Chapter 8 · Chemistry
Reducing and Oxidising Aldehydes and Ketones
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Reduction is easy for both aldehydes and ketones, but in oxidation the aldehyde reacts at once and the ketone resists. The reason is one H atom.
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Tollens' and Fehling's TestsWarm a clear liquid and a mirror forms in the test tube. How Tollens' and Fehling's tests tell aldehydes from ketones.Chemistry · simulation
Key points
- NaBH₄ or LiAlH₄ reduce aldehydes to primary alcohols and ketones to secondary alcohols.
- Clemmensen (Zn–Hg, conc. HCl) and Wolff-Kishner (NH₂NH₂, KOH) turn C=O into CH₂.
- An aldehyde has an H on the carbonyl carbon, so it oxidises easily to a carboxylic acid with the same carbons.
- A ketone has no such H: it needs strong conditions.
- Ketone oxidation breaks a C–C bond and gives a mixture of acids with fewer carbons.
What each reagent does
| Reagent | Aldehyde | Ketone |
|---|---|---|
| NaBH₄ / LiAlH₄ | 1° alcohol | 2° alcohol |
| Clemmensen, Wolff-Kishner | Alkane | Alkane |
| Mild oxidant | Carboxylic acid | No reaction |
| Strong oxidant | Carboxylic acid | Mixture of acids, fewer carbons |
Try it yourself
Goal: Predict the product of each compound and reagent, and see why a ketone resists mild oxidation.
- Choose a compound.
- Choose a reagent and predict the product.
- Press React and compare.
- Try a mild and a strong oxidant on a ketone.
Quick quiz
3 questions. Choose an answer for each, then submit to see your score and the explanations.
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