Qualitative chemical tests for aldehydes ketones Fehling's Benedict's Tollen's reagent iodoform reaction equations conditions observations 24DNPH advanced A level organic chemistry revision notes doc brown

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Advanced Level Organic Chemistry: The oxidation of aldehydes & ketones, tests

Part 5. The chemistry of ALDEHYDES and KETONES

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 5.7 The oxidation of aldehydes and ketones and chemical tests with Tollen's reagent and Fehlings and Benedict's solutions plus the iodoform reaction

INDEX of ALDEHYDES and KETONES revision notes

All Advanced A Level Organic Chemistry Notes

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Oxidation of aldehydes and ketones

Readily oxidisable aldehydes gives a relatively stable carboxylic acid e.g.

using a mixture of dil. sulfuric acid and potassium dichromate(VI), even at room temperature you see an orange to green colour change in the aqueous reagent.

Ketones are not usually readily oxidised by this reagent, so it will often distinguish an aldehyde from a ketone, BUT, lots of other organic compounds are readily oxidised by acidified dichromate solution e.g. primary and secondary alcohols, so its not a useful test for aldehydes.

(i) ethanal ==> ethanoic acid

+ [O] ===>

(ii) 2-methylpropanal ==> 2-methylpropanoic acid

+ [O] ===>

(iii) butanal ===> butanoic acid

+ [O] ===>

(iv) pentanal ==> pentanoic acid

+ [O] ===>

The redox half-equation for oxidising an aldehyde to a carboxylic acid is:

R-CHO + H₂O ===> R-COOH + 2H+ + 2e^-

The redox half-equation for oxidising a primary alcohol to an aldehyde is:

R-CH₂OH ===> R-CHO + 2H+ + 2e^-

Ketones are NOT readily oxidised because it would involve breaking strong bonds in the carbon chain.

Aldehydes are readily oxidised by:

Potassium dichromate(VI)/sulfuric acid solution.

This gives the free carboxylic acid:

RCHO + [O] ===> RCOOH

The colour changes from the orange of the dichromate(VI) ion to the green chromium(III) ion.

Cr₂O₇^2-(aq) == reduced ==> Cr³⁺(aq)

(i) reduction half-equation: Cr₂O₇^2–_(aq) + 14H⁺_(aq) + 6e^– ===> 2Cr³⁺_(aq) + 7H₂O_(l)

(ii) oxidation half-equation for ethanal: CH₃CHO(aq) + H₂O(l) ===> CH₃COOH(aq) + 2H⁺(aq) + 2e^-

To obtain the fully balanced redox equation you add (i) + 3 x (iii)

(think 6 electron change to balance 1 : 3 ratio of equations (i) and (ii)

3CH₃CHO(aq) + Cr₂O₇^2–_(aq) + 8H⁺_(aq) ===> 3CH₃COOH(aq) + 2Cr³⁺_(aq) + 4H₂O_(l)

More details of this reaction can be found on ...

Part 4.5 Controlled oxidation of alcohols/aldehydes with selected oxidising agents

Potassium manganate(VII) in acidic, neutral or alkaline solution.

In acid/neutral conditions you get the free carboxylic acid:

RCHO + [O] ===> RCOOH

In acid conditions the purple manganate(VII) ion MnO₄^- is reduced to the almost colourless Mn²⁺ ion.

Under neutral and alkaline conditions the purple manganate(VII) ion is reduced to the insoluble brown precipitate of

In alkaline conditions you get the carboxylate anion - as the salt of the carboxylic acid:

RCHO + [O] + OH^- ===> RCOO^- + H₂O

Unlike ketones, aldehydes are oxidised to the carboxylate ion when testing with Fehling's solution or Tollen's reagent (see below).

Non of the above oxidations and observations are particularly useful for identifying aldehydes or ketones, although the observations can distinguish aldehydes from ketones.

