MP Board · Class 12 · Chemistry · BiomoleculesWhat are carbohydrates? Classify them based on their behavior towards hydrolysis. Differentiate between reducing and non-reducing sugars with suitable examples, and discuss the cyclic structure of D-glucose.

Introduction to Carbohydrates\nCarbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones, or compounds 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 Based on Hydrolysis\nCarbohydrates are classified into three major categories based on their behavior towards hydrolysis:

• Monosaccharides: These are the simplest carbohydrates that cannot be hydrolyzed further into simpler polyhydroxy aldehydes or ketones. Examples include Glucose, Fructose, and Ribose.
• Oligosaccharides: These carbohydrates yield 2 to 10 monosaccharide units upon hydrolysis. Depending on the number of units, they are further classified as disaccharides (e.g., Sucrose, Lactose), trisaccharides, etc.
• Polysaccharides: These are polymeric carbohydrates of high molecular mass that yield a large number of monosaccharide units on hydrolysis. Examples include Starch, Cellulose, and Glycogen.

Reducing and Non-Reducing Sugars

• Reducing Sugars: Carbohydrates that contain free aldehydic or ketonic groups and can reduce Tollens' reagent and Fehling's solution are called reducing sugars. All monosaccharides and most disaccharides (like maltose and lactose) are reducing sugars.
• Non-Reducing Sugars: Carbohydrates that do not have free aldehydic or ketonic groups and cannot reduce Tollens' or Fehling's reagents are called non-reducing sugars. Sucrose is a classic example because the reducing groups of glucose and fructose are involved in glycosidic bond formation.

Cyclic Structure of D-Glucose\nAlthough D-glucose exhibits many reactions typical of aldehydes, it fails to undergo certain characteristic aldehyde reactions (such as Schiff's test) and does not form a bisulfite addition product. This led to the conclusion that the aldehyde group is involved in hemiacetal formation.

• In glucose, the $-\text{OH}$ group at C-5 reacts with the aldehydic group at C-1 to form a six-membered cyclic hemiacetal ring, known as a pyranose ring.
• This cyclic structure gives rise to two stereoisomers:
  • $\alpha$-D-Glucose: In which the $-\text{OH}$ group at C-1 is on the right/down side.
  • $\beta$-D-Glucose: In which the $-\text{OH}$ group at C-1 is on the left/up side.
• These two isomers are called anomers, and C-1 is known as the anomeric carbon.