Q.Which one given below is a non-reducing sugar? (NEET Phase – I)

Step 1. A sugar reduces Tollens'/Fehling's reagent (i.e. is a 'reducing sugar') only if it has at least one free carbonyl (anomeric) carbon available to be oxidised.

Step 2. Glucose (a) is a free monosaccharide with its aldehyde carbon completely unreacted, so it is reducing. Maltose (c) links two glucose units via an α-1,4 bond that consumes only ONE unit's anomeric carbon, leaving the other free -- so maltose is reducing. Lactose (d) links galactose and glucose via a β-1,4 bond that likewise consumes only the galactose's anomeric carbon, leaving the glucose unit's aldehyde free -- so lactose is reducing too.

Step 3. Sucrose (b), however, links C1 of α-D-glucose to C2 of β-D-fructose -- and these are precisely the two carbons that, in the free monosaccharides, are the anomeric (carbonyl-derived) carbons. Because the glycosidic bond consumes BOTH anomeric carbons at once, neither monosaccharide unit in sucrose has a free carbonyl left to reduce Tollens'/Fehling's reagent.

For each disaccharide, identify which carbon(s) form the glycosidic bond; if both units' anomeric carbons are consumed, the sugar is non-reducing, otherwise it is reducing.

• Assuming any disaccharide 'sugar' must automatically be reducing just because glucose itself is -- reducing power depends specifically on whether a free anomeric carbon survives the glycosidic bond, not on which monosaccharides are present.