Question

Fehling's reagent is a mixture of

• A. aqueous \( CuSO_4 \) and ammoniacal \( AgNO_3 \) solution
• B. aqueous \( CuSO_4 \) and 2,4-DNP
• C. aqueous \( KOH \) and ammoniacal \( AgNO_3 \) solution
• D. aqueous \( CuSO_4 \) and alkaline sodium potassium tartrate
• E. aqueous \( KOH \) and alkaline sodium potassium tartrate

Hint:

Fehling’s reagent detects aldehydes.

Answer 1

The Correct Option is D

Step 1: Understanding Fehling's Reagent:
Fehling's reagent is a chemical test used to differentiate between water-soluble carbohydrate and ketone functional groups, and as a test for reducing sugars and non-reducing sugars. It is specifically used to test for aliphatic aldehydes. It is always prepared fresh by mixing two separate solutions just before use.
Step 2: Identifying the Components:
The two solutions that constitute Fehling's reagent are:
Fehling's solution A: An aqueous solution of copper(II) sulfate (CuSO\(_4\)), which is deep blue.
Fehling's solution B: An aqueous solution of sodium potassium tartrate (also known as Rochelle's salt, NaKC\(_4\)H\(_4\)O\(_6\)\(\cdot\)4H\(_2\)O) and a strong alkali, typically sodium hydroxide (NaOH). This solution is colorless.
When these two solutions are mixed, the tartrate ions chelate (form a complex) with the Cu\(^{2+}\) ions, preventing them from precipitating as copper(II) hydroxide in the alkaline medium. The resulting deep blue solution contains the active species, the bis(tartrato)cuprate(II) complex, which acts as the oxidizing agent.
Step 3: Evaluating the Options:
(A) Ammoniacal AgNO\(_3\) is Tollen's reagent.
(B) 2,4-DNP (2,4-Dinitrophenylhydrazine) is Brady's reagent, used to test for carbonyl groups in general.
(C) This is a mix of an alkali and Tollen's reagent components.
(D) This correctly identifies the two main components: aqueous CuSO\(_4\) (Fehling's A) and alkaline sodium potassium tartrate (Fehling's B).
(E) This only describes Fehling's solution B.
Step 4: Final Answer:
Fehling's reagent is a mixture of aqueous CuSO\(_4\) and alkaline sodium potassium tartrate.