Amines
Amines covers classification, basicity, preparation, distinguishing tests and diazonium chemistry. Basicity order and test reactions are frequently asked.
Basicity and tests
In aqueous solution, methylamines are basic in the order (CH3)2NH > CH3NH2 > (CH3)3N > NH3; solvation and steric effects compete. Aromatic amines are weaker bases because the lone pair is delocalised into the ring. Hinsberg test: primary amines form a soluble sulfonamide with KOH, secondary form an insoluble one, tertiary do not react. Carbylamine test is positive only for primary amines.
Preparation and reactions
Hofmann bromamide degradation converts an amide to a primary amine with one fewer carbon. Gabriel synthesis gives primary amines. Aniline with NaNO2/HCl at 273-278 K gives benzenediazonium chloride. Diazonium salts undergo Sandmeyer reactions and coupling with phenol or aniline to form azo dyes.
Worked example
Aniline is treated with NaNO2 and HCl at 0-5 C to form benzenediazonium chloride. This salt is coupled with phenol in alkaline solution to give p-hydroxyazobenzene, an orange dye. The reasoning is that the diazonium ion is a strong electrophile and attacks the activated phenol ring at the para position.
Traps
Using the gas-phase basicity order for aqueous amines, expecting tertiary amines to react in the Hinsberg test, and forgetting the low temperature for diazotisation are common errors.