CBSE · Class 12 · Chemistry · BiomoleculesDefine carbohydrates and classify them based on their behavior towards hydrolysis. Explain the cyclic hemiacetal structure of D-glucose and mention its open-chain structure limitations.

Definition of Carbohydrates\nCarbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones, or compounds that produce such units on hydrolysis. They are commonly known as saccharides.

Classification Based on Hydrolysis

1. Monosaccharides:
   • These are carbohydrates that cannot be hydrolyzed further into simpler polyhydroxy aldehyde or ketone units.
   • Examples: Glucose, fructose, ribose.
2. Oligosaccharides:
   • These carbohydrates yield 2 to 10 monosaccharide units on hydrolysis.
   • Further classified as disaccharides (e.g., sucrose, lactose), trisaccharides, etc.
3. Polysaccharides:
   • These are polymeric carbohydrates formed by joining a large number of monosaccharide units through glycosidic linkages.
   • Examples: Starch, cellulose, glycogen.

Cyclic Structure of D-Glucose

• The open-chain structure of glucose fails to explain certain reactions, such as the absence of a free aldehyde group test with Schiff's reagent and the existence of two stereoisomeric forms of glucose ($\alpha$-D-glucose and $\beta$-D-glucose).
• This behavior is explained by the formation of a cyclic hemiacetal structure. The $-\text{OH}$ group at $\text{C-5}$ adds to the aldehydic group ($\text{C-1}$) to form a six-membered cyclic hemiacetal ring, commonly known as pyranose structure.

Limitations of Open-Chain Structure

1. Glucose does not give the Schiff’s test and does not form a hydrogensulphite addition product with $\text{NaHSO}_3$, despite having an aldehyde group.
2. The pentaacetate of glucose does not react with hydroxylamine, indicating the absence of a free $-\text{CHO}$ group.
3. D-glucose exists in two distinct crystalline forms, $\alpha$ and $\beta$, which exhibit mutarotation.