ISOMERISM,EPIMERISM,ANOMERISM

ISOMERISM:

§Compounds having the same molecular formula but different structures, i.e., different spatial arrangements, are called isomers/ Stereoisomers.

§The ability to exist as isomers is called isomerism.

§There are five types of isomerism :

1. Ketose-Aldose isomerism

§Glucose & Fructose have the same molecular formula, C₆H₁₂O₆, but they differ in their structure concerning their functional groups

§Glucose is an aldose sugar with an aldehyde group at the first carbon

§Fructose is a ketose sugar with a ketose group at the second carbon.

2. D and L isomerism

§The spatial arrangement of –H & -OH groups is vital while writing monosaccharides' structure because of the asymmetric carbon atom.

§D and L isomerism depend on the H and OH groups' orientation around the penultimate carbon atom.

§E.g., in glucose at the C5 position, if the –OH group is present on the right side, it is called D-isomer. If it is present on the left side, it is called L-isomer.

As D and L isomers are mirror images of each other and are also called Enantiomers

 3. Optical Isomerism

§ The capacity of a substance to rotate the plane-polarized light passing through it is called optical activity.

§ The presence of asymmetrical carbon atoms in the compound causes optical activity.

§ A solution of optical isomer will be rotated either to the right or the left when a beam of plane-polarized light passes through it. If it turns to the right, it is said to be dextrorotatory (d) or (+), and if it rotates to the left is said to be levorotatory (l) or (-).

§ When an equal amount of D and L isomers are present, the activity of each isomer cancel one another & the resulting mixture has no optical activity and is called a racemic mixture

4.Epimerism

When two monosaccharides differ from each other in their configuration around a single asymmetric carbon other than a reference carbon atom, they are called epimers

For example,

§Galactose and Mannose differ from glucose in the configuration of groups (H and OH) around c-4 and c-2, respectively, and are two epimers of glucose.

§But galactose and Mannose are not epimers of each other

5.Anomerism

§ Stereoisomers that do not mirror images of each other are called diastereoisomers

§Special pair of diastereomeric aldoses or ketoses that differ only in the carbonyl carbon (C1 for aldoses and C2 for ketoses) are called anomers.

§In solution, an open-chain form of sugar cyclizes into rings. When glucose cyclizes, an additional asymmetric center is created.

§Carbon-1 of glucose in an open-chain structure becomes an asymmetric carbon in the ring form and is called an anomeric carbon atom

§ Because of this, two-ring structures can be formed. These are:

§  α-D-glucose

§  β -D-glucose.

§The designation α implies that the hydroxyl group attached to c-1 is below the ring's plane,β means that it is above the ring's plane.