Oxidation of an aldehyde with acidified potassium permanganate gives
Correct answer: C. a carboxylic acid with the same number of carbon atoms
- A. a primary alcohol
- B. a ketone
- C. a carboxylic acid with the same number of carbon atoms
- D. carbon dioxide only
Explanation
The hydrogen on the carbonyl carbon is replaced by a hydroxyl group, so ethanal becomes ethanoic acid with no change in the length of the chain. This ready oxidation is why aldehydes act as reducing agents and give positive results with Tollens and Fehling's reagents. Ketones cannot be oxidised in this way without breaking the carbon skeleton.
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About Reactivity of Aldehydes and Ketones
The polar carbonyl group makes aldehydes and ketones undergo nucleophilic addition, reduction and condensation reactions. Aldehydes are generally more reactive and oxidise readily to carboxylic acids, whereas ketones resist mild oxidation; reactions with hydrogen cyanide, sodium bisulfite and 2,4-DNP help identify their behaviour.
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