Q.37 Fehling’s solution
(A) contains a copper complex of tartaric acid
(B) forms a brick‑red precipitate with glucose
(C) gives a purple precipitate with aldehydes
(D) is used as a test reagent for reducing sugars
Fehling’s solution is a key reagent in qualitative analysis for detecting reducing sugars and aldehydes, with option (D) being the correct choice as it serves as the standard test for reducing sugars like glucose.
Option Analysis
Fehling’s solution consists of two parts: Fehling A (copper(II) sulfate) and Fehling B (potassium sodium tartrate with sodium hydroxide), forming a deep blue copper-tartrate complex when mixed.
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(A) Incorrect: It contains a copper complex, but specifically of potassium sodium tartrate (Rochelle salt), not tartaric acid alone; tartrate ions stabilize Cu²⁺ to prevent hydroxide precipitation.
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(B) Correct but incomplete: It forms a brick-red precipitate of Cu₂O with glucose (a reducing sugar), due to oxidation of the aldehyde group to gluconic acid and reduction of Cu²⁺ to Cu⁺.
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(C) Incorrect: Aldehydes give a red Cu₂O precipitate, not purple; purple indicates other tests like Schiff’s reagent.
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(D) Correct: Widely used as the primary test reagent for reducing sugars, distinguishing them from non-reducing ones via the red precipitate in alkaline medium.
Reaction Mechanism
Reducing sugars with free aldehyde/ketone groups reduce Cu²⁺ to Cu₂O:
\ceRCHO+2Cu2++5OH−−>RCOO−+Cu2O+3H2O
The tartrate complex ensures solubility, and the test requires heating for the color change from blue to brick-red.
Fehling’s solution test for reducing sugars ranks as a cornerstone in biochemistry and organic chemistry, enabling precise detection of compounds like glucose via a distinctive brick-red precipitate.
Composition Breakdown
Fehling’s solution mixes Fehling A (CuSO₄) and B (NaOH + potassium sodium tartrate) to form a stable Cu²⁺-tartrate complex.
Tartrate prevents Cu(OH)₂ precipitation, maintaining alkalinity for the redox reaction.
Test Procedure
Heat sample with freshly mixed Fehling’s solution; blue color shifts to brick-red Cu₂O for positives.
Positive with aldehydes and reducing sugars (glucose, fructose); negative with ketones.
Applications in Exams
Ideal for CSIR NET questions on carbohydrate tests, distinguishing reducing vs. non-reducing sugars.
Complements Benedict’s test but uses tartrate over citrate.
