Q. 13.44 - An Introduction to General, Organic, and Biological Chemistry

Question

Indicate whether each disaccharide in the following figure is a reducing sugar or not.

a.

 

b.

Step-by-Step Solution

Answer

Part a) Structure (a) is a reducing sugar.

Part b) Structure (b) is not a reducing sugar

Step 1: Definition for reducing sugars (Part a)

• Monosaccharides are regarded as reducing sugars because the aldose group in them can diminish the Benedict's reagent. 
• Ketoses tautomerize to aldoses in a solution, lowering sugars as well. 

Step 2: Given information (Part a)

To find whether each disaccharide is reducing sugar or not.

Step 3: Explanation (Part a)

• Structure (a) is a reducing sugar because it contains a free aldose group.

• If the sample is a reducing sugar, the cupric ions are reduced to cuprous ions and cuprous oxide is formed, resulting in a positive Benedict reaction. 
• Although polysaccharides do not contain aldose.

Step 4: Given information (Part b)

To find whether each disaccharide is reducing sugar or not.

Step 5: Explanation (Part b)

• Structure (b) is sucrose. 
• Sucrose has a "direct" bond that forms a glycosidic bond between two anomer carbons. 
• These are carbon from glucose and carbon from fructose.
• Because it lacks free anomer hydroxyl groups, it is not a reducing sugar. 
• Therefore, sucrose does not test positive in the Benedict test.