Which of the following reaction is not shown by ketones ?
Correct answer: D. reaction with Fehling solution
- A. reaction with HCN
- B. reaction with NaHSO3
- C. reaction with 2 4 dinitrophenyl hydrazine
- D. reaction with Fehling solution
Explanation
Ketones form addition products with HCN and NaHSO3 and condensation products with 2,4-dinitrophenylhydrazine because their carbonyl group remains chemically reactive. They generally do not reduce Fehling solution, so no brick-red Cu2O precipitate forms. Students may choose the 2,4-DNP reaction because it is often associated with aldehydes, but it is actually a test for both aldehydes and ketones.
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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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