Aldehydes, Ketones, Carboxylic Acids and Amines
Domain 12: Aldehydes, Ketones, Carboxylic Acids and Amines
This domain tests carbonyl addition, oxidation tests, acid strength, and amine basicity, including diazonium chemistry.
Core rules
- Aldehydes are oxidised by Tollens' reagent (silver mirror), Fehling's solution (red Cu2O for aliphatic aldehydes), and Benedict's solution; ketones are not. This is the key test to distinguish them.
- Nucleophilic addition to carbonyl is favoured by electron-withdrawing groups and less steric hindrance. Aldol condensation requires an α-hydrogen; Cannizzaro disproportionation occurs for aldehydes without α-H (HCHO, PhCHO).
- Haloform reaction: methyl ketones and ethanol/acetaldehyde give CHI3 with I2/NaOH; useful for identifying CH3CO–.
- Carboxylic acids are acidic because the carboxylate ion is resonance-stabilised; electron-withdrawing substituents increase Ka (trichloroacetic acid > acetic acid).
- Amines are basic at nitrogen; in water, methylamine is more basic than ammonia. Amides are far less basic because the lone pair is delocalised into the carbonyl. Hinsberg test distinguishes 1°, 2°, 3° amines.
- Diazonium salts undergo Sandmeyer and coupling reactions.
Worked example
Propanal and propanone both have the formula C3H6O, but Tollens' reagent distinguishes them. Propanal is an aldehyde and is oxidised to propanoate, reducing Ag+ to Ag metal (silver mirror). Propanone is a ketone and has no aldehyde hydrogen, so no silver mirror forms.
Traps
- Using Tollens' test for ketones.
- Forgetting Fehling's test is for aliphatic aldehydes, not aromatic ones.
- Expecting Cannizzaro for acetaldehyde, which has α-H.
- Treating amides as strongly basic amines.
- Confusing Hinsberg reagent with Tollens' reagent.