Aldehydes and ketones

· Aldehydes and ketones contain the carbonyl functional group, C=O.
· Aldehyde functional group: RCHO; carbonyl carbon is bonded to H and R.
· Ketone functional group: RCOR′; carbonyl carbon is bonded to two carbon groups.
· The C=O bond is polar: oxygen is δ−, carbonyl carbon is δ+, so the carbonyl carbon is attacked by nucleophiles.
· Main reaction type in this topic: nucleophilic addition.

This image shows the aldehyde functional group, where the carbonyl carbon is bonded to one hydrogen atom and one R group. This distinguishes aldehydes from ketones in structure and in oxidation tests. Source

Production from alcohols

· Aldehydes are made by oxidising primary alcohols.
· Reagents: acidified K₂Cr₂O₇ or acidified KMnO₄.
· Conditions for aldehydes: distillation to remove the aldehyde before further oxidation.
· General: primary alcohol + [O] → aldehyde + H₂O.
· Ketones are made by oxidising secondary alcohols.
· Reagents: acidified K₂Cr₂O₇ or acidified KMnO₄.
· Conditions for ketones: distillation.
· General: secondary alcohol + [O] → ketone + H₂O.
· Exam tip: tertiary alcohols are not oxidised under these conditions.

This page helps visualise why aldehydes are easily oxidised but ketones resist oxidation. It is useful for linking preparation reactions to identification tests. Source

Reduction to alcohols

· Aldehydes are reduced to primary alcohols.
· Ketones are reduced to secondary alcohols.
· Reagents: NaBH₄ or LiAlH₄.
· Reaction type: reduction and nucleophilic addition of hydride.
· General aldehyde reaction: RCHO + 2[H] → RCH₂OH.
· General ketone reaction: RCOR′ + 2[H] → RCH(OH)R′.
· Exam tip: state both the reagent and the type of alcohol formed.

Reaction with HCN

· Aldehydes and ketones react with HCN to form hydroxynitriles.
· Reagents/conditions: HCN, KCN catalyst, heat.
· KCN provides CN⁻, the nucleophile that attacks the δ+ carbonyl carbon.
· Product contains both –OH and –CN on the former carbonyl carbon.
· Ethanal forms 2-hydroxypropanenitrile.
· Propanone forms 2-hydroxy-2-methylpropanenitrile.
· This reaction is important because it forms a new C–C bond and increases carbon chain length.

The diagram shows CN⁻ adding to the carbonyl carbon followed by protonation to form a hydroxynitrile/cyanohydrin. It directly supports the CIE requirement to know the product of aldehydes and ketones with HCN/KCN. Source

Mechanism: nucleophilic addition of HCN

· Step 1: CN⁻ attacks the δ+ carbonyl carbon.
· The C=O π bond breaks, and the electron pair moves onto oxygen.
· This forms a tetrahedral alkoxide ion intermediate.
· Step 2: the alkoxide ion accepts H⁺ from HCN/water.
· Product: hydroxynitrile.
· Curly arrow rule: arrows show movement of electron pairs, starting from a lone pair or bond.
· Key mechanism phrase: nucleophilic addition across the C=O bond.

This mechanism shows the two essential exam arrows: CN⁻ attacking the carbonyl carbon and the π bond moving to oxygen. It is a clear model for drawing the CIE nucleophilic addition mechanism. Source

Testing for carbonyl compounds: 2,4-DNPH

· 2,4-dinitrophenylhydrazine, written as 2,4-DNPH, tests for carbonyl compounds.
· Positive result with aldehydes and ketones: orange/yellow precipitate.
· This confirms the presence of a C=O group in an aldehyde or ketone.
· It does not distinguish aldehydes from ketones.
· Exam phrase: 2,4-DNPH detects the presence of a carbonyl compound.

Distinguishing aldehydes from ketones

· Aldehydes are easily oxidised; ketones are not easily oxidised.
· Tollens’ reagent: aldehydes give a silver mirror; ketones give no reaction.
· Fehling’s reagent: aldehydes give a brick-red precipitate; ketones give no reaction.
· With acidified K₂Cr₂O₇, aldehydes turn orange to green; ketones show no change.
· These tests allow you to deduce whether an unknown carbonyl compound is an aldehyde or ketone.
· Exam tip: if 2,4-DNPH is positive and Tollens’/Fehling’s is negative, the compound is a ketone.

Tri-iodomethane test

· Alkaline iodine, I₂(aq)/OH⁻, tests for the CH₃CO– group.
· Positive result: yellow precipitate of tri-iodomethane, CHI₃.
· Aldehyde/ketone structure tested: CH₃CO–R.
· Product also includes a carboxylate ion, RCO₂⁻.
· Important exception: ethanal, CH₃CHO, gives a positive result because it contains the required CH₃CO– pattern after oxidation/reaction pathway.