Aldehydes, Ketones and Carboxylic Acids
Aldehydes, Ketones and Carboxylic Acids tests nucleophilic addition, oxidation, condensation and acidity. Functional-group tests are the most examinable part.
Carbonyl reactions
Aldehydes and ketones undergo nucleophilic addition. Aldehydes are easily oxidised; ketones resist mild oxidation. Tollens' reagent gives a silver mirror with aldehydes; Fehling's solution gives a red precipitate with aliphatic aldehydes but not aromatic aldehydes. Iodoform test is positive for CH3CO- groups and for ethanol or acetaldehyde. Aldol condensation requires alpha-hydrogens; Cannizzaro reaction occurs for aldehydes without alpha-hydrogens.
Carboxylic acids
Acidity depends on stabilisation of the carboxylate ion. Electron-withdrawing groups increase acidity; electron-donating groups decrease it. Carboxylic acids react with NaHCO3 to give CO2, which distinguishes them from phenols. HVZ reaction halogenates the alpha carbon; decarboxylation removes CO2.
Worked example
Propanal and propanone are isomers. Warm Tollens' reagent: propanal gives a silver mirror, propanone does not. Iodoform test: propanone gives a yellow precipitate of CHI3, propanal does not. The reasoning is that propanal has an aldehydic C-H that is oxidised, while propanone has a CH3CO- group but no aldehydic hydrogen.
Traps
Using Fehling's test for benzaldehyde, expecting ketones to reduce Tollens' reagent, and forgetting that NaHCO3 distinguishes carboxylic acids from phenols are common mistakes.