Lesson 9 of 12 · 6 min
Diazonium salts: making and handling
NCERT §9.7–9.8
The diazotisation tank is packed in ice. A thermometer taped to the side is checked every few minutes, and the operator will not let it creep above 278 K.
The lesson in notes
In short
Diazonium salts have the formula R–N₂⁺X⁻, where R is an aryl group and X⁻ may be Cl⁻, Br⁻, HSO₄⁻, BF₄⁻ and so on. The –N₂⁺ group is the diazonium group.
Names add 'diazonium' to the parent hydrocarbon and then name the anion: C₆H₅N₂⁺Cl⁻ is benzenediazonium chloride and C₆H₅N₂⁺HSO₄⁻ is benzenediazonium hydrogensulphate.
Primary aliphatic amines give very unstable alkyldiazonium salts. Arenediazonium salts last for a short time in cold solution (273–278 K); resonance with the ring explains their extra stability.
Diazotisation is the conversion of a primary aromatic amine into its diazonium salt. Benzenediazonium chloride is made from aniline and nitrous acid (NaNO₂ + HCl) at 273–278 K: C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O.
Being unstable, the salt is usually used as soon as it is made rather than stored.
Benzenediazonium chloride is a colourless crystalline solid, readily soluble in water. It is stable in the cold, reacts with water on warming and decomposes easily when dry. Benzenediazonium fluoroborate is insoluble in water and stable at room temperature.