Aldehydes, Ketones, Carboxylic Acids and Organic Compounds Containing Nitrogen
Aldehydes, Ketones, Carboxylic Acids and Organic Compounds Containing Nitrogen
This domain tests nucleophilic addition, oxidation, condensation and the basicity of amines.
Core rules
- Aldehydes are more reactive than ketones: less steric hindrance and less electron donation by alkyl groups.
- Tollens' reagent oxidises aldehydes (silver mirror), not ketones. Fehling's solution oxidises aliphatic aldehydes, not benzaldehyde.
- The iodoform test is positive for CH₃CO– groups and for ethanol or acetaldehyde.
- Cannizzaro reaction needs an aldehyde with no α-hydrogen.
- Carboxylic acids are stronger acids than phenols; electron-withdrawing groups increase acidity.
- Amine basicity in water: 2° > 1° > 3° for methylamines; aniline is much weaker because the lone pair is delocalised into the ring.
Worked example Distinguish propanal and propanone. Warm each with Tollens' reagent: propanal gives a silver mirror, propanone does not. Confirm with Fehling's: propanal gives a red Cu₂O precipitate; propanone gives no change.
Traps
- Expecting benzaldehyde to give a positive Fehling's test.
- Using Cannizzaro for acetaldehyde, which has α-hydrogens and undergoes aldol condensation instead.
- Ranking aniline as more basic than ammonia; it is less basic.