Four compounds A, B, C and D were heated individually with Tollen’s reagent. It was found that all compounds other than D formed a silver mirror on the inside of their test tubes. Identify the compound D.
Answer & explanation
Correct answer: option 3
The correct answer is option 3. Acetone.
Tollen's reagent consists of a solution of silver nitrate \((AgNO_3)\) in aqueous ammonia, forming the diamminesilver(I) complex, \([Ag(NH_3)_2]^+\). This reagent is specifically used to detect aldehydes.
Reaction Mechanism:
1. Aldehyde Oxidation: Aldehydes \((RCHO)\) are easily oxidized to carboxylic acids \((RCOOH)\) in the presence of an oxidizing agent.
2. Reduction of Tollen's Reagent: During this oxidation process, the silver ion in Tollen's reagent \((Ag^+)\) is reduced to metallic silver \((Ag)\), which deposits on the surface of the test tube, forming a characteristic silver mirror.
Specific Compounds:
1. Acetaldehyde \((CH_3CHO)\):
Functional Group: Aldehyde.
Reaction with Tollen's Reagent: Acetaldehyde is oxidized to acetic acid \((CH_3COOH)\), and Tollen's reagent is reduced to metallic silver.
Observation: Formation of a silver mirror.
2. Butyraldehyde \((CH_3(CH_2)_2CHO)\):
Functional Group: Aldehyde.
Reaction with Tollen's Reagent: Butyraldehyde is oxidized to butyric acid \((CH_3(CH_2)_2COOH)\), and Tollen's reagent is reduced to metallic silver.
Observation: Formation of a silver mirror.
3. Acetone \((CH_3COCH_3)\):
Functional Group: Ketone.
Reaction with Tollen's Reagent: Ketones generally do not react with Tollen's reagent because they are not easily oxidized. Tollen's reagent specifically oxidizes aldehydes but not ketones.
Observation: No formation of a silver mirror.
4. Benzaldehyde \((C_6H_5CHO)\):
Functional Group: Aromatic Aldehyde.
Reaction with Tollen's Reagent: Benzaldehyde is oxidized to benzoic acid \((C_6H_5COOH)\), and Tollen's reagent is reduced to metallic silver.
Observation: Formation of a silver mirror.
Conclusion:
Given the observations: Acetaldehyde, Butyraldehyde, and Benzaldehyde all form a silver mirror with Tollen's reagent because they are aldehydes and get oxidized. Acetone does not form a silver mirror because it is a ketone, which does not react with Tollen's reagent.