Topic 15 · page 7 of 28

Oxidation tests for aldehydes

Aldehydes are easily oxidised to carboxylic acids and ketones are not. Tollens' reagent, Fehling's or Benedict's solution and acidified dichromate(VI) all react with aldehydes and not with ketones.

14 min read15.8(i)

Read first: Naming aldehydes and ketones

Why aldehydes oxidise and ketones do not 15.8(i)

The carbonyl carbon of an aldehyde has a hydrogen atom bonded to it. An oxidising agent can replace that hydrogen with an –OH group, which turns the aldehyde into a carboxylic acid.

CHX3CHO+[O]→CHX3COOH \ce{CH3CHO + [O] -> CH3COOH}

The oxidising agent can be written as [O] in equations, as the specification allows. A ketone has no hydrogen on the carbonyl carbon, only two C–C bonds. Oxidising it would mean breaking a carbon–carbon bond, which needs much harsher conditions, so a ketone does not react with these mild reagents.

This difference gives three tests that separate aldehydes from ketones.

Three pairs of test tubes after warming, each with ethanal on the left and propanone on the right. Tollens' reagent: a silver mirror forms with ethanal and nothing happens with propanone. Fehling's solution: a brick-red precipitate forms with ethanal and the solution stays blue with propanone. Acidified potassium dichromate(VI): orange turns green with ethanal and stays orange with propanone.
Three oxidation tests. Only the aldehyde gives a positive result in each pair.

Tollens' reagent 15.8(i)

Tollens' reagent is ammoniacal silver nitrate: silver nitrate solution with just enough ammonia added to dissolve the precipitate. It contains the diamminesilver(I) ion, [Ag(NHX3)X2]X+\ce{[Ag(NH3)2]+}. It is prepared fresh.

Warm the sample with the reagent in a hot water bath. An aldehyde reduces the silver(I) ion to silver metal, which coats the tube as a silver mirror.

CHX3CHO+2 [Ag(NHX3)X2]X++3 OHX−→CHX3COOX−+2 Ag+4 NHX3+2 HX2O \ce{CH3CHO + 2[Ag(NH3)2]+ + 3OH- -> CH3COO- + 2Ag + 4NH3 + 2H2O}

With a ketone there is no change and the solution stays colourless.

Fehling's and Benedict's solutions 15.8(i)

Fehling's solution and Benedict's solution are blue alkaline solutions containing copper(II) ions held in a complex so that they do not form a precipitate of copper(II) hydroxide. Warm the sample with either solution. An aldehyde reduces the copper(II) ions to copper(I) oxide, a brick-red precipitate.

CHX3CHO+2 CuX2++5 OHX−→CHX3COOX−+CuX2O+3 HX2O \ce{CH3CHO + 2Cu^2+ + 5OH- -> CH3COO- + Cu2O + 3H2O}

With a ketone the solution stays blue.

Acidified dichromate(VI) 15.8(i)

Warm the sample with acidified potassium dichromate(VI), which is orange. An aldehyde is oxidised to the acid and the chromium is reduced from orange CrX2OX7X2−\ce{Cr2O7^2-} (chromium in the +6 state) to green CrX3+\ce{Cr^3+} (+3 state). A ketone gives no change.

Test Aldehyde, for example ethanal Ketone, for example propanone
Tollens' reagent, warm silver mirror no change
Fehling's or Benedict's, warm blue solution to brick-red precipitate stays blue
acidified dichromate(VI), warm orange to green stays orange

Check yourself

  1. State what you see when ethanal is warmed with (a) Tollens' reagent, (b) Fehling's solution and (c) acidified potassium dichromate(VI).
  2. Write an equation using [O] for the oxidation of propanal.
  3. Explain why propanone does not react with Tollens' reagent.
  4. An unknown liquid turns acidified potassium dichromate(VI) from orange to green. Explain why this does not prove that it is an aldehyde, and suggest a second test that would.
  5. A colourless liquid gives a brick-red precipitate with Benedict's solution. Name the functional group that it contains, and the organic product of the reaction.

Answers cover these until you have tried the questions

  1. (a) A silver mirror forms on the inside of the tube. (b) The blue solution turns to a brick-red precipitate. (c) The orange solution turns green.
  2. CHX3CHX2CHO+[O]→CHX3CHX2COOH\ce{CH3CH2CHO + [O] -> CH3CH2COOH}. The aldehyde is oxidised to propanoic acid.
  3. A ketone has no hydrogen atom on the carbonyl carbon. Oxidising it would require breaking a C–C bond, which these mild reagents cannot do, so there is no reaction.
  4. Primary and secondary alcohols are also oxidised by acidified dichromate(VI) and also turn it green, so the result fits an alcohol as well as an aldehyde. Warm the liquid with Tollens' reagent: a silver mirror shows an aldehyde, because alcohols do not give one.
  5. The liquid contains an aldehyde group, –CHO. The organic product is a carboxylic acid, which in the alkaline solution is present as its carboxylate ion.