Aldehydes and ketones both react with 2,4-dinitrophenylhydrazine to give
Correct answer: D. an orange crystalline precipitate
- A. a silver mirror
- B. a colourless solution
- C. carbon dioxide
- D. an orange crystalline precipitate
Explanation
This reagent, often called Brady's reagent, condenses with any carbonyl group to give a bright orange or yellow solid, so it detects the presence of the group without distinguishing aldehyde from ketone. The precipitate has a sharp melting point characteristic of the parent compound, which allows identification. Tollens or Fehling's is then used to decide which of the two classes it is.
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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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