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: Understanding the Concept
Fehling's test is used to distinguish between aldehydes and ketones. Aldehydes are easily oxidized to carboxylate ions, while the $Cu^{2+}$ ions in the Fehling's solution are reduced.
Step 2: Key Formula or Approach
The overall redox reaction is: \[ R-CHO + 2Cu^{2+} + 5OH^- \to R-COO^- + Cu_2O \downarrow + 3H_2O \]
Step 3: Detailed Explanation
1. Oxidation: The aldehyde ($R-CHO$) is oxidized to a carboxylate ion ($R-COO^-$). 2. Reduction: The deep blue complex of $Cu^{2+}$ (Cupric ions) is reduced to $Cu^+$ (Cuprous ions). 3. Precipitation: The cuprous ions combine with oxygen to form Cuprous oxide ($Cu_2O$), which is an insoluble solid with a characteristic reddish-brown (brick red) color.
Step 4: Final Answer
The reddish-brown precipitate is due to the formation of Cuprous oxide.