An unknown organic compound Y forms a brick-red precipitate when warmed with Fehling's sol… — JAMB UTME

An unknown organic compound $Y$ forms a brick-red precipitate when warmed with Fehling's solution and also gives a yellow precipitate of triiodomethane ( $CHI_3$ ) when treated with iodine in sodium hydroxide solution. Which of the following compounds is $Y$ ?

EthanalAnswer
B
Propan-2-one
C
Propanal
D
Methanal

Answer

Ethanal

Ethanal (

CH_3CHO

) is an alkanal, so it reduces copper(II) ions in Fehling's solution to copper(I) oxide (

Cu_2O

), producing a brick-red precipitate. Furthermore, because its carbonyl group is bonded directly to a methyl group (

CH_3C=O

), it undergoes halogenation and cleavage in alkaline iodine to produce a yellow precipitate of triiodomethane (

CHI_3

Step-by-Step Solution

Analyze the Fehling's solution reaction.

Fehling's test distinguishes alkanals (aldehydes) from alkanones (ketones). A positive test (brick-red

Cu_2O

precipitate) indicates

Y

must be an alkanal.

Alkanals are easily oxidized to alkanoic acids, whereas alkanones resist mild oxidation.

Analyze the triiodomethane (iodoform) reaction.

A positive triiodomethane test (yellow

CHI_3

precipitate) requires a methyl carbonyl group (

CH_3C=O

) or a secondary alcohol structure (

CH_3CH(OH)-

Iodine in aqueous alkali oxidizes and iodinates compounds containing the methyl carbonyl structural unit.

Combine the structural requirements.

The compound must be both an alkanal (

-CHO

) and contain a methyl carbonyl group (

CH_3C=O

). Ethanal (

CH_3CHO

) is the only alkanal that possesses a

CH_3C=O

group.

Other alkanals like methanal (

HCHO

) or propanal (

CH_3CH_2CHO

) lack the

CH_3C=O

structural unit.

Key Concept

Distinction tests for carbonyl compounds: Fehling's test and Iodoform test

Estimated Time:1m 0s

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