Preparation of Diazonium Salt - QS Study
QS Study

The salt having two N atom containing cation  (-N≡N↔N≡N:) in the positive part of benzene ring & inorganic mono-valet anions (Cl, Br, NO3, HSO4, BF4 etc.) in the negative part is known as Diazonium salt. The process by which primary amine reacts with HNO2 (in presence of mineral acid) at low temperature (0-5 C) to form diazonium salt is called diazotization.

Laboratory Preparation:

Principle: Generally at temperature of 0 – 5 C, when an aqueous solution of Sodium of nitrite added to aniline dissolved in HCL, benzene diazonium chloride is formed. It is a three process. At first aniline hydrochloride salt is formed by the reaction at aniline and HCl. On the other hand, the reaction of NaNO2 and mineral acid produce HNO2, which acts with aniline hydrochloride at 0 – 5 C forms diazonium chloride. HNO2 is very unstable. That is why nitrous acid is prepared in the reacting vessel by the reaction of NaNO2 and mineral acid.

NaNO2 + HCl → NaCl + HNO2

Addition of the solution of NaNO2 is stopped when one drop of the resulting solution starch iodide paper blue. A pale yellow colored solution of benzene diazonium chloride is formed

Method of preparation: At first around bottom flask with HCI is taken and aniline is solved into it. Then an aqueous solution of sodium nitrite is added to the flask when benzene diazonium chloride is formed.

Though diazonium salt is a crystalline compound, it cannot be separated from the reaction mixture by crystallization. It cannot be done because it creates explosion. So, it used as a solution of reaction mixture.