Question
Distinguish between an aldose and a ketose with one specific chemical test, and identify whether glucose and fructose are aldoses or ketoses. (NEET 2022)
Solution — Step by Step
An aldose has the carbonyl group (C=O) at C1 — i.e., it’s an aldehyde sugar. A ketose has the carbonyl at an internal position (typically C2) — i.e., it’s a ketone sugar.
Glucose: CHO at C1 → aldose (aldohexose, six carbons).
Fructose: C=O at C2 → ketose (ketohexose, six carbons).
Aldoses, with their free aldehyde group, reduce Tollens’ reagent (silver mirror test) and Fehling’s solution (red Cu₂O precipitate). Free ketones don’t reduce these reagents.
Despite being a ketose, fructose does give positive Tollens’ and Fehling’s tests. The reason: in alkaline conditions, fructose tautomerises to glucose and mannose via the Lobry de Bruyn–van Ekenstein rearrangement. The aldose forms then reduce the reagent.
To truly distinguish aldose from ketose without alkali, use Seliwanoff’s test. Ketoses give a red colour rapidly; aldoses give a faint pink only after prolonged heating.
Final answer: Glucose is an aldose, fructose is a ketose. Use Seliwanoff’s test (rapid red for ketose) to distinguish them reliably.
Why This Works
Tollens’ and Fehling’s tests rely on free aldehyde groups, but they fail to discriminate aldoses from ketoses because alkaline conditions interconvert them. Seliwanoff’s test (with HCl, not base) avoids this rearrangement and gives a clean diagnostic.
NEET likes to test exactly this nuance — it’s the difference between a 1-mark MCQ correct answer and a trap option.
Alternative Method
Look at the open-chain Fischer projection. Aldoses have CHO at the top; ketoses have a CH₂OH at the top with C=O at the second position.
Cyclic sugars (pyranose, furanose) hide the aldehyde or ketone as a hemiacetal/hemiketal. The chain form re-emerges in solution, so chemical tests still work.
Common Mistake
Concluding that fructose is an aldose because it gives a positive Fehling’s test. The test isn’t strict — alkaline interconversion is the catch. Always specify Seliwanoff for a clean distinction.