Chemistry Preparation and Physical Properties of Diazonium Salts

Topics Covered :

● Diazonium Salts
● Method of Preparation
● Physical Properties

Diazonium Salts :

`=>` The diazonium salts have the general formula `color{red}(R overset(+)N_2 overset(-)X)` where `color{red}(R)` stands for an aryl group and `color{red}(overset(-)X)` ion may be `color{red}(Cl^(-), Br^(-), HSO_4^(-), BF_4^(-))`, etc.

`=>` They are named by suffixing diazonium to the name of the parent hydrocarbon from which they are formed, followed by the name of anion such as chloride, hydrogensulphate, etc.

`=>` The `color{red}(overset(+)N_2)` group is called diazonium group.

● For example, `color{red}(C_6H_5 overset(+)N_2 overset(-)Cl)` is named as benzenediazonium chloride and `color{red}(C_6H_5N_2^(+)HSO_4^(-))` is known as benzenediazonium hydrogensulphate.

`=>` Primary aliphatic amines form highly unstable alkyldiazonium salts.

`=>` Primary aromatic amines form arenediazonium salts which are stable for a short time in solution at low temperatures (`273-278 K`).

`=>` The stability of arenediazonium ion is explained on the basis of resonance. See fig.

Method of Preparation of Diazoniun Salts :

`=>` Benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at `273-278K`.

`=>` Nitrous acid is produced in the reaction mixture by the reaction of sodium nitrite with hydrochloric acid.

`=>` The conversion of primary aromatic amines into diazonium salts is known as diazotisation.

`=>` Due to its instability, the diazonium salt is not generally stored and is used immediately after its preparation.

`color{red}(C_6H_5NH_2+NaNO_2+2HCl overset(273-278K)→ C_6H_5N_2Cl +NaCl +2H_2O)`

Physical Properties :

`=>` Benzenediazonium chloride is a colourless crystalline solid.

`=>` It is readily soluble in water and is stable in cold but reacts with water when warmed.

`=>` It decomposes easily in the dry state.

`=>` Benzenediazonium fluoroborate is water insoluble and stable at room temperature.