Q.Explain Tollen's test used for distinguishing propanal and propanone.
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The Tollens Test: The Silver Mirror That Spots an Aldehyde
Imagine you have two unlabelled bottles — one contains an aldehyde, the other a ketone. They look identical, smell similar, and both are carbonyl compounds. How do you tell them apart in a single, dramatic step? The Tollens test gives you a visible, unmistakable answer: a beautiful silver mirror coating the inside of your test tube.
The Intuition: Why Aldehydes Are Special
The key difference lies in the carbonyl carbon. In an aldehyde, that carbon is bonded to at least one hydrogen atom. In a ketone, it is bonded to two carbon groups. That hydrogen on the aldehyde is not just any hydrogen — it is weakly acidic and, more importantly, easily oxidised. The aldehyde can lose this hydrogen and become a carboxylic acid, donating electrons in the process.
Tollens reagent is a solution of silver ions (Ag+) held in a complex with ammonia. Silver ions are excellent oxidising agents — they want to gain electrons and become neutral silver metal. When an aldehyde meets these silver ions, the aldehyde gets oxidised (loses its hydrogen) and the silver ions get reduced (gain electrons), forming metallic silver.
Think of it as a trade: the aldehyde gives away electrons (gets oxidised), and the silver ions accept those electrons (get reduced). The silver atoms then clump together on the glass surface, forming that characteristic mirror.
The Precise Statement
Tollens test is a qualitative chemical test that distinguishes aldehydes from ketones. When an aldehyde is warmed with ammoniacal silver nitrate (Tollens reagent), the aldehyde is oxidised to a carboxylate ion, and the silver(I) ions are reduced to metallic silver, which deposits as a shiny mirror on the walls of the test tube. Ketones do not react under these mild conditions.
RCHO+2[Ag(NH3)2]++3OH−⟶RCOO−+2Ag↓+4NH3+2H2O
How the Reagent Is Made
You cannot just use silver nitrate directly — the silver ions would precipitate as silver oxide in the alkaline conditions needed for the reaction. So you first dissolve silver nitrate in water, add sodium hydroxide to get a brown precipitate of silver oxide, and then add just enough ammonia to redissolve that precipitate. The result is a clear solution containing the complex ion [Ag(NH3)2]+.
Never store Tollens reagent. It forms explosive silver nitride (Ag3N) on standing. Prepare it fresh, use it immediately, and destroy any leftover by adding dilute acid.
The Procedure in the Lab
- Clean a test tube thoroughly — any grease will ruin the mirror.
- Add a few drops of the unknown compound.
- Add about 2 mL of freshly prepared Tollens reagent.
- Warm the mixture gently in a water bath (do not boil). …
Tollens' reagent oxidises aldehydes because they carry an oxidisable hydrogen on the carbonyl carbon, reducing Ag+ to metallic silver; a ketone's carbonyl carbon has no such hydrogen, so it cannot be oxidised this way. …
Tollens' reagent (ammoniacal AgNO3) oxidises aldehydes (which have an oxidisable C-H bond on the carbonyl carbon) to carboxylate, depositing metallic silver as a mirror; ketones like propanone have no such H and are not oxidised, so no mirror forms.
Tollens' reagent is prepared by adding aqueous ammonia to silver nitrate solution until the precipitate of Ag2O just dissolves, giving the diammine silver(I) complex [Ag(NH3)2]+ OH-.
When propanal (CH3CH2CHO, an aldehyde) is warmed gently with Tollens' reagent in a clean test tube, the aldehyde is oxidised to the corresponding carboxylate (propanoate) while Ag+ is reduced to metallic silver (Ag0), which deposits as a bright, shiny silver mirror on the inner wall of the test tube:
CH3CH2CHO + 2[Ag(NH3)2]+ + 3OH- -> CH3CH2COO- + 2Ag(s, mirror) + 4NH3 + 2H2O
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Showing the 12 most recent of 18 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following gives silver mirror with Tollen's reagent?(a) Formic acid(b) Acetic acid(c) Acetone(d) Ethyl alcohol
›Reveal solutionSolution
Formic acid is unusual among carboxylic acids in giving Tollens' test because it structurally contains an aldehyde-like H–C=O unit.
