Chemistry Aldehydes & Ketones MDCAT 2010
PMDC Verified Question 94 of 94
Brick red precipitate are formed when aldehyde reacts with:
A
Sodium borohydride
B
Fehling solution
C
Sodium bisulphate
D
Formaldehyde
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: Fehling solution
Concept:

Aliphatic aldehydes are easily oxidized by mild oxidizing agents such as Fehling's solution, whereas ketones are not.

Formula:

$$ RCHO + 2Cu^{2+} + 5OH^- \longrightarrow RCOO^- + Cu_2O \downarrow + 3H_2O $$

Solution:

  • Fehling's solution contains complexed Copper(II) ions.


  • When heated with an aliphatic aldehyde, the aldehyde is oxidized to a carboxylic acid.


  • Simultaneously, the blue \( Cu^{2+} \) ions are reduced to Copper(I) oxide (\( Cu_2O \)), which precipitates as a distinct brick-red color.


Why other options are incorrect:

Sodium borohydride is a reducing agent. Sodium bisulphate forms white crystalline addition products. Formaldehyde is itself an aldehyde, not a test reagent.

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