How to identify epimers and anomers in carbohydrates (sugars)?

The isomerism that is caused by the non-similar arrangements of atoms or functional groups belonging to an atom in space is called Stereoisomers. These types of isomers have the same constitutions, but different geometric arrangements of atoms. Stereoisomers can be broadly classified into two types, namely enantiomers and diastereomers.

Enantiomers and diastereomers are two types of stereoisomers.

  • Enantiomers include mirror images and non-superimposable chiral centres.
  • Diastereomers contain non-superimposable chiral centres, but are not mirror images. Depending on the number of stereocenters, there could be far more than 2.

Anomers and epimers are both diastereomers, but an epimer is a stereoisomer that differs in configuration at any single stereogenic centre, while an anomer is actually an epimer that differs in configuration at the acetal/hemiacetal carbon.

Epimer

An epimer is one of a pair of stereoisomers that differ in configuration at only one chiral (stereogenic) centre.

  • An epimer is one of a pair of stereoisomers that differ in configuration only at the chiral centre.
  • All other stereocenters (if there are any) are the same in both molecules.
  • If the pair of molecules has only 1 stereocenter, then the epimers are enantiomers, whereas when the molecules have 2 or more stereocenters, the epimers are referred to as diastereomers.

Anomer

An anomer is actually an epimer (also a cyclic saccharide) which differs in configuration, particularly at the acetal or hemiacetal carbon (refer to the image below to differentiate between acetal and hemiacetal carbons).

  • An anomer is a kind of stereoisomer.
  • The anomers are saccharides or glycosides that are epimers, which are distinct from each other in the configuration at C-2.
  • If they are ketoses, or in the configuration of C-1, if they are aldoses.

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