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The Fehling Test — From Intuition to Precision
Imagine you have a bowl of copper sulphate solution — a clear, beautiful blue. Now imagine you could somehow pull oxygen atoms away from the copper, changing its chemical state. The blue would vanish, and a brick-red solid would appear. That colour change is the heart of the Fehling test.
The test is a chemical detective. It asks one simple question: Is this compound an aliphatic aldehyde? (An aldehyde where the aldehyde group is attached to a carbon chain, not directly to a benzene ring.) If the answer is yes, the blue solution turns into a red precipitate. If no, the blue stays blue.
The Chemistry Behind the Colour
Fehling's solution is actually two solutions kept separate until use. Fehling A is copper(II) sulphate (CuSOX4) — the source of the blue CuX2+ ions. Fehling B is a mixture of sodium potassium tartrate (Rochelle salt) and sodium hydroxide. The tartrate keeps the copper ions from precipitating as copper hydroxide when you add the base; it forms a soluble deep-blue complex.
When you mix equal parts of A and B, you get the familiar deep-blue Fehling's reagent. The copper is in the +2 oxidation state, dissolved as a tartrate complex.
Now add an aliphatic aldehyde and heat gently. The aldehyde gets oxidised to a carboxylic acid. In doing so, it donates electrons to the CuX2+ ions, reducing them to CuX+. These CuX+ ions combine with hydroxide ions from the solution to form cuprous hydroxide, which on heating decomposes to give red cuprous oxide (CuX2O).
The reaction looks like this:
R−CHO+2CuX2++5OHX−R−COOX−+CuX2O ↓+3HX2O
The red precipitate is the unmistakable signal.
Aromatic aldehydes (like benzaldehyde) do not give this test. The aldehyde group attached directly to a benzene ring is less reactive toward oxidation under these mild conditions. This is a key distinction in organic analysis.
The Precise Statement
Fehling test: A reagent prepared by mixing equal volumes of Fehling A (aqueous CuSOX4) and Fehling B (aqueous sodium potassium tartrate + NaOH) is added to the compound and heated. Aliphatic aldehydes reduce the CuX2+ ions to CuX+, producing a brick-red precipitate of cuprous oxide (CuX2O). Ketones, aromatic aldehydes, and most other functional groups do not react.
Aliphatic aldehyde+Fehling’s reagentΔred CuX2O precipitate
What the Test Tells You (and What It Doesn't) …
(i) Fehling's solution is a mild oxidising test reagent for aldehydes: acetaldehyde reduces its blue Cu2+ complex to a red-brown Cu2O precipitate. (ii) Phosphorus pentachloride swaps the –OH of a carboxylic acid for –Cl, converting it directly to the corresponding acid chloride. …
Fehling's test: aldehyde reduces Cu²⁺ to red Cu₂O. PCl₅ swaps the –OH of a carboxylic acid for –Cl, giving the acid chloride.
- Acetaldehyde + Fehling's solution: Fehling's solution contains Cu²⁺ ions complexed with tartrate (as a deep-blue complex). On warming with an aldehyde such as acetaldehyde, the aldehyde is oxidised to the corresponding carboxylate, while Cu²⁺ is reduced to Cu⁺, which precipitates as red-brown cuprous oxide (Cu₂O): CH3CHO+2Cu2++5OH−→CH3COO−+Cu2O↓(red-brown)+3H2O This is a positive test used to distinguish aldehydes from ketones — ketones (other than some special cases) do not give this reduction.
- Acetic acid + phosphorus pentachloride: …
- CBSE 2025Set ANNUAL1 markMCQQ.Which of the following does not reduce Fehling's solution -(a) CH3CHO(b) HCHO(c) CH3COOH(d) HCOOH
›Reveal solutionSolution
Only compounds with a free −CHO group reduce Fehling's solution; a carboxylic acid without −CHO does not.
Fehling's solution is reduced by aldehydes (which get oxidised to carboxylate while Cu2+ is reduced to Cu2O, a brick-red precipitate). CH3CHO and HCHO are aldehydes, so they reduce it. HCOOH (formic acid) also reduces Fehling's solution because it still contains an aldehydic −CHO-type hydrogen (it can be regarded …
- CBSE 2025Set JZ1 markMCQQ.Reagent, which do not react with acetone and benzaldehyde(a) Sodium hydrogen sulphite(b) Phenyl hydrazine(c) Fehling's solution(d) Grignard reagent
›Reveal solutionSolution
Fehling's solution reacts with neither acetone (a ketone) nor benzaldehyde (an aromatic aldehyde), so it is the odd reagent — option (c).
Concept. We need a reagent that reacts with neither acetone (CH3COCH3, a ketone) nor benzaldehyde (C6H5CHO, an aromatic aldehyde).
Checking each reagent:
- (a) Sodium hydrogen sulphite (NaHSO3) forms crystalline bisulphite addition products with aldehydes and with reactive methyl ketones like acetone → reacts with both.
