Tollens' Test and Fehling's Test: How to Tell Aldehydes from Ketones
Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
Aldehydes are easy to oxidise, ketones are not. So two mild tests can spot an aldehyde: Tollens' test gives a shiny silver mirror on the tube, and Fehling's test gives a red-brown precipitate. Ketones give no change in either test. Memory hook: "Aldehyde = A for Ag mirror; Ketone = No Kolor change."
Tollens' test gives a silver mirror; Fehling's test gives a red-brown Cu2O precipitate. Both oxidise aldehydes to carboxylate ions (R-COO-) but leave ketones unchanged.
Your doubts, answered
What is the difference between Tollens' test and Fehling's test?
Both tests find aldehydes, but they look different. Tollens' test uses ammoniacal silver nitrate [Ag(NH3)2]+ (Tollens' reagent). The aldehyde reduces Ag+ to silver metal, which coats the tube as a shiny SILVER MIRROR. Fehling's test uses blue Cu2+ (copper sulphate + Rochelle salt). The aldehyde reduces Cu2+ to a RED-BROWN precipitate of Cu2O (copper(I) oxide). One key extra rule: aromatic aldehydes like benzaldehyde give a positive Tollens' test but do NOT respond to Fehling's test. So Fehling's is mainly for aliphatic aldehydes.
Why do aldehydes give these tests but ketones do not?
An aldehyde has an H atom directly on the carbonyl carbon (R-CHO). That C-H bond is easy to break, so aldehydes are easily oxidised to carboxylic acids (as the carboxylate ion in these alkaline tests). A ketone has two carbon groups on the carbonyl carbon (R-CO-R') and no such H. To oxidise a ketone you must break a strong C-C bond, which mild reagents cannot do. So ketones give NO silver mirror and NO red precipitate. This is exactly why these are called tests to 'distinguish aldehydes from ketones'.
What are Fehling's solution A and B made of?
Fehling's reagent is two solutions mixed just before use. Solution A is aqueous copper sulphate (CuSO4) — this is the blue Cu2+ source. Solution B is an alkaline solution of sodium potassium tartrate, also called Rochelle salt. Solution B keeps the copper dissolved (as a complex) and makes the medium basic. NEET asks this directly: 'Fehling's solution A is aqueous copper sulphate.' Do not swap A and B.
What exactly does the aldehyde turn into in these tests?
In both tests the medium is alkaline (basic), so the aldehyde is oxidised to the carboxylate ANION (R-COO-), not the neutral carboxylic acid. In Tollens', Ag+ is reduced to Ag (silver mirror). In Fehling's, Cu2+ is reduced to Cu2O (red-brown solid). A common mistake is writing the product as R-COOH; because OH- is present, the answer is the carboxylate ion R-COO-.
Does benzaldehyde give the Fehling test?
No. Benzaldehyde is an AROMATIC aldehyde, and aromatic aldehydes do NOT reduce Fehling's reagent. But benzaldehyde DOES give a positive Tollens' test (silver mirror). So if a question shows benzaldehyde, expect: Tollens' yes, Fehling's no. This one-line exception is a favourite NEET trap.
⚠️ The NEET trap ✗ Tollens' reagent oxidises both the -CHO and the -CO-CH3 groups on the ring, so both become -COO-. ✓ Tollens' reagent is a MILD oxidant. It oxidises only the aldehyde -CHO to carboxylate -COO- (with a silver mirror) and leaves the ketone -CO-CH3 untouched. So 2-acetylbenzaldehyde gives 2-acetylbenzoate: acetyl stays intact, only CHO becomes COO-. 🧠 Mild reagent = touches only the easy aldehyde. Ketone survives Tollens' and Fehling's every time.
Real NEET questions
NEET 2024
Fehling's solution 'A' is:
A · alkaline copper sulphate
B · alkaline solution of sodium potassium tartrate (Rochelle's salt)
C · aqueous sodium citrate
D · aqueous copper sulphate ✓
Solution: Fehling's reagent is made from two solutions mixed just before use. Solution A is aqueous copper sulphate (CuSO4), the blue Cu2+ source. Solution B is the alkaline solution of sodium potassium tartrate (Rochelle salt). Do not swap them: A = CuSO4, B = Rochelle salt. On heating with an aliphatic aldehyde, Cu2+ is reduced to a red-brown precipitate of Cu2O.
NEET 2023
Identify the major product when 2-acetylbenzaldehyde (a benzene ring bearing -CO-CH3 and -CHO on adjacent carbons) is treated with 2[Ag(NH3)2]+ and 3OH- on heating.
A · Ring bearing -CH(OH)CH3 and -CH2OH
B · Ring bearing -CO-CH3 (intact) and -CH2OH
C · Ring bearing -CO-CH3 (intact) and -COO- (2-acetylbenzoate) ✓
D · Ring bearing -CH(OH)CH3 and -COO-
Solution: Tollens' reagent [Ag(NH3)2]+ is a mild oxidant. In the basic medium it oxidises the aldehyde -CHO to the carboxylate -COO- and deposits a silver mirror. It does NOT touch the ketone -CO-CH3 and does NOT reduce anything. So only -CHO becomes -COO-; the acetyl group stays intact, giving 2-acetylbenzoate. Option C.
NEET 2017
X (C2H6O) is heated over Cu at 573 K to give A. A gives a silver mirror with [Ag(NH3)2]+/OH-, an aldol product Y on heating with OH-, and Z with H2N-NH-CO-NH2. Identify A, X, Y and Z.
A · A = methoxymethane, X = ethanoic acid, Y = acetate ion, Z = hydrazine
B · A = methoxymethane, X = ethanol, Y = ethanoic acid, Z = semicarbazide
C · A = ethanal, X = ethanol, Y = but-2-enal, Z = semicarbazone ✓
D · A = ethanol, X = acetaldehyde, Y = butanone, Z = hydrazone
Solution: Cu at 573 K dehydrogenates a primary alcohol to an aldehyde, so X = ethanol and A = ethanal (CH3CHO). The positive Tollens' test (silver mirror) confirms A is an aldehyde. Aldol of ethanal then heating gives CH3-CH=CH-CHO, but-2-enal (Y). Ethanal + semicarbazide gives the semicarbazone (Z). Option C.
Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Do both tests work for aromatic aldehydes like benzaldehyde?
Only Tollens'. Benzaldehyde gives a silver mirror with Tollens' reagent but does not reduce Fehling's reagent. Fehling's is reliable mainly for aliphatic aldehydes.
What colour is the precipitate in Fehling's test?
A red-brown (reddish) precipitate of copper(I) oxide, Cu2O. The starting Fehling's solution is deep blue because of Cu2+; the colour change from blue to red-brown means an aldehyde is present.
What is the formula of Tollens' reagent?
Tollens' reagent is ammoniacal silver nitrate, written as the diamminesilver(I) ion [Ag(NH3)2]+. It is freshly prepared before use because it can become unstable on standing.
Why is the product a carboxylate ion and not a carboxylic acid?
Both tests are run in alkaline (basic) medium (OH- and NH3 present). Any -COOH formed immediately loses its proton, so the real product is the carboxylate anion R-COO-. Write R-COO-, not R-COOH, in NEET answers.
Can these tests oxidise ketones at all?
No. Tollens' and Fehling's are mild oxidants. They cannot break the C-C bonds needed to oxidise a ketone, so ketones give no silver mirror and no red precipitate. This is what makes the tests useful for telling aldehydes and ketones apart.
