Aldehydes and ketones: Class 12 Chemistry

Key points

Aldehydes (R-CHO) and ketones (R-CO-R') both contain the carbonyl group C=O. In an aldehyde the carbonyl carbon is bonded to at least one hydrogen, so the group is at the end of the chain. In a ketone it is bonded to two carbon atoms. The carbonyl carbon is sp² hybridised, the group is planar with bond angles of about 120°, and the C=O bond is polar, with a partial positive charge on carbon and a partial negative charge on oxygen.

IUPAC names: aldehydes end in al (methanal HCHO, ethanal CH₃CHO) and ketones end in one (propanone CH₃COCH₃, butanone CH₃COC₂H₅). The carbonyl carbon of an aldehyde is number 1. In ketones the chain is numbered to give the C=O the lowest number, as in pentan-2-one. Common names are formaldehyde, acetaldehyde and acetone.

Preparation of aldehydes and ketones:

• Oxidation of primary alcohols gives aldehydes and of secondary alcohols gives ketones. To stop at the aldehyde the product is distilled off as it forms.
• Ozonolysis of alkenes gives carbonyl compounds.
• Hydration of alkynes: ethyne with dilute H₂SO₄ and HgSO₄ gives ethanal, and other alkynes give ketones.
• Friedel-Crafts acylation of benzene gives aromatic ketones such as phenylethanone.
• Dry distillation of calcium salts of carboxylic acids gives ketones, for example calcium ethanoate gives propanone.

Aldehydes and ketones are polar, so their boiling points are higher than those of alkanes of similar mass. They are lower than those of alcohols because they cannot form hydrogen bonds with each other. The lower members dissolve in water, since the oxygen forms hydrogen bonds with water. Methanal is a gas, ethanal boils at about 21 °C, and propanone is a liquid that boils at 56 °C.

Nucleophilic addition is the typical reaction. The nucleophile attacks the partly positive carbon, the pi bond breaks and the oxygen gets a negative charge, then takes a proton. Aldehydes are more reactive than ketones, because they have less steric hindrance, and because alkyl groups in ketones push electrons toward carbon and reduce its positive charge.

Examples of nucleophilic addition:

• HCN gives a cyanohydrin, which adds one carbon: CH₃CHO + HCN → CH₃CH(OH)CN.
• NaHSO₃ gives a white crystalline addition product with aldehydes and methyl ketones.
• Grignard reagents give alcohols: methanal gives a primary, other aldehydes a secondary and ketones a tertiary alcohol.
• 2,4-DNPH (2,4-dinitrophenylhydrazine) gives a yellow or orange precipitate with all aldehydes and ketones, and is a test for the carbonyl group.

Oxidation and reduction:

• Aldehydes are easily oxidised to carboxylic acids, even by mild oxidising agents. Ketones resist oxidation and need strong agents which break the chain.
• Reduction with H₂ and Ni, LiAlH₄ or NaBH₄ gives primary alcohols from aldehydes and secondary alcohols from ketones.
• Clemmensen reduction (Zn/Hg and conc. HCl) turns C=O into CH₂.

Tests that separate aldehydes from ketones:

• Tollens' test: warm with ammoniacal silver nitrate. An aldehyde gives a silver mirror, and a ketone does not. RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O.
• Fehling's test: warm with Fehling's solution. An aliphatic aldehyde gives a red-brown precipitate of Cu₂O, and a ketone gives none. Benzaldehyde does not respond.
• Iodoform test: compounds with CH₃CO- (ethanal and methyl ketones) or CH₃CH(OH)- give a yellow precipitate of CHI₃ with iodine and NaOH.

Aldol condensation: aldehydes and ketones with an alpha hydrogen react in dilute alkali. Two molecules of ethanal join to form 3-hydroxybutanal (an aldol), which loses water on heating to give but-2-enal. Aldehydes with no alpha hydrogen, such as methanal and benzaldehyde, undergo the Cannizzaro reaction in concentrated alkali, where one molecule is oxidised to the acid salt and another is reduced to alcohol.

Uses: methanal as a 40% solution called formalin, used to preserve specimens and to make Bakelite and other resins. Ethanal is used to make ethanoic acid and other chemicals. Propanone is a common solvent (nail polish remover) for paints and plastics. Benzaldehyde is used in perfumes and flavours.

Study tip: Learn the three distinguishing tests with their exact colours, and know which compounds give iodoform. For mechanism questions, always draw the attack of the nucleophile on the carbonyl carbon and the movement of electrons to oxygen. Aldol and Cannizzaro are often asked together, so remember the alpha hydrogen rule that separates them.

Definitions and flashcards

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Short questions with model answers

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Numericals with worked steps

Common mistakes

• Saying ketones give a positive Tollens' or Fehling's test. Only aldehydes do (and Fehling's only aliphatic ones).
• Writing the aldehyde group as -COH. The correct form is -CHO.
• Thinking 2,4-DNPH separates aldehydes from ketones. It only confirms a carbonyl group.
• Numbering ketones from the wrong end, so the C=O does not get the lowest number.
• Forgetting that the iodoform test needs a CH₃CO- group, so propanal and benzaldehyde do not give it.

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