Chapter 8 · Chemistry
Aldehyde or Ketone? The Distinguishing Tests
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Two colourless compounds, both with C=O. How Tollens', Fehling's and the iodoform test tell an aldehyde from a ketone, and why they work.
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Uses of Aldehydes and Ketones: From Formalin to VanillaWhere aldehydes and ketones turn up in daily life: formalin and Bakelite from formaldehyde, acetic acid from acetaldehyde, acetone as a solvent, and vanillin, cinnamaldehyde and benzaldehyde as smells and flavours, each explained from its C=O group.Chemistry
Key points
- Aldehyde: H on the carbonyl carbon (R–CHO). Ketone: carbon on both sides (R–CO–R′).
- Aldehydes are easily oxidised; ketones resist mild oxidation. The tests use this difference.
- Tollens' test: aldehydes reduce Ag⁺ to metallic Ag, a silver mirror. Ketones generally do not.
- Fehling's test: aliphatic aldehydes reduce blue Cu²⁺ to brick-red Cu₂O. Aromatic aldehydes (benzaldehyde) generally do not.
- Iodoform test (I₂/NaOH): compounds with CH₃CO– give yellow CHI₃: methyl ketones, and ethanal, the one aldehyde that does.
Why the silver mirror forms
In Tollens' test the aldehyde is oxidised to a carboxylate ion while silver ions gain electrons and are reduced to metallic silver, which coats the glass:
RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O
Fehling's test works the same way with copper: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O.
Choosing tests
| Compound | Tollens' | Fehling's | Iodoform |
|---|---|---|---|
| Propanal CH₃CH₂CHO | + | + | – |
| Benzaldehyde C₆H₅CHO | + | – | – |
| Ethanal CH₃CHO | + | + | + |
| Propanone CH₃COCH₃ | – | – | + |
| Pentan-3-one C₂H₅COC₂H₅ | – | – | – |
| Acetophenone C₆H₅COCH₃ | – | – | + |
Start with Tollens' for aldehyde versus ketone; use Fehling's to separate aliphatic from aromatic aldehydes, and iodoform to find a CH₃CO– group. Results are general rules under ordinary conditions; α-hydroxy ketones are an exception to the Tollens'/Fehling's rule.
Try it yourself
Goal: Identify each mystery carbonyl compound as an aldehyde or a ketone with as few tests as possible, and see why each result follows from its structure.
- Read the mystery compound's formula.
- Run one test and read the tube.
- Run more tests only if you need them.
- Decide: aldehyde or ketone.
- Try benzaldehyde, ethanal and acetophenone: which single test would have fooled you?
Quick quiz
5 questions. Choose an answer for each, then submit to see your score and the explanations.
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