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

Concept: Fehling's test is an important qualitative test used to distinguish aldehydes from ketones. Fehling's reagent contains a complex of copper(II) ions in an alkaline medium. When an aldehyde is heated with Fehling's solution, the aldehyde gets oxidised to the corresponding carboxylate ion, while copper(II) ions are reduced to copper(I) oxide \((Cu_2O)\), which appears as a brick-red precipitate. \[ RCHO \longrightarrow RCOO^- \] \[ Cu^{2+} \longrightarrow Cu_2O \] The appearance of the brick-red precipitate indicates a positive Fehling's test.

Step 1: Examining the Assertion. The Assertion states that all aliphatic aldehydes give a positive Fehling's test. Aliphatic aldehydes are readily oxidised by Fehling's reagent. For example, \[ HCHO,\; CH_3CHO,\; C_2H_5CHO \] and other aliphatic aldehydes produce the characteristic brick-red precipitate of copper(I) oxide. Therefore, the Assertion is true.

Step 2: Understanding the chemical change occurring in Fehling's test. During the reaction, the aldehyde loses electrons and undergoes oxidation. The copper(II) ions present in Fehling's solution gain electrons and undergo reduction. Thus, \[ \text{Aldehyde} \rightarrow \text{Oxidised} \] and \[ Cu^{2+} \rightarrow Cu^+ \] is reduced. Hence, it is the reagent that is reduced, not the aldehyde.

Step 3: Examining the Reason carefully. The Reason states: quote Aliphatic aldehydes are reduced by Fehling's reagent. quote This statement is incorrect. In reality, aldehydes are oxidised by Fehling's reagent. The copper(II) ions present in Fehling's reagent are reduced. Therefore, the Reason is false.

Step 4: Determining the correct Assertion–Reason relationship. Assertion: True Reason: False Hence the correct option is \[ \boxed{\text{Assertion is true, but Reason is false}} \] \[ \boxed{\text{Option (C)}} \]