Aldehydes, Ketones and Carboxylic Acids
Aldehydes, Ketones and Carboxylic Acids means nucleophilic addition, the named reactions that distinguish carbonyl compounds, and the ranking of acid strength.
Core rules
- Aldehydes are more reactive than ketones: less steric crowding and less electron release by alkyl groups onto the carbonyl carbon.
- Tollens' reagent (ammoniacal AgNO₃) gives a silver mirror with aliphatic and aromatic aldehydes alike; Fehling's solution is reduced only by aliphatic aldehydes, so benzaldehyde fails.
- Iodoform test (I₂/NaOH) is positive for CH₃CO– groups: ethanal, propanone, acetophenone.
- Aldol condensation requires an α-hydrogen. Benzaldehyde has none, so with concentrated NaOH it undergoes Cannizzaro disproportionation to benzyl alcohol and sodium benzoate.
- Clemmensen (Zn-Hg/HCl) and Wolff–Kishner (N₂H₄/KOH) both reduce C=O to CH₂.
- Acidity: ClCH₂COOH > HCOOH > CH₃COOH. Electron-withdrawing groups raise Ka, alkyl groups lower it.
- Hell–Volhard–Zelinsky reaction: α-halogenation of a carboxylic acid with X₂ and red phosphorus.
Worked example
Distinguish propanal from propanone. Add Tollens' reagent and warm: propanal gives a silver mirror, propanone gives nothing. The iodoform test is useless here because both compounds contain a CH₃CO– or CH₃CH(OH)– unit and both give yellow iodoform. Only the test that differentiates them earns the mark.
Traps that cost marks
- Saying Fehling's test detects all aldehydes, including benzaldehyde.
- Forgetting the α-hydrogen requirement for aldol and haloform reactions.
- Applying Cannizzaro to aldehydes that possess α-hydrogens.
- Reversing the acidity trend on adding alkyl groups.
- Assuming ketones resist oxidation entirely - vigorous reagents such as hot acidified KMnO₄ cleave them.