- (b) Phenylhydrazine condenses with the >C=O of both aldehydes and ketones to give phenylhydrazones → reacts with both. …
- CBSE 2025Set A1 markQ.Fill in the blank: Fehling reagent comprises of two solutions, fehling solution A and ______.
›Reveal solutionSolution
Fehling's reagent is made of two separate solutions, A and B, mixed just before use.
Fehling's reagent is used as a test to distinguish aldehydes (especially aliphatic aldehydes) from ketones. It consists of:
- Fehling solution A: an aqueous solution of copper sulphate (CuSO4).
- Fehling solution B: an alkaline solution of sodium potassium tartrate (Rochelle salt) and sodium hydroxide. …
- CBSE 2025Set ANNUAL1 markMCQQ.The compound failing to reduce Fehling's solution is:(a) Acetaldehyde(b) Acetic acid(c) Formic acid(d) Formaldehyde
›Reveal solutionSolution
Fehling's test is positive for compounds containing a free -CHO group (or one that can tautomerise to expose one); acetic acid (CH3COOH) has no such group and fails the test, unlike formic acid, acetaldehyde, and formaldehyde.
Fehling's solution (an alkaline Cu2+ complex) is reduced by aldehydes to a brick-red precipitate of Cu2O, with the aldehyde being oxidised to a carboxylic acid.
- Acetaldehyde (CH3CHO): has a -CHO group — reduces Fehling's solution readily.
- Formaldehyde (HCHO): has a -CHO group — reduces Fehling's solution readily (and is oxidised all the way to CO2/formic acid). …
- CBSE 2024Set ANNUAL1 markMCQQ.The reagent which does not react with both acetone and benzaldehyde :(a) NaHSO3(b) Phenyl hydrazine(c) Fehling solution(d) Tollen's reagent
›Reveal solutionSolution
Fehling's solution is a test specific to aliphatic aldehydes; it gives a negative result with ketones (like acetone) AND with aromatic aldehydes (like benzaldehyde) — so it is the one reagent that reacts with neither.
Checking each reagent against BOTH acetone and benzaldehyde:
- NaHSO3 (sodium bisulfite): forms a bisulfite addition product with both methyl ketones (acetone) and aldehydes (benzaldehyde) — reacts with both.
- Phenylhydrazine: forms a phenylhydrazone with any aldehyde or ketone via nucleophilic addition-elimination at the carbonyl — reacts with both. …
- CBSE 2024Set ANNUAL1 markQ.Acetaldehyde is a hydrocarbon containing oxygen, only name the product when it reacts with Fehling solution.
›Reveal solutionSolution
Fehling's solution (an alkaline Cu2+ complex) is reduced by aldehydes like acetaldehyde to a brick-red Cu2O precipitate — a classic aldehyde test.
Fehling's solution consists of Fehling A (CuSO4 solution) and Fehling B (alkaline sodium potassium tartrate). When an aldehyde such as acetaldehyde (CH3CHO) is warmed with Fehling's solution, the Cu2+ ion (as the deep-blue tartrate complex) oxidises the aldehyde to the corresponding carboxylate (acetate) ion, while itself getting reduced from Cu2+ to Cu+, which precipitates as a reddish-brown/brick-red solid of Cu2O.
…
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following reduces Fehling solution?(a) Acetic Acid(b) Benzoic Acid(c) Oxalic Acid(d) Formic Acid
›Reveal solutionSolution
Formic acid reduces Fehling's solution because it contains an aldehydic (-CHO) group.
Formic acid, HCOOH, structurally contains an aldehyde-like -CHO group attached to -OH, so it behaves like an aldehyde and gives a positive Fehling's test (reducing Cu2+ of Fehling's solution to a red precipitate of Cu2O). Acetic …
- CBSE 2023Set ANNUAL1 markQ.The aqueous solution of ________ is called fehling solution A.
›Reveal solutionSolution
Fehling's reagent is made of two separately-stored solutions; solution A is aqueous copper sulphate.
Fehling's solution is prepared fresh by mixing two solutions just before use: Fehling's solution A = aqueous solution of copper sulphate (CuSO4), and Fehling's solution B = alkaline solution of sodium potassium tartrate (Roche …
- CBSE 2023Set ANNUAL1 markQ.What is Fehling solution B?
›Reveal solutionSolution
Fehling's reagent is prepared by mixing two solutions, A and B, just before use; solution B is alkaline sodium potassium tartrate.
Fehling's solution is used as a test for reducing sugars (aldehydes, especially aliphatic aldehydes, and reducing carbohydrates). It is made of two separate solutions that are mixed in equal volumes immediately before the test, because the mixture does not keep well:
- Fehling's solution A: an aqueous solution of copper sulphate (CuSO4).
- Fehling's solution B: an alkaline solution of sodium potassium tartrate (Rochelle salt, NaKC4H4O6) dissolved in sodium hydroxide (NaOH). …
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