Question

What are reducing sugars?

Hint:

Free anomeric OH = Reducing sugar. Locked anomeric carbon (as in sucrose) = Non-reducing sugar.

Answer 1

Step 1: Definition.
Reducing sugars are carbohydrates that can reduce mild oxidising agents such as Tollen's reagent (ammoniacal $AgNO_3$) and Fehling's solution (alkaline $CuSO_4$).
Step 2: Structural requirement.
A sugar is reducing if it has a free anomeric carbon, i.e., a hemiacetal or hemiketal group in equilibrium with an open-chain form containing a free $-CHO$ or $-CO$ group. This free carbonyl acts as the reducing agent.
Step 3: Examples.
All monosaccharides (glucose, fructose, galactose) are reducing sugars. Among disaccharides, maltose and lactose are reducing (free anomeric $-OH$ present). Sucrose is non-reducing (both anomeric carbons engaged in glycosidic linkage).
Step 4: Conclusion.
Reducing sugars possess a free anomeric $-OH$ that opens to a free aldehyde or ketone, enabling them to reduce Tollen's and Fehling's reagents. Examples: glucose, fructose, maltose, lactose.