A level Chemistry · Edexcel 9CH0 · Topic 17B
Carbonyl compounds
Key points
- Aldehydes (–CHO) and ketones (C=O between two carbons) contain a polar carbonyl group. They cannot hydrogen bond with each other, so have lower boiling points than alcohols, but small ones dissolve in water.
- Oxidation: aldehydes are oxidised to carboxylic acids by acidified dichromate (orange → green); ketones are not.
- Distinguishing tests: Fehling's/Benedict's (blue → brick-red precipitate) and Tollens' reagent (silver mirror) are positive for aldehydes only.
- Reduction with LiAlH₄ in dry ether: aldehydes → primary alcohols, ketones → secondary alcohols.
- Nucleophilic addition of HCN (KCN with acid) gives hydroxynitriles; the CN⁻ attacks the δ+ carbon.
- 2,4-dinitrophenylhydrazine gives an orange precipitate with any carbonyl; melting point of the purified solid identifies it.
- Iodoform test: iodine in alkali gives a pale yellow precipitate (CHI₃) with CH₃CO– groups and with CH₃CH(OH)– alcohols.
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Aldehydes and ketones both contain the carbonyl group, C=O, but an aldehyde has a hydrogen on the carbonyl carbon and a ketone has two carbons. That one hydrogen is why aldehydes can be oxidised and ketones cannot, and it is the basis of…
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