Arene Diazonium Salt (Aryl Diazonium Salt)

The diazonium salt has general formula ArN2+X–.

Ar is stand for the aryl group

X– = Cl–, Br–, HSO4–, BF4–

These are named as Parent hydrocarbon + diazonium +corresponding anion name of a compound

For example

C6H5N2+Cl– -Benzene diazonium