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17.1 Aldehydes and ketones

Syllabus
9701–2028–2029
Topic
17.1
Level
AS

Learning objectives

17.1.1Preparation of aldehydes and ketones• Recall preparation of aldehydes and ketones- (a) the oxidation of primary alcohols using acidified K 2Cr2O7 or acidified KMnO4 and distillation to produce aldehydes- (b) the oxidation of secondary alcohols using acidified K 2Cr2O7 or acidified KMnO4 and distillation to produce ketones17.1.2Describe• Describe:: (a) the reduction of aldehydes and ketones using NaBH4 or LiAl H4 to produce alcohols; (b) the reaction of aldehydes and ketones with HCN, KCN as catalyst, and heat to produce hydroxynitriles as exemplified by ethanal and propanone17.1.3Reactions of hydrogen cyanide• Describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones in 17 .1.2(b)17.1.42,4-DNPH carbonyl test• Describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH reagent) to detect the presence of carbonyl compounds17.1.5The nature (aldehyde or ketone) of an unknown• Deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of simple tests (Fehling’s and Tollens’ reagents; ease of oxidation)17.1.6Iodoform test for CH3CO- group• Deduce the presence of a CH3CO – group in an aldehyde or ketone, CH3CO–R, from its reaction with alkaline I2(aq) to form a yellow precipitate of tri-iodomethane and an ion, RCO2 –

Distil an oxidised alcohol to obtain an aldehyde or ketone

Alcohol starting material Oxidant and apparatus Carbonyl product
primary, RCH₂OH acidified K₂Cr₂O₇ or acidified KMnO₄; heat and distil aldehyde, RCHO
secondary, R₂CHOH acidified K₂Cr₂O₇ or acidified KMnO₄; heat and distil ketone, R₂CO

RCHX2OH+[O]→RCHO+HX2O\ce{RCH2OH + [O] -> RCHO + H2O}

RX2CHOH+[O]→RX2CO+HX2O\ce{R2CHOH + [O] -> R2CO + H2O}

Distillation separates the carbonyl product as it forms. This is essential for an aldehyde because leaving it in hot oxidising mixture allows further oxidation to a carboxylic acid; it also matches the stated preparation condition for ketones.

A tertiary alcohol does not produce an aldehyde or ketone by this oxidation pattern. Do not reflux a primary alcohol when the target is the aldehyde.

Reduce C=O to an alcohol or add HCN to make a hydroxynitrile

Carbonyl starting material Reagent and conditions Product class Structural change
aldehyde NaBH₄ or LiAlH₄ primary alcohol C=O → CH–OH
ketone NaBH₄ or LiAlH₄ secondary alcohol C=O → CH–OH
aldehyde or ketone HCN, KCN catalyst, heat hydroxynitrile CN and OH add to the former carbonyl carbon

CHX3CHO+2 [H]→CHX3CHX2OH\ce{CH3CHO + 2[H] -> CH3CH2OH}

CHX3CHO+HCN→CHX3CH(OH)CN\ce{CH3CHO + HCN -> CH3CH(OH)CN}

CHX3COCHX3+HCN→(CHX3)X2C(OH)CN\ce{CH3COCH3 + HCN -> (CH3)2C(OH)CN}

The CN carbon becomes part of the product skeleton, so HCN addition increases the carbon count by one. Reduction adds no carbon: an aldehyde gives a primary alcohol and a ketone gives a secondary alcohol, not a tertiary alcohol.

CN⁻ attacks the δ+ carbon in nucleophilic addition

The C=O bond is polar: carbon is δ+ and oxygen is δ−. CN⁻ is the nucleophile because the carbon lone pair can attack the electron-deficient carbonyl carbon.

Step 1: draw a full curly arrow from the lone pair on the carbon end of :C≡N⁻ to the carbonyl carbon, and simultaneously draw another from the C=O π bond to oxygen. This forms a tetrahedral alkoxide intermediate bearing O⁻ and CN.

Step 2: the O⁻ lone pair takes H from HCN; draw O⁻ → H and H–CN → CN. The hydroxynitrile forms and CN⁻ is regenerated, which explains KCN's catalytic role.

CHX3CHO+HCN→KCN,  heatCHX3CH(OH)CN\ce{CH3CHO + HCN ->[KCN,\ heat] CH3CH(OH)CN}

Attack occurs through carbon, giving a nitrile –C≡N, not through nitrogen. Curly arrows begin at electron pairs—a lone pair or π bond—not at the δ+ carbon or proton.

2,4-DNPH detects an aldehyde or ketone carbonyl

Add 2,4-dinitrophenylhydrazine reagent (2,4-DNPH) to the sample. Formation of a yellow or orange precipitate of a 2,4-dinitrophenylhydrazone is a positive result for an aldehyde or ketone C=O group.

The test establishes that one of these carbonyl classes is present, but both aldehydes and ketones respond. A second oxidation-based test is required to decide which class the unknown belongs to.

A positive 2,4-DNPH result does not identify the carbon skeleton and is not a general test for alcohols or carboxylic acids. Record the precipitate, not merely a vague colour change in solution.

Aldehydes reduce Fehling’s and Tollens’ reagents; ketones do not

Warmed reagent Aldehyde observation Ketone observation Meaning
Fehling’s solution blue solution gives brick-red Cu₂O precipitate remains blue; no precipitate aldehyde is oxidised while Cu²⁺ is reduced
Tollens’ reagent silver mirror / grey Ag deposit no silver deposit aldehyde is oxidised while Ag⁺ is reduced

First establish a carbonyl with 2,4-DNPH, then use either mild oxidation test. A positive Fehling’s or Tollens’ result identifies an aldehyde; a valid negative result for a DNPH-positive compound supports a ketone.

Aldehydes are readily oxidised to carboxylates in these alkaline test reagents and to carboxylic acids on acidification. Ketones lack the aldehydic H and are not readily oxidised under the same mild conditions.

A negative result is useful only when fresh reagent, warming and a DNPH-confirmed carbonyl are established. It is not proof of a ketone for an arbitrary unknown.

A yellow iodoform precipitate reveals CH₃CO–

Warm the aldehyde or ketone with alkaline aqueous iodine. A yellow precipitate of triiodomethane, CHI₃, is a positive result for the methyl-carbonyl group CH₃CO–R.

CHX3COR+3 IX2+4 OHX−→CHIX3(s)+RCOX2X−+3 IX−+3 HX2O\ce{CH3COR + 3I2 + 4OH- -> CHI3(s) + RCO2- + 3I- + 3H2O}

The CH₃ side becomes CHI₃ and the remaining acyl fragment becomes RCO₂⁻. Propanone has R = CH₃, so it gives CHI₃ and ethanoate, CH₃CO₂⁻; ethanal has R = H and gives methanoate, HCO₂⁻.

Ethanal is the only aldehyde with the required CH₃CO– arrangement; methyl ketones also qualify. Propanal, CH₃CH₂CHO, lacks this connectivity and gives no yellow precipitate.