Amines
Amines tests classification, preparation, basicity, Hinsberg and carbylamine tests, diazonium salt reactions and coupling.
Core rules
- Basicity in water: aliphatic amines > ammonia > aromatic amines; aniline is weak because the lone pair is delocalised into the ring.
- Hinsberg reagent (benzenesulphonyl chloride) distinguishes 1°, 2° and 3° amines.
- Carbylamine test is positive only for primary amines.
- Diazonium salts are unstable above 5 °C and are used in Sandmeyer, Gattermann and coupling reactions.
- Azo coupling: diazonium salt + phenol gives an orange dye.
Worked example
Arrange ethylamine, ammonia and aniline in order of basicity in water. Ethylamine is most basic because the ethyl group donates electron density. Ammonia is next. Aniline is least basic because the nitrogen lone pair participates in resonance with the benzene ring. Reasoning: basicity depends on lone-pair availability, and solvation also matters in water.
Traps
Assuming the same basicity order in gas and aqueous phases, using the carbylamine test for secondary amines, and forgetting that diazonium salts must be kept cold. Hinsberg's reagent gives a sulphonamide with 1° and 2° amines but not with 3° amines.