What are Anomers

Answer:

Definition: The anomers are the epimers i.e. these are cyclic glycosides or monosaccharides which consist of different configurations at C-2 position and at C-1 position if they bear ketone and aldehyde groups in the open chain of the sugar moiety respectively. An anomeric carbon is the carbon which consists of a ketone group at C-2 position or an aldehyde group at C-1 position in the open chain of the sugar moiety therefore anomers existence needs to have cyclic sugar moiety as well as it should be achiral. The phenomenon of converting the one form of anomer into another form is known as anomerisation.

Stability and Physical Properties of Anomers: All anomers differ in their stability and physical properties. They possess different structures so their stabilising as well as destabilising properties also differ. The parameters which affects stability are;

• Anomeric Effect- Electron withdrawing group (especially nitrogen or oxygen atoms) in the axial ring helps stabilise the anomer. This anomeric effect in polar solvents (like water) stopped.

• Hydrogen bonds- The hydrogen bond in the ring between other and anomeric groups leads to anomer stabilisation.

• 1,3-diaxial interactions- The anomer usually gets destabilised by this interaction in the axial ring. In pyranoses (saccharide of 6-membered ring with 1 Oxygen atom and 5 carbon atoms) or other 6-membered ring shows prominent effect.

• Dipolar Repulsion- This repulsion in the ring between other and anomeric groups leads to anomer destabilisation.

Types of Anomers: There are 2 types of anomers depending on their configurational arrangements in the ring; alpha and beta anomers like R or S configuration in enantiomers. They both are relatively stereo descriptors. The centre of anomers in R1R2C(OH)OR, (hemiacetals; R1 & R2= Hydrogen or some other organic atoms) is the anomeric carbon of position C-1. The anomers are nomenclatured with the names of alpha and beta on this basis of the difference in the configuration of the structure.