Why does glucose give a positive Fehling's/Tollens' test but table sugar (sucrose) doesn't?
These tests need a free aldehyde (or α-hydroxy ketone) group that can be oxidised.
These tests need a free aldehyde (or α-hydroxy ketone) group that can be oxidised. Glucose has a free aldehyde group in its open-chain form, so it reduces the reagents (a reducing sugar). In sucrose, the reactive groups of glucose and fructose are locked together in the glycosidic bond joining them, leaving no free reducing group — so sucrose is non-reducing and gives a negative test until it's hydrolysed back into its monosaccharides.
Key point
A free aldehyde/ketone group makes a sugar reducing; sucrose locks both away, so it's non-reducing.
Common mistake
assuming all sweet sugars behave the same in these tests.
Memory tip
free carbonyl → reducing sugar; tied up in a glycosidic bond (sucrose) → non-reducing.
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