Diazonium Salts


The diazonium salts have the general formula where R stands for an aryl group and  ion may be Cl Br, , , 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 group is called diazonium group. For example,  is named as benzenediazonium chloride and C6H5N2+HSO4 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.


Structure of Diazonium Salt



Preparation of Diazonium 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.


Physical Properties of diazonium salts

•    It is a colourless crystalline solid.

•    It is readily soluble in water and is stable in cold but reacts with water when warmed.

•    It has tendency to explode when dry.


Chemical Reactions of diazonium salts

The reactions of diazonium salts can be broadly divided into two categories, namely (A) reactions involving displacement of nitrogen and (B) reactions involving retention of diazo group.


Gatterman reaction

chlorine or bromine can be introduced in the benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of copper powder. This is referred as Gatterman reaction.


Sandmeyer reaction

The Cl, Br and CN nucleophiles can easily be introduced in the benzene ring in the presence of Cu(I) ion. This reaction is called Sandmeyer reaction.



Reaction of diazonium salt with potassium iodide

Iodine is not easily introduced into the benzene ring directly, but, when the diazonium salt solution is treated with potassium iodide, iodobenzene is formed.


Reaction of diazonium salt with fluoroboric acid

When arenediazonium chloride is treated with fluoroboric acid, arene diazonium fluoroborate is precipitated which on heating decomposes to yield aryl fluoride.



Reaction of diazonium salt with hypophosphorous acid

Certain mild reducing agents like hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively.


Reaction of diazonium salt with water

If the temperature of the diazonium salt solution is allowed to rise upto 283 K, the salt gets hydrolysed to phenol.


Coupling reactions

The azo products obtained have an extended conjugate system having both the aromatic rings joined through the –N=N– bond. These compounds are often coloured and are used as dyes. Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to form p-hydroxyazobenzene. This type of reaction is known as coupling reaction. Similarly the reaction of diazonium salt with aniline yields p-aminoazobenzene. This is an example of electrophilic substitution reaction.


Importance of Diazonium Salts in Synthesis of  Aromatic Compounds

Diazonium salts are versatile intermediates in the introduction of various functional groups (-F, -Cl, -Br, -I, -CN, -OH, -NO2) into the aromatic ring. They allow for the preparation of substituted aromatic compounds that cannot be obtained by direct halogenation or nucleophilic substitution reactions. For example, aryl fluorides and iodides cannot be prepared by direct halogenation, but can be introduced via diazonium salts. Similarly, the cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene, but can be easily obtained from diazonium salt. Overall, the use of diazonium salts as intermediates enables the synthesis of a wide range of substituted aromatic compounds that would otherwise be difficult to prepare.