Question
How can you distinguish between glucose and fructose using chemical tests? Both give positive Tollens’ test — so which test can actually tell them apart?
(NEET 2022, similar pattern)
Solution — Step by Step
Both glucose and fructose give positive results with:
- Tollens’ test (silver mirror) — because both are reducing sugars
- Fehling’s test (red precipitate of Cu₂O) — same reason
- Benedict’s test — same reason
These tests detect the reducing ability of the sugar, which both glucose (aldehyde group) and fructose (ketone group, but tautomerises to aldose in basic medium) possess.
Seliwanoff’s reagent = resorcinol + concentrated HCl
- Fructose (ketohexose): gives a deep cherry red colour within 1-2 minutes
- Glucose (aldohexose): gives a faint pink colour only after prolonged heating (5+ minutes)
This test distinguishes ketoses from aldoses. Fructose is a ketose and reacts rapidly; glucose is an aldose and reacts slowly.
Bromine water () in acidic conditions:
- Glucose: decolourises bromine water (the aldehyde group gets oxidised to gluconic acid)
- Fructose: does NOT decolourise bromine water (ketones are not oxidised by Br₂ water under mild conditions)
This is because bromine water is a mild oxidising agent — it oxidises aldehydes but not ketones in acidic medium. Unlike Tollens’ and Fehling’s (which work in basic medium where fructose tautomerises), bromine water works in acidic/neutral medium where no tautomerisation occurs.
Why This Works
The key to understanding these tests is the enediol tautomerisation. In basic solution (Tollens’, Fehling’s), fructose converts to glucose through an enediol intermediate — so both behave identically.
In acidic or neutral conditions (bromine water), no tautomerisation occurs. Glucose still has its aldehyde group and gets oxidised. Fructose keeps its ketone group, which bromine water cannot oxidise. This is why bromine water is the definitive test to distinguish them.
Alternative Method — Summary Table
| Test | Glucose | Fructose | Can Distinguish? |
|---|---|---|---|
| Tollens’ (silver mirror) | Positive | Positive | No |
| Fehling’s (red ppt) | Positive | Positive | No |
| Seliwanoff’s (resorcinol + HCl) | Slow/faint pink | Rapid cherry red | Yes |
| Bromine water | Decolourises | No reaction | Yes |
For NEET, the most commonly asked distinguishing test is bromine water. The logic: aldehyde = oxidised by Br₂ water, ketone = not oxidised. If the question says “distinguish glucose from fructose,” write bromine water as your first choice — it’s the cleanest and most specific test.
Common Mistake
Students often write “Tollens’ test can distinguish glucose from fructose because glucose has an aldehyde group and fructose has a ketone group.” This sounds logical but is WRONG. In the alkaline medium of Tollens’ reagent, fructose tautomerises to glucose (via an enediol intermediate), so both give a positive silver mirror test. Never use Tollens’ or Fehling’s test to distinguish aldoses from ketoses.