates a new chiral centre at "C-1".

(from www.chem.ucalgary.ca

The carbon atom that generates the new chiral centre ("C-1") is called the anomeric carbon.

Anomers are special cases — they are epimers that differ in configuration only at the anomeric carbon.

For example, α-D-glucose and β-D-glucose are anomers.

(from wikispaces.psu.edu

The α form has the anomeric "OH" group at "C-1" on the opposite side of the ring from the "CH"_2"OH" group at "C-5".

The β form has the anomeric "OH" group on the same side as the "CH"_2"OH".

In D-fructose, the carbonyl group is at "C-2".

(fromweb.pdx.edu)

Here, "C-2" is the anomeric carbon.

(from science.uvu.edu

α-D-Fructofuranose and β-D-fructofuranose are anomers.