Which of the following is non-reducing sugar ?

Question

Accepted Answer

Sucrose is the correct answer as a non-reducing sugar. Here's a detailed explanation:

1. **Why Sucrose is a Non-Reducing Sugar:**
   - Sucrose is a disaccharide composed of glucose and fructose units linked by a glycosidic bond. Specifically, it is an α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside linkage.
   - In sucrose, the anomeric carbon atoms of both glucose and fructose are involved in the glycosidic bond. This linkage prevents the free aldehyde or ketone group from being available to act as a reducing agent.
   - As a result, sucrose does not have a free anomeric carbon, which is necessary for a sugar to exhibit reducing properties. Therefore, it is classified as a non-reducing sugar.

2. **Why the Other Options are Incorrect:**

- **Fructose:**
     - Fructose is a monosaccharide with a ketone group. It can tautomerize to form an aldehyde group in solution, allowing it to act as a reducing sugar.
     - The presence of a free ketone group in its open-chain form enables it to reduce other compounds, which is a characteristic of reducing sugars.

- **Glucose:**
     - Glucose is a monosaccharide with an aldehyde group in its open-chain form. This aldehyde group can participate in redox reactions, making glucose a reducing sugar.
     - The presence of a free aldehyde group allows glucose to reduce other substances, which is a hallmark of reducing sugars.

- **Lactose:**
     - Lactose is a disaccharide composed of glucose and galactose units. The glucose unit in lactose has a free anomeric carbon, which allows it to act as a reducing sugar.
     - The presence of this free anomeric carbon in the glucose unit enables lactose to participate in redox reactions, classifying it as a reducing sugar.

In summary, sucrose is the only option among the given choices that is a non-reducing sugar due to its unique glycosidic linkage that prevents the presence of a free anomeric carbon.