Tollens' reagent ([Ag(NH3)2]+OH−, ammoniacal silver nitrate) is reduced to metallic silver (depositing as a bright 'silver mirror' on the test tube) by compounds that can be oxidised easily, classically aldehydes (R−CHO→R−COOH, with Ag+→Ag).
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- CBSE 2026Set ANNUAL1 markMCQQ.Tollen's reagent is used to distinguish between(a) Alcohol and ether(b) Aldehydes and ketones(c) Carboxylic acid and esters(d) 1 degree, 2 degree and 3 degree amines
›Reveal solutionSolution
Tollen's test distinguishes aldehydes from ketones because only aldehydes are easily oxidised, reducing the diamminesilver(I) complex to metallic silver (a bright mirror).
Tollen's reagent is an ammoniacal solution of silver nitrate, containing the complex ion [Ag(NH3)2]+. When warmed gently with an aldehyde, the aldehyde is oxidised to a carboxylate ion, while Ag+ is reduced to metallic silver, which deposits as a shiny 'silver mirror' on the test-tube wall:
RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag(s) + 4NH3 + 2H2O
…
- CBSE 2026Set ANNUAL1 markQ.Among aldehydes or ketones which gives positive Tollen's Test?
›Reveal solutionSolution
Tollens' reagent (ammoniacal silver nitrate) oxidizes aldehydes to carboxylate ions while itself being reduced to metallic silver (silver mirror); ketones do not respond (with the exception of α-hydroxy ketones).
Tollens' reagent, [Ag(NH3)2]+, is a mild oxidizing agent. Aldehydes are readily oxidized (they have an oxidizable C–H bond on the carbonyl carbon):
RCHO+2[Ag(NH3)2]++3OH−→RCOO−+2Ag↓+4NH3+2H2O
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- CBSE 2024Set FZ1 markMCQQ.Which of the following compound is identified by Tollen's Reagent?(a) Alcohol(b) Aldehyde(c) Ketone(d) Carboxylic acid
›Reveal solutionSolution
Tollen's reagent gives a silver mirror only with aldehydes → option (b).
Concept. Tollen's reagent is a mild oxidising agent, the diamminesilver(I) ion [Ag(NH3)2]+. Aldehydes are readily oxidised to carboxylate; in doing so they reduce Ag+ to metallic silver, which deposits as a bright silver mirror.
Reaction.
RCHO+2[Ag(NH3)2]++3OH−⟶RCOO−+2Ag↓+4NH3+2H2O …
- CBSE 2024Set ANNUAL1 markQ.What is Tollen's reagent?
›Reveal solutionSolution
Tollen's reagent is a solution of silver nitrate in excess ammonia, [Ag(NH3)2]+OH-; it selectively oxidises aldehydes and deposits metallic silver as a mirror on the container wall.
Preparation: AgNO3 solution is treated with dilute NaOH to precipitate Ag2O, then dissolved in excess aqueous ammonia to give the soluble complex ion [Ag(NH3)2]+ (diamminesilver(I) ion), along with OH-.
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- CBSE 2023Set F1 markMCQQ.Which of the following is Tollen's reagent?(a) [Ag(NH3)2]+ ion(b) Cu(OH)2(c) Ag2O(d) CuO
›Reveal solutionSolution
Tollen's reagent is ammoniacal silver nitrate, containing the diamminesilver(I) ion, [Ag(NH3)2]+.
Tollen's reagent is prepared by adding aqueous ammonia to silver nitrate until the initial precipitate just dissolves, forming the complex ion [Ag(NH3)2]+ (diamminesilver(I)). Aldehydes reduce it, depositing metallic silver as a bright silver mirror (the silver …
- CBSE 2023Set ANNUAL1 markMCQQ.Tollen's reagent is(a) mixture of CuSO4 and NaOH(b) ammoniacal silver nitrate solution(c) a solution of copper sulphate, sodium citrate and sodium carbonate(d) anhydrous ZnCl2 and conc. HCl
›Reveal solutionSolution
Tollen's reagent is AgNO3 dissolved in excess ammonia, which forms the diamminesilver(I) complex used to oxidise aldehydes.
Tollen's reagent is prepared by adding aqueous ammonia to silver nitrate solution until the initially formed brown precipitate of Ag2O just redissolves, giving [Ag(NH3)2]+OH- (ammoniacal silver nitrate). Aldehydes reduce this complex to metallic silver, which deposits as a bright 'silver mirror' on the test-tube wall, while the aldehyde is …
- CBSE 2023Set TERM21 markQ.Freshly prepared ammonical silver nitrate solution is called ……………
›Reveal solutionSolution
Tollens' reagent is used to test for aldehydes via the 'silver mirror test'.
Tollens' reagent is prepared by adding aqueous ammonia to silver nitrate solution until the initially formed brown precipitate of Ag2O just dissolves, forming the complex ion [Ag(NH3)2]+OH− (diamminesilver(I) hydroxide) — this freshly-prepared solution is called Tollens' reagent.
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- CBSE 2023Set ANNUAL1 markMCQQ.The compounds CH₃CH₂CHO and CH₃CH₂COOH cannot be distinguished by -(i) Tollen's test(ii) Fehling's test(iii) NaHCO₃ test(iv) Hinsberg's test
›Reveal solutionSolution
Hinsberg's test is a test for classifying amines, not carbonyl compounds — it is simply irrelevant to this pair, so neither compound gives a diagnostic result and the two cannot be told apart by it.
Propanal (CH3CH2CHO) and propanoic acid (CH3CH2COOH) CAN be distinguished by:
- Tollens' test: the aldehyde reduces Tollens' reagent to give a silver mirror; the carboxylic acid does not.
- Fehling's test: the aldehyde gives a brick-red precipitate of Cu2O; the carboxylic acid does not.
- NaHCO₃ test: the carboxylic acid gives brisk effervescence of CO2 (it's acidic enough to liberate CO2 from bicarbonate); the aldehyde does not react. …
- CBSE 2022Set ANNUAL1 markMCQQ.Tollen's reagent is used to identify –(a) aldehydes(b) ketones(c) carboxylic acids(d) alcohols
›Reveal solutionSolution
Tollen's reagent (ammoniacal silver nitrate) is used to identify aldehydes via the silver-mirror test.
Tollen's reagent, [Ag(NH3)2]⁺OH⁻, oxidizes an aldehyde to the corresponding carboxylate ion while Ag⁺ is reduced to metallic silver, which deposits as a bright silver mirror on the walls of the test tube …
- CBSE 2022Set ANNUAL1 markQ.What is Tollen's reagent?
›Reveal solutionSolution
Tollen's reagent = ammoniacal AgNO3, containing [Ag(NH3)2]+, a mild oxidant that gives a silver mirror with aldehydes.
Tollen's reagent is prepared by adding aqueous ammonia to silver nitrate solution until the initial brown precipitate of Ag2O just dissolves, forming the soluble complex [Ag(NH3)2]+.
It is a weak oxidising agent. When warmed with an aldehyde, the aldehyde is oxidised to the corresponding carboxylate ion, while Ag+ is reduced to metallic silver, which deposits as a shiny mirror on the inner wall of the tube:
…
- CBSE 2021Set ANNUAL1 markMCQQ.Aldehydes and ketones can be distinguished by –(a) Tollen's reagent(b) Sodium bicarbonate test(c) FeCl3 solution(d) Carbylamine test
›Reveal solutionSolution
Aldehydes reduce Tollen's reagent (ammoniacal AgNO3) to give a silver mirror; ketones do not.
Tollen's reagent is a mild oxidising agent, [Ag(NH3)2]+. Aldehydes are easily oxidised to carboxylate ions because the carbonyl carbon still carries an H atom, so they reduce Ag+ to metallic silver, which deposits as a bright silver mirror on the test-tube wall:
RCHO+2[Ag(NH3)2]++3OH−→RCOO−+2Ag↓+4NH3+2H2O
…
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