Benzaldehyde gives a positive test with Tollens reagent but a negative one with Fehling's solution. This shows that
Correct answer: D. aromatic aldehydes are oxidised less readily than aliphatic aldehydes
- A. benzaldehyde is a ketone
- B. benzaldehyde contains no carbonyl group
- C. Fehling's solution reacts only with aromatic compounds
- D. aromatic aldehydes are oxidised less readily than aliphatic aldehydes
Explanation
Fehling's solution is the milder oxidising agent of the two, so it fails with aromatic aldehydes whose carbonyl group is stabilised by conjugation with the ring, while the stronger Tollens reagent still succeeds. Benzaldehyde is definitely an aldehyde, as the silver mirror confirms. This exception is a favourite examination point precisely because it looks like a contradiction.
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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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679 free Aldehydes and Ketones MCQs from Chemistry, each with the correct answer and an explanation. Unlimited attempts, no account needed.
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