Question

Assertion (A) : All aliphatic aldehydes give a positive Fehling’s test. Reason (R) : Aliphatic aldehydes are reduced by Fehling’s reagent.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

In Fehling's test: \[ \text{Aldehyde} \rightarrow \text{Oxidised} \] \[ Cu^{2+} \rightarrow Cu_2O \] (reduced to brick-red precipitate) Always remember: Aldehyde is oxidised, Fehling's reagent is reduced.

Answer 1

The Correct Option is C

Step 1: Recall the chemistry of Fehling's test.
Fehling's reagent contains $Cu^{2+}$ ions in alkaline solution. When warmed with an aldehyde, the aldehyde is oxidised to a carboxylate ion while $Cu^{2+}$ is reduced to $Cu_2O$, giving a brick-red precipitate.
Step 2: Assess the Assertion.
All aliphatic aldehydes (e.g., $HCHO$, $CH_3CHO$) are readily oxidised by Fehling's reagent and give the characteristic brick-red precipitate. The Assertion is true.
Step 3: Identify the error in the Reason.
The Reason states that aliphatic aldehydes are reduced by Fehling's reagent. This is incorrect. In the reaction, the aldehyde is oxidised (loses electrons); it is $Cu^{2+}$ in the reagent that is reduced to $Cu^+$. The Reason is false.
\[ \boxed{\text{Option (C): Assertion true, Reason false}} \]