Q.Sucrose does not reduce Fehling's solution or Tollens' reagent, i.e. it is a non-reducing sugar. Explain why, in terms of the glycosidic linkage between its two monosaccharide units.

A sugar behaves as a reducing sugar only if at least one of its monosaccharide units has a free anomeric carbon that can reopen from its cyclic form back into an open-chain aldehyde (or an oxidisable, isomerisable ketone) form, which is the species actually oxidised by Fehling's solution or Tollens' reagent.

In sucrose, the glycosidic bond connecting the glucose unit and the fructose unit runs specifically from C1 of glucose (glucose's anomeric carbon) to C2 of fructose (fructose's anomeric carbon). Because this one linkage uses up the anomeric carbon of both monosaccharide units at once, neither ring retains a free anomeric $-\text{OH}$ that could reopen back to a carbonyl form. With no ring in the molecule able to reopen to expose an oxidisable carbonyl group, sucrose cannot red …

Check whether a disaccharide's glycosidic linkage involves one anomeric carbon or both: if both are used up in the one bond, neit …

Assuming all disaccharides are automatically non-reducing -- most (like maltose) have only one anomeric carbon tied up in the glycosidic bond, leaving the other free, so they remain reducing sugar …