Aldehydes, Ketones and Carboxylic Acids
Aldehydes, Ketones and Carboxylic Acids tests nucleophilic addition, oxidation, named tests, aldol and Cannizzaro reactions, HVZ reaction and acidity.
Core rules
- Aldehydes are more reactive than ketones toward nucleophilic addition because of less steric hindrance and less electron donation by alkyl groups.
- Tollens' test: silver mirror with aldehydes. Fehling's test: red precipitate with aliphatic aldehydes, not aromatic ones.
- Iodoform test is positive for CH3CO- groups and ethanol.
- Aldol condensation needs an alpha hydrogen; Cannizzaro disproportionation occurs with aldehydes lacking alpha hydrogen (e.g. benzaldehyde).
- Carboxylic acids are acidic because the carboxylate ion is resonance-stabilised; electron-withdrawing groups increase acidity.
Worked example
Why does benzaldehyde undergo Cannizzaro but not aldol condensation? Benzaldehyde has no alpha hydrogen, so it cannot form an enolate for aldol addition. In concentrated alkali it disproportionates to benzyl alcohol and benzoate. Reasoning: the alpha hydrogen is the structural requirement for aldol; its absence channels the reaction to Cannizzaro.
Traps
Using Fehling's test for benzaldehyde, forgetting that iodoform requires a CH3CO- group, and missing that HVZ introduces halogen at the alpha carbon. Carboxylic acids are weaker than mineral acids but stronger than alcohols and phenols.