Aldehydes, Ketones, Carboxylic Acids and Amines
This domain tests nucleophilic addition at the carbonyl group, the classic distinguishing tests, acidity patterns in carboxylic acids, and the basicity and identification of amines.
Aldehydes and ketones
Nucleophilic addition is the defining reaction (HCN, NaHSO₃, Grignard reagents, alcohols). Tollens and Fehling reagents are reduced by aliphatic aldehydes; benzaldehyde gives Tollens but not Fehling. Iodoform test requires a CH₃CO– group (or CH₃CH(OH)–). Cannizzaro disproportionation applies only to aldehydes without α-hydrogen (HCHO, C₆H₅CHO), while aldol condensation requires α-hydrogen. Clemmensen (Zn–Hg/HCl) and Wolff–Kishner (NH₂NH₂/KOH) both convert C=O to CH₂.
Carboxylic acids
In water HCOOH > CH₃COOH > CH₃CH₂COOH; electron-withdrawing groups increase acidity (ClCH₂COOH > CH₃COOH) and benzoic acid is more acidic than acetic acid. Reactions include esterification, acid-chloride formation with SOCl₂, anhydride and amide formation, decarboxylation (CH₃COONa + NaOH/CaO → CH₄) and HVZ halogenation.
Amines
Aqueous basicity: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃, a balance of inductive donation and solvation of the conjugate acid; aniline is far weaker because the lone pair is delocalised into the ring. Hinsberg test: 1° gives a soluble sulphonamide, 2° an insoluble one, 3° no reaction; carbylamine test is specific to 1° amines. Hofmann bromamide converts an amide to a 1° amine with one carbon less; Gabriel phthalimide gives pure 1° amines but fails with aryl halides; diazotisation must be kept at 273–278 K.
Worked example
Distinguish propanal from propanone. Warm each with I₂/NaOH: propanone has a CH₃CO– group and gives a yellow precipitate of CHI₃, whereas propanal (CH₃CH₂CHO) has no CH₃CO– group and gives none. Tollens reagent provides the complementary test: propanal gives a silver mirror, propanone does not.
Traps that cost marks
- Applying Cannizzaro to acetaldehyde, which has α-hydrogens and undergoes aldol reaction instead.
- Expecting benzaldehyde to reduce Fehling solution.
- Ranking amine basicity in the gas phase (where 3° wins) rather than in water.
- Writing Sandmeyer coupling without a Cu(I) salt.