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• 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.

To find whether each disaccharide is reducing sugar or not.

• 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.

To find whether each disaccharide is reducing sugar or not.

• 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.