Amines
Amines tests classification, basicity, preparation, diazonium chemistry and coupling reactions.
Classification and basicity
Amines are 1°, 2° or 3° by the number of carbon groups on nitrogen. In water, basicity depends on lone-pair availability and solvation of the conjugate acid. Cyclohexylamine > ammonia > aniline because the aniline lone pair is delocalised into the ring. Electron-withdrawing groups lower basicity; electron-donating groups raise it.
Preparation and reactions
Amines are prepared by reduction of nitro compounds, ammonolysis of halides, and Gabriel phthalimide synthesis (which gives pure 1° amines). Hinsberg test uses benzenesulphonyl chloride: 1° gives a soluble sulphonamide, 2° an insoluble one, 3° no reaction. Carbylamine test is positive for 1° amines.
Diazonium salts
Aniline + NaNO2 + HCl at 273–278 K gives benzenediazonium chloride. It undergoes Sandmeyer reactions (CuCl/HCl gives chlorobenzene), coupling with phenol in alkaline medium to give an azo dye, and replacement by H, OH, CN or halide. Diazonium salts are unstable above about 5 °C and are kept cold.
Worked example
Distinguish aniline from N-methylaniline and N,N-dimethylaniline. Add benzenesulphonyl chloride: aniline (1°) gives a clear solution after NaOH, N-methylaniline (2°) gives an insoluble solid, and N,N-dimethylaniline (3°) gives no reaction. The reasoning is the number of N–H hydrogens available to form the sulphonamide.
Mark-losing traps
Assuming 3° amines are always most basic in water, forgetting that aniline does not undergo Friedel–Crafts alkylation readily, warming diazonium salts too soon, and confusing coupling (with phenol) with Sandmeyer substitution are common mistakes.