What is the color of the precipitate when Brady's reagent is used to test a benzaldehyde?
Correct answer: D. Red
- A. Green
- B. Brown
- C. Magenta
- D. Red
Explanation
Brady's reagent (2,4-dinitrophenylhydrazine) reacts with the carbonyl group of benzaldehyde. It forms a 2,4 -dinitrophenylhydrazone, which is an insoluble red-orange/red precipitate. This confirms the presence of an aldehyde or ketone group.
This question appeared on the NUMS 2019 (cancelled sitting) paper, which you can sit online with every answer explained.
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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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