Aldehydes, Ketones and Carboxylic Acids
Expect nucleophilic addition reasoning, distinguishing tests, named reactions dependent on α-hydrogens, and acidity trends of substituted acids.
Reactivity and addition
- nucleophilic addition order: HCHO > RCHO > R2CO (steric and electronic reasons)
- addition of HCN gives cyanohydrins; Grignard reagents give alcohols
- Tollens' reagent (ammoniacal AgNO3) oxidises aldehydes, giving a silver mirror; ketones do not react
- Fehling's solution gives a red Cu2O with aliphatic aldehydes, but aromatic aldehydes fail it
- iodoform test: CH3CO– or CH3CH(OH)– groups give yellow CHI3
Named reactions
- aldol condensation needs dilute NaOH and an α-hydrogen
- Cannizzaro reaction needs concentrated NaOH and no α-hydrogen (HCHO, benzaldehyde)
- Clemmensen (Zn–Hg/HCl) and Wolff–Kishner (NH2NH2/KOH) both reduce C=O to CH2
- HVZ reaction: carboxylic acids with Cl2/red P give α-halo acids
Carboxylic acids
- acidity: Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > HCOOH > CH3COOH, because –I groups stabilise the anion; alkyl groups are electron-releasing
- benzoic acid is stronger than acetic acid
- soda lime decarboxylation removes CO2 and gives one carbon fewer
Worked example
A compound gives a yellow precipitate with I2/NaOH and a silver mirror with Tollens' reagent. The iodoform test fixes a CH3CO– group, and the silver mirror fixes an aldehyde - so the compound is acetaldehyde, CH3CHO.
Traps that cost marks
- Using Fehling's test for benzaldehyde.
- Writing Cannizzaro for a compound that has α-hydrogens.
- Reversing the acidity order of chloroacetic acids.