Question

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

a.

b.

Short answer

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

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

Step-by-step solution

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.

Given information (Part a)

To find whether each disaccharide is reducing sugar or not.

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.

Given information (Part b)

To find whether each disaccharide is reducing sugar or not.

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.