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Zorluk: OrtaAlkanals and Alkanones: Oxidation, Reduction, and Distinction Tests

An unknown organic compound YY forms a brick-red precipitate when warmed with Fehling's solution and also gives a yellow precipitate of triiodomethane (CHI3CHI_3) when treated with iodine in sodium hydroxide solution. Which of the following compounds is YY?

  1. EthanalCevap
  2. B
    Propan-2-one
  3. C
    Propanal
  4. D
    Methanal

Cevap

Ethanal
Ethanal (CH3CHOCH_3CHO) is an alkanal, so it reduces copper(II) ions in Fehling's solution to copper(I) oxide (Cu2OCu_2O), producing a brick-red precipitate. Furthermore, because its carbonyl group is bonded directly to a methyl group (CH3C=OCH_3C=O), it undergoes halogenation and cleavage in alkaline iodine to produce a yellow precipitate of triiodomethane (CHI3CHI_3).

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1
Analyze the Fehling's solution reaction.
Fehling's test distinguishes alkanals (aldehydes) from alkanones (ketones). A positive test (brick-red Cu2OCu_2O precipitate) indicates YY must be an alkanal.
Alkanals are easily oxidized to alkanoic acids, whereas alkanones resist mild oxidation.
2
Analyze the triiodomethane (iodoform) reaction.
A positive triiodomethane test (yellow CHI3CHI_3 precipitate) requires a methyl carbonyl group (CH3C=OCH_3C=O) or a secondary alcohol structure (CH3CH(OH)−CH_3CH(OH)-).
Iodine in aqueous alkali oxidizes and iodinates compounds containing the methyl carbonyl structural unit.
3
Combine the structural requirements.
The compound must be both an alkanal (−CHO-CHO) and contain a methyl carbonyl group (CH3C=OCH_3C=O). Ethanal (CH3CHOCH_3CHO) is the only alkanal that possesses a CH3C=OCH_3C=O group.
Other alkanals like methanal (HCHOHCHO) or propanal (CH3CH2CHOCH_3CH_2CHO) lack the CH3C=OCH_3C=O structural unit.

Anahtar Kavram

Distinction tests for carbonyl compounds: Fehling's test and Iodoform test
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Alkanals and Alkanones: Oxidation, Reduction, and Distinction Tests

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