Aldehydes, Ketones and Carboxylic Acids
Aldehydes, ketones and carboxylic acids tests nucleophilic addition, named reactions, acidity and qualitative tests.
Nucleophilic addition
The carbonyl carbon is sp2 and electrophilic. Aldehydes are more reactive than ketones because of less steric hindrance and less electron donation by alkyl groups. HCN gives cyanohydrins; NaHSO3 gives bisulphite adducts; Grignard reagents give alcohols; reduction gives alcohols or hydrocarbons.
Named reactions
Aldol condensation needs an α-hydrogen and gives a β-hydroxy carbonyl, then an α,β-unsaturated carbonyl on dehydration. Cannizzaro reaction occurs with aldehydes lacking α-hydrogen (e.g., benzaldehyde) in strong base. Clemmensen (Zn-Hg/HCl) and Wolff–Kishner (NH2NH2/KOH) reduce carbonyls to alkanes. Hell–Volhard–Zelinsky halogenates the α-carbon of carboxylic acids.
Tests and acidity
Tollens reagent gives a silver mirror with aldehydes, including benzaldehyde; Fehling's solution is reduced by aliphatic aldehydes but not benzaldehyde. Iodoform test is positive for CH3CO– groups and for acetaldehyde/ethanol. Carboxylic acids are more acidic than phenols because the carboxylate ion is resonance-stabilised; electron-withdrawing groups increase acidity.
Worked example
Propanal and propanone both have formula C3H6O. Add Tollens reagent: propanal gives a silver mirror, propanone does not. Add iodine and NaOH: propanone gives yellow iodoform, propanal does not. The reasoning is based on the aldehyde C–H oxidation and the methyl ketone enolate reaction.
Mark-losing traps
Using Fehling's test for benzaldehyde, assuming all aldehydes give iodoform, forgetting the α-hydrogen requirement for aldol condensation, and ranking phenol above a carboxylic acid are frequent errors.