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Qualitative ORGANIC functional group tests for aldehydes and ketones
CHEMICAL TEST FOR	TEST METHOD	OBSERVATIONS	TEST CHEMISTRY and comments
Aldehydes chemical test (R–CHO, R = H, alkyl or aryl) to distinguish from ketones (R₂C=O, R = alkyl or aryl) and also reducing sugars. Note (1) Test (b)(i) and (ii) can be used to distinguish aldehydes (reaction) and ketones (no reaction). (2) Aromatic aldehydes do NOT give a positive result with (b)(ii) Benedict's or Fehling's reagent). (3) Reducing sugars may also give a positive test with (b)(i) and (ii) reagents e.g. glucose (aldohexose) but not fructose? (ketohexose)?	(a) Add a few drops of the suspected carbonyl compound to Brady's reagent (2,4–dinitrophenylhydrazine solution) Often quoted as just 24DNPH !	(a) A yellow–orange precipitate forms with both types of carbonyl compound. Specific identification The precipitate may be collected, recrystallised and from it's unique melting point of the crystals, the aldehyde or ketone can be identified from data tables.	The aldehyde or ketone 2,4–dinitrophenylhydrazone is formed R₂C=O + (NO₂)₂C₆H₃NHNH₂ ==> (NO₂)₂C₆H₃NHN=CR₂ + H₂O (R = H, alkyl or aryl) This tells you it's an aldehyde or ketone, but you can't distinguish them, read on below! For details of the 24DNPH reaction Addition - elimination reactions of aldehydes & ketones
	(b)(i) warm a few drops of the compound with Tollens' reagent [ammoniacal silver nitrate] (b)(ii) simmer with Fehling's or Benedict's solution [a blue tartrate complex of Cu²⁺_(aq)] Tollen's reagent is made by adding a few drops of sodium hydroxide to silver nitrate solution. Dilute ammonia is added until the precipitate just dissolve.	(b) Only the aldehyde produces ... (i) A silver mirror on the side of the test tube. (ii) The deep blue solution produces a brown or brick red precipitate of copper(I) oxide if its an aldehyde Ketones do not give a response to tests with Tollens' reagent, Fehling's solution or Benedict's solution - no colour changes and no precipitates.	Aldehydes are stronger reducing agents than ketones and reduce the metal ion and are oxidised in the process to a carboxylic acid i.e. RCHO + [O] ==> RCOOH (i) reduction of silver(I) ion to silver metal RCHO + 2Ag⁺ + H₂O ==> RCOOH + 2Ag + 2H⁺ (ii) reduction of copper(II) to copper(I) i.e. the blue solution of the Cu²⁺ complex changes to the brown/brick red colour of insoluble copper(I) oxide Cu₂O. RCHO + 2Cu²⁺ + 2H₂O ==> RCOOH + Cu₂O + 4H⁺ For (b)(i)/(ii) technically the oxidation is RCHO + [O] + OH^- ===> RCOO^- + H₂O because the reagents are alkaline. With (b)(i)/(ii) no reactions with ketones.
Iodoform test The formation of CHI₃, triiodomethane (or old name '*iodoform*'.	NaOH_(aq) is added to a solution of iodine in potassium iodide solution until most of the colour has gone. The organic compound is warmed with this solution.	A yellow solid is formed with the smell of an antiseptic, CHI₃, tri–iodomethane, melting point 119^oC.	This reaction is given by the aldehyde ethanal CH₃CHO and all ketones with the '2–one' structure R–CO–CH₃ ('methyl ketones'). More details below. Its a combination of halogenation and oxidation and is not a definitive test for any functional group, it just indicates a possible part of a molecule's structure.

Extra notes on the iodoform reaction

To make the reagent, sodium hydroxide solution is added to a solution of iodine in potassium iodide solution until most of the colour has gone.

You can also use a mixture of potassium iodide dissolved in sodium chlorate(I) solution.

The organic compound is warmed with this solution.

Only the aldehyde ethanal (CH₃CHO) and all the ketones with the CH₃CO- end grouping give the iodoform reaction.

It is a complicated reaction, in which the hydrogen atoms of the methyl group of the CH₃CO grouping are replaced by hydrogen atoms.

The sodium hydroxide/iodine reagent causes the C-C bond of the CH₃CO grouping to break in the process of forming the pale yellow precipitate of CHI₃ (triiodomethane, 'iodoform'), that smells like an antiseptic.

The following equilibrium is found in the NaOH/I₂ reagent:

I₂ + 2OH- IO^- + I- H₂O

The iodate(I) ion, IO^-, substitutes I for H forming the CI₃CO grouping.

The electron withdrawing effect of the three iodine atoms weakens the C-C sigma bond allowing for the formation of CHI₃.

For R = H or alkyl e.g. CH₃, CH₂CH₃ etc., the overall equation is:

CH₃COR + 3I₂ + 4NaOH ===> CHI₃ + RCOONa + 3NaI + 3H₂O

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Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of ALDEHYDE and KETONE revision notes

All Advanced Organic Chemistry Notes

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