MP Board · Class 12 · Chemistry · BiomoleculesWhat are carbohydrates? How are they classified? Write the structural differences between D-glucose and D-fructose, and explain the concept of anomeric carbon and mutarotation with respect to glucose.

Introduction to Carbohydrates\nCarbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones, or substances that yield such units on hydrolysis. They are commonly known as saccharides and serve as primary energy sources and structural elements in living organisms.

Classification of Carbohydrates\nCarbohydrates are classified based on their behavior on hydrolysis into three major categories:

1. Monosaccharides: These cannot be hydrolyzed further into simpler polyhydroxy aldehydes or ketones. Examples include glucose, fructose, and ribose.
2. Oligosaccharides: These yield 2 to 10 monosaccharide units on hydrolysis. They are further classified as disaccharides, trisaccharides, etc., based on the number of units they yield. Examples include sucrose, lactose, and maltose.
3. Polysaccharides: These are polymeric carbohydrates containing a large number of monosaccharide units linked by glycosidic linkages. Examples include starch, cellulose, and glycogen.

Structural Differences between D-Glucose and D-Fructose

• Glucose: It is an aldohexose containing an aldehyde group ($-CHO$) at carbon-1 and five hydroxyl groups. Its formula is $C_6H_{12}O_6$.
• Fructose: It is a ketohexose containing a ketonic group ($-C=O$) at carbon-2 and five hydroxyl groups. Its formula is also $C_6H_{12}O_6$.

Anomeric Carbon and Mutarotation

• Anomeric Carbon: In cyclic structures of monosaccharides, the C-1 carbon (in aldoses) or C-2 carbon (in ketoses) becomes asymmetric due to ring closure. This specific carbon is known as the anomeric carbon. The $\alpha$ and $\beta$ forms of glucose are stereoisomers that differ in configuration at the anomeric carbon (C-1).
• Mutarotation: It is the spontaneous change in the specific optical rotation of an optically active compound in solution until an equilibrium value is established. For instance, freshly prepared aqueous solution of $\alpha$-D-glucose shows a specific rotation of $+112^\circ$, which gradually changes and drops to a constant value of $+52.5^\circ$. Similarly, $\beta$-D-glucose shows an initial specific rotation of $+19^\circ$ and rises to the same equilibrium value of $+52.5^\circ$. This interconversion through an open-chain form is called mutarotation.