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Difficulty: EasyAlkanals and Alkanones: Oxidation, Reduction, and Distinction Tests

Which of the following reagents reacts with propanal to produce a silver mirror, but shows no reaction with propanone?

  1. Ammoniacal silver nitrate solutionAnswer
  2. B
    Fehling's solution
  3. C
    Acidified potassium dichromate(VI) solution
  4. D
    Lithium tetrahydridoaluminate(III) in dry ether

Answer

Ammoniacal silver nitrate solution
Ammoniacal silver nitrate solution (Tollen's reagent) acts as a mild oxidizing agent. It oxidizes propanal to propanoate ions while Ag+Ag^+ ions are reduced to elemental silver, depositing as a silver mirror on the container walls. Propanone resists oxidation by mild oxidizing agents and yields no silver deposit.

Step-by-Step Solution

1
Identify the functional groups of the given compounds
Propanal (CH3CH2CHOCH_3CH_2CHO) is an alkanal (aldehyde), whereas propanone (CH3COCH3CH_3COCH_3) is an alkanone (ketone).
Alkanals possess a terminal carbonyl group with a hydrogen atom that is easily oxidized, while alkanones lack this hydrogen atom.
2
Determine the specific reagent that yields a silver mirror
Tollen's reagent (ammoniacal silver nitrate solution, [Ag(NH3)2]+[Ag(NH_3)_2]^+) is reduced by alkanals to form metallic silver (Ag(s)Ag(s)), which deposits on the glass wall as a silver mirror.
Alkanones cannot be easily oxidized by mild oxidizing agents like Tollen's reagent under standard conditions.

Key Concept

Distinction between alkanals and alkanones using Tollen's reagent
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Topics

Alkanals and Alkanones: Oxidation, Reduction, and Distinction Tests

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