Functional Group Test (aldehydes)
Among the following organic liquids, which one will give a bright silver mirror with Tollens' reagent but no reaction with neutral ferric chloride solution?
Select the correct option:
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About This Question
- Subject
- chemistry
- Chapter
- principles related to practical chemistry
- Topic
- functional group test (aldehydes)
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
Propanal
Tollens' reagent is an ammoniacal silver nitrate solution containing the diamminesilver(I) ion, which is a mild oxidising agent specific to easily oxidised carbonyl compounds. Aldehydes reduce the silver(I) complex to metallic silver, depositing a bright silver mirror on the tube wall, while being themselves oxidised to carboxylate ions. Propanal, an aldehyde, therefore gives a positive silver mirror test. Neutral ferric chloride gives a violet or coloured complex only with phenols, so propanal shows no reaction with it. Option Phenol is wrong because it does not reduce Tollens' reagent but does give a colour with neutral ferric chloride, the opposite behaviour. Option Propan-2-ol is wrong because secondary alcohols are not oxidised by the mild Tollens' reagent and give no mirror. Option Propanone is a ketone, which lacks the hydrogen on the carbonyl carbon needed for the gentle oxidation and so fails the Tollens' test. This selective oxidation is the NCERT basis for distinguishing aldehydes from ketones and alcohols. Plausibility check: only the aldehydic C-H bond permits oxidation to a carboxylic acid under such mild conditions, uniquely identifying propanal.
This easy difficulty chemistry question is from the chapter principles related to practical chemistry, covering the topic of functional group test (aldehydes). It appeared in the 2025 exam.
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