Question

Enumerate the reactions of D-glucose which cannot be explained by its open chain structure.

What happens when D-glucose is treated with the following reagents? (i) HI (ii) Bromine water (iii) HNO₃

Carbohydrates are essential for life in both plants and animals. Carbohydrates are used as storage molecules as starch in plants and glycogen in animals. Chemically they are polyhydroxy aldehydes or ketones. On the basis of their behaviour on hydrolysis, carbohydrates are classified as monosaccharides, oligosaccharides and polysaccharides. All monosaccharides are reducing sugars, i.e., they are oxidized by Tollens' reagent and Fehling's solution. A monosaccharide like glucose is aldohexose and its molecular formula was found to be C₆H₁₂O₆. After reacting with different reagents like HI, H₂N – OH, Bromine water, (CH₃CO)₂O, etc. its structure was found to contain one aldehyde group, one primary alcoholic group, (– CH₂OH) and four secondary alcoholic groups (> CHOH). Despite having the aldehyde group, glucose does not give some of the reactions of aldehyde group like Schiff's test, NaHSO₃ addition. This explains the existence of glucose in two cyclic hemiacetal forms which differ only in the configuration of the hydroxyl group at C – 1.