Organic Chemistry: Aldehydes, Ketones, Carboxylic Acids, Amines and Biomolecules
This section covers aldehydes, ketones, carboxylic acids, amines and biomolecules: nucleophilic addition, oxidation and condensation reactions, acidity, basicity and tests, plus carbohydrates, proteins, enzymes, vitamins and nucleic acids. Distinguishing tests are frequently asked.
Core rules
- Aldehydes are oxidised by Tollens' reagent (silver mirror) and Fehling's solution; ketones are not, except α-hydroxy ketones.
- Iodoform test is positive for CH₃CO– groups and for ethanol/acetaldehyde after oxidation.
- Carboxylic acids are stronger acids than phenols and alcohols because the carboxylate ion is resonance-stabilised.
- Amine basicity in water: 2° > 1° > 3° > NH₃ for methylamines, influenced by inductive effect, solvation and steric effects.
- Proteins are polyamides; carbohydrates include reducing and non-reducing sugars; nucleic acids are polynucleotides.
Worked example
Distinguish propanal from propanone. Warm each with Tollens' reagent: propanal gives a silver mirror because it is an aldehyde and is oxidised to propanoate; propanone gives no silver mirror. Then use I₂/NaOH: propanone gives a yellow precipitate of iodoform because it contains CH₃CO–, while propanal does not. The two tests together identify both compounds.
Traps
Do not say Fehling's oxidises aromatic aldehydes such as benzaldehyde; do not use Tollens' for ordinary ketones; do not rank amine basicity without specifying solvent; do not confuse peptide and glycosidic bonds; remember that sucrose is non-reducing.