Aldehydes and ketones
Carbonyl compounds contain the C=O group. In an aldehyde the carbonyl carbon is at the end of the chain (–CHO), as in ethanal. In a ketone it is in the middle (R–CO–R′), as in propanone. The C=O bond is polar, so the carbon is attacked by nucleophiles: the typical reaction is nucleophilic addition.
| Reaction | Reagent | Product |
|---|---|---|
| Addition of HCN | HCN or NaCN with acid | Hydroxynitrile (cyanohydrin) |
| Reduction | NaBH₄ or LiAlH₄ | Aldehyde → primary alcohol; ketone → secondary alcohol |
| Condensation | 2,4-dinitrophenylhydrazine (2,4-DNPH) | Orange or yellow precipitate (test for any carbonyl; its melting point identifies the compound) |
| Iodoform test | I₂ and NaOH | Yellow precipitate for CH₃CO– compounds and CH₃CH(OH)– alcohols |
Distinguishing aldehydes from ketones
| Test | Aldehyde | Ketone |
|---|---|---|
| Tollens' reagent (ammoniacal silver nitrate), warm | Silver mirror | No change |
| Fehling's or Benedict's solution, warm | Brick-red precipitate of copper(I) oxide (aliphatic aldehydes) | No change |
| Acidified dichromate | Orange to green (oxidised to a carboxylic acid) | No change |
Try itMaking a 2,4-DNPH derivative
A carbonyl compound reacts with 2,4-dinitrophenylhydrazine in a 1 : 1 ratio. Choose the mass of the carbonyl compound, its molar mass and the molar mass of the derivative. Find the maximum mass of derivative.
Uses
| Compound | Use |
|---|---|
| Methanal (formaldehyde), 40% solution | Preserving specimens; making resins and plastics |
| Ethanal | Making ethanoic acid and other chemicals |
| Propanone (acetone) | Solvent, nail polish remover |
| Benzaldehyde | Almond flavouring |
| Cinnamaldehyde | Flavour of cinnamon |
Key terms
- Carbonyl group
- The C=O group.
- Nucleophilic addition
- Addition of a nucleophile across the C=O bond.
- Tollens' reagent
- Ammoniacal silver nitrate; gives a silver mirror with aldehydes.
- 2,4-DNPH
- A reagent that gives an orange precipitate with aldehydes and ketones.
Summary
- Aldehydes (–CHO) and ketones (C=O in the chain) undergo nucleophilic addition.
- Reduction gives alcohols; 2,4-DNPH detects any carbonyl.
- Only aldehydes are oxidised easily: Tollens', Fehling's and dichromate tests.
- The iodoform test detects CH₃CO– groups.