How the carbonyl group makes aldehydes and ketones react
Aldehydes (R–CHO) and ketones (R–CO–R′) both contain the carbonyl group, C=O, which is polar: the carbon is slightly positive and is attacked by nucleophiles. Typical nucleophilic addition reactions include the addition of HCN to form cyanohydrins and of sodium bisulphite. Aldehydes are generally more reactive than ketones because they have less steric hindrance and fewer electron-donating alkyl groups.
Aldehydes are easily oxidised, so they give positive Tollens’ test (silver mirror) and Fehling’s test (red precipitate), while ketones do not. Methyl ketones give the iodoform test. Aldehydes without α-hydrogen, such as HCHO, undergo the Cannizzaro reaction, while those with α-hydrogen undergo aldol condensation. Clemmensen and Wolff–Kishner reductions convert C=O to CH₂. Carboxylic acids, R–COOH, are more acidic than phenols; electron-withdrawing groups such as Cl increase their acidity, and they form esters, amides and acid chlorides.
Why it matters
Formaldehyde preserves biological specimens and makes resins, acetone is a common solvent in nail polish remover, and vanillin and cinnamaldehyde give vanilla and cinnamon their flavours. Acetic acid is the acid in vinegar, and esters give fruits their smell. Carbonyl reactions are central to making medicines, perfumes and plastics.
Key terms
- Carbonyl group
- A carbon atom double-bonded to an oxygen atom, C=O.
- Nucleophilic addition
- A reaction where a nucleophile adds to the positive carbon of a carbonyl group.
- Tollens’ test
- A test in which aldehydes reduce ammoniacal silver nitrate to form a silver mirror.
- Aldol condensation
- A reaction in which two carbonyl compounds with α-hydrogen join to form a β-hydroxy carbonyl compound.