Question
Identify whether each of the following is a reducing or non-reducing sugar, and justify in one line: glucose, fructose, sucrose, maltose.
Solution — Step by Step
A reducing sugar has a free anomeric carbon (the carbon attached to two oxygens in the cyclic form) that can open into an aldehyde/ketone group, which then reduces Tollens’ or Fehling’s reagent.
Glucose is an aldohexose with one free anomeric carbon (in either α or β form, the ring opens easily to expose the aldehyde). Reducing.
Fructose is a ketohexose. Even though its native form has a ketone (not aldehyde), under basic conditions (Fehling’s, Tollens’) it tautomerizes to glucose/mannose via an enediol intermediate. Reducing. This surprises students every year.
Sucrose is an α,β-1,2-glycosidic linkage between glucose and fructose. Both anomeric carbons are tied up in the linkage, so neither sugar can open up. Non-reducing.
Maltose is an α-1,4-glycosidic linkage between two glucose units. Only one of the two anomeric carbons is in the linkage; the other is free. Reducing.
Final Answer: Reducing — glucose, fructose, maltose. Non-reducing — sucrose.
Why This Works
The reducing/non-reducing test boils down to one question: is there at least one free anomeric carbon? Monosaccharides have one (always free → always reducing). Disaccharides depend on the linkage: if both anomeric carbons are in the glycosidic bond, neither can open up (sucrose is the canonical example). Otherwise, at least one is free, and the sugar is reducing.
The tautomerization of fructose to glucose under basic conditions is the standard trap NEET sets every year. Memorize: ketoses still reduce Fehling’s and Tollens’ reagents.
Alternative Method
Look at the structural formula and trace each anomeric C. If both are in the O–C–O–C bridge of a disaccharide, non-reducing. Otherwise, reducing. Same logic, structural lens.
Marking fructose as non-reducing because “it’s a ketone, not an aldehyde” is the most common error. Under basic conditions, the keto form converts to an aldehyde via tautomerization, making fructose reducing. Don’t trust the structure of the native form alone.
NEET asks “which of these is non-reducing?” with sucrose as the trap-free correct answer at least once every two years. Memorize: only sucrose (among common disaccharides) is non-reducing.