Reducing vs Non-Reducing Sugars: Complete Analysis

Definition: A reducing sugar has a free anomeric carbon that can exist in the open-chain (hemiacetal/hemiketal) form, exposing a free aldehyde or alpha-hydroxy ketone group capable of reducing oxidising agents.

Tests for reducing sugars:

• Tollens' test: $Ag^{+}$ (in ammonia) → $Ag^{0}$ (silver mirror)
• Fehling's test: $Cu^{2+}$ (deep blue) → $Cu_{2}O$ (brick-red precipitate)
• Benedict's test: Similar to Fehling's ($Cu^{2+}$ → $Cu_{2}O$)

Reducing sugar equilibrium: $\text{cyclic form (hemiacetal)} \rightleftharpoons \text{open-chain form (aldehyde)} \rightarrow \text{reduces oxidant}$

Key disaccharides — NEET table:

Critical NEET trap: Fructose IS a reducing sugar even though it is a ketose (ketohexose). Under alkaline conditions of Fehling's/Tollens' tests, the alpha-hydroxy ketone undergoes keto-enol tautomerism:

$\text{D-Fructose (ketose)} \xrightarrow{\text{base}} \text{Enediol intermediate} \rightarrow \text{D-Glucose + D-Mannose (aldoses)}$

The aldoses generated reduce the reagent. Therefore "ketoses are non-reducing" is FALSE for free fructose.