MP Board · Class 12 · Chemistry · BiomoleculesDefine carbohydrates and classify them based on their behavior towards hydrolysis. Explain the cyclic structure of glucose with the help of Haworth projections and anomeric carbons.

Definition of 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 in living organisms.

Classification Based on Hydrolysis

1. Monosaccharides: These are carbohydrates that cannot be hydrolyzed further into smaller units of polyhydroxy aldehydes or ketones. Examples include glucose, fructose, and galactose.
2. Oligosaccharides: These carbohydrates yield 2 to 10 monosaccharide units on hydrolysis. They are further classified into disaccharides, trisaccharides, etc., depending on the number of monosaccharides. Example: Sucrose, Maltose, Lactose.
3. Polysaccharides: These are polymeric carbohydrates containing a large number of monosaccharide units joined together by glycosidic linkages. Examples include starch, glycogen, and cellulose.

Cyclic Structure of Glucose and Haworth Projections\nAlthough glucose reacts as an aldehyde (shows reactions with Tollen's reagent, Fehling's solution, and hydroxylamine), certain evidence (such as its failure to give Schiff's test and the existence of two crystalline forms, $\alpha$-D-glucose and $\beta$-D-glucose) proved that the straight-chain structure is inadequate.

• Hemiacetal Formation: The open-chain structure of D-glucose undergoes an intramolecular nucleophilic addition reaction. The alcoholic group at C-5 attacks the carbonyl carbon at C-1, forming a stable six-membered ring known as a pyranose ring.
• Anomeric Carbons: The C-1 carbon (the carbonyl carbon in the open-chain form) becomes chiral in the cyclic structure and is referred to as the anomeric carbon. The two cyclic stereoisomers formed are called anomers ($\alpha$ and $\beta$).
• Haworth Projections: In the Haworth structure, the six-membered pyranose ring is represented as a planar hexagon viewed edge-on. For $\alpha$-D-glucopyranose, the $-\text{OH}$ group at C-1 points downwards, while for $\beta$-D-glucopyranose, the $-\text{OH}$ group at C-1 points upwards.