Question

On heating an aldehyde with Fehling's reagent, a reddish-brown precipitate is obtained due to the formation of:

• A. Cupric oxide
• B. Cuprous oxide
• C. Carboxylic acid
• D. Silver
• E. Copper acetate

Hint:

Fehling's test is a qualitative test for aldehydes, where the aldehyde reduces copper(II) to cuprous oxide, forming a reddish-brown precipitate.

Answer 1

The Correct Option is B

Step 1:Understand the reaction with Fehling's reagent.
Fehling's reagent is a mixture of copper(II) sulfate (\( \text{CuSO}_4 \)), sodium tartrate, and sodium hydroxide. When an aldehyde is heated with Fehling's reagent, the aldehyde is oxidized, and copper(II) ions are reduced.
Step 2:Oxidation and reduction process.
The aldehyde group (\( \text{RCHO} \)) is oxidized to a carboxyl group (\( \text{RCOOH} \)), while the copper(II) ions are reduced to cuprous ions (\( \text{Cu}^+ \)). The reaction can be represented as: \[ \text{RCHO} + \text{Cu}^{2+} \xrightarrow{\text{Fehling's reagent}} \text{RCOOH} + \text{Cu}_2\text{O} \, (\text{reddish-brown precipitate}) \] The reddish-brown precipitate is cuprous oxide (\( \text{Cu}_2\text{O} \)).
Step 3:Conclusion.
Thus, the reddish-brown precipitate formed when an aldehyde is heated with Fehling's reagent is cuprous oxide.