Question
How can you distinguish between an aldehyde and a ketone using (a) Tollens’ test and (b) Fehling’s test? Write the reactions involved.
(NCERT Class 12, Chapter 12)
Solution — Step by Step
Reagent: Tollens’ reagent — ammoniacal silver nitrate, .
Aldehydes reduce Ag to metallic silver, forming a shiny silver mirror on the test tube walls:
Ketones do not react — no silver mirror forms.
Result: Silver mirror = aldehyde present. No mirror = ketone (or no carbonyl compound).
Reagent: Fehling’s solution — a mixture of Fehling’s A (CuSO) and Fehling’s B (sodium potassium tartrate + NaOH). Contains Cu ions as a deep blue complex.
Aldehydes reduce Cu to CuO (cuprous oxide), which appears as a reddish-brown precipitate:
Ketones do not react — the solution remains blue.
Result: Reddish-brown precipitate = aldehyde. Blue solution unchanged = ketone.
| Test | Aldehyde | Ketone |
|---|---|---|
| Tollens’ | Silver mirror on walls | No reaction |
| Fehling’s | Reddish-brown precipitate (CuO) | No reaction (stays blue) |
Why This Works
Both tests rely on the reducing property of aldehydes. The -CHO group can be easily oxidised to -COOH (or -COO in basic medium). During this oxidation, the metal ion (Ag or Cu) is reduced.
Ketones lack the terminal hydrogen on the carbonyl carbon, making them much harder to oxidise under mild conditions. They cannot reduce Tollens’ or Fehling’s reagents, which is why these tests distinguish the two.
Alternative Method — Benedict’s Test
Benedict’s reagent (copper citrate complex in alkaline solution) works similarly to Fehling’s. It gives a green → yellow → orange → reddish-brown colour change with aldehydes depending on concentration. Used commonly in biochemistry to test for reducing sugars.
Exception to remember: Aromatic aldehydes (like benzaldehyde, CHCHO) give a positive Tollens’ test but a negative Fehling’s test. Fehling’s test works only with aliphatic aldehydes. This distinction is frequently tested in CBSE and NEET.
Common Mistake
Students often say “Fehling’s test distinguishes all aldehydes from all ketones.” This is not fully correct — aromatic aldehydes like benzaldehyde do NOT give a positive Fehling’s test (though they give positive Tollens’ test). For a foolproof distinction, Tollens’ test is more reliable than Fehling’s test.