Question

Structures of four disaccharides are given below. Among the given disaccharides, the non-reducing sugar is: 

• A. Maltose
• B. Lactose
• C. Sucrose
• D. Cellobiose

Hint:

To identify non-reducing sugars: - Check if both anomeric carbons are involved in glycosidic linkage. - If yes → non-reducing sugar (e.g., sucrose). - If no → reducing sugar (e.g., maltose, lactose, cellobiose).

Answer 1

The Correct Option is C

Concept: Reducing sugars are those which have a free anomeric carbon atom (free \(-OH\) group on the hemiacetal carbon) that can open up to form an aldehyde or ketone group, allowing them to reduce reagents like Tollen’s or Fehling’s solution. Non-reducing sugars lack a free anomeric carbon because both anomeric carbons are involved in glycosidic linkage. 
Step 1: Analyze each disaccharide. 
- \(\mathbf{Maltose}\): Formed from two glucose units with \(\alpha(1 \to 4)\) linkage. One anomeric carbon is free → reducing sugar. 
- \(\mathbf{Lactose}\): Formed from glucose and galactose with \(\beta(1 \to 4)\) linkage. One anomeric carbon is free → reducing sugar. 
- \(\mathbf{Sucrose}\): Formed from glucose and fructose with \(\alpha(1 \to 2)\) and \(\beta(2 \to 1)\) linkage. Both anomeric carbons are involved in glycosidic bond → non-reducing sugar. 
- \(\mathbf{Cellobiose}\): Formed from two glucose units with \(\beta(1 \to 4)\) linkage. One anomeric carbon is free → reducing sugar. 
Step 2: Conclusion. Among the given disaccharides, sucrose is the only non-reducing sugar. \[ \text{Answer: Sucrose (Option 3)} \]