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

When propanal (CH3CH2CHOCH_3CH_2CHO) is warmed with Fehling's solution, a brick-red precipitate is formed. What is the chemical formula of this precipitate, and what organic product is formed from the reaction?

  1. Cu2OCu_2O and propanoic acidAnswer
  2. B
    CuOCuO and propan-1-ol
  3. C
    Cu2OCu_2O and propanone
  4. D
    Cu(OH)2Cu(OH)_2 and propanoic acid

Answer

The precipitate formed is copper(I) oxide (Cu2OCu_2O) and the organic oxidation product is propanoic acid.
Fehling's solution contains alkaline solution of copper(II) sulfate complexed with tartrate ions. When heated with an alkanal like propanal, the aldehyde group is oxidized to a carboxylic acid (propanoic acid), while Cu2+Cu^{2+} is reduced to insoluble brick-red copper(I) oxide (Cu2OCu_2O).

Step-by-Step Solution

1
Identify the functional group and reaction type
Propanal is an alkanal containing the terminal carbonyl group (−CHO-CHO). Warming with Fehling's solution causes a redox distinction test.
Alkanals act as reducing agents and readily undergo oxidation, whereas Fehling's solution contains complexed Cu2+Cu^{2+} ions.
2
Determine the inorganic reduction product (precipitate)
The deep blue Cu2+Cu^{2+} ions are reduced to copper(I) oxide (Cu2OCu_2O), which precipitates as a insoluble brick-red solid.
The reduction half-reaction is 2Cu2++2OH−+2e−→Cu2O+H2O2Cu^{2+} + 2OH^- + 2e^- → Cu_2O + H_2O.
3
Determine the organic oxidation product
Propanal (CH3CH2CHOCH_3CH_2CHO) is oxidized to propanoic acid (CH3CH2COOHCH_3CH_2COOH).
Oxidation of an alkanal inserts an oxygen atom into the C−HC-H bond of the aldehyde group without altering the carbon backbone length.

Key Concept

Fehling's distinction test for alkanals and their oxidation products
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Topics

Alkanals and Alkanones: Oxidation, Reduction, and Distinction Tests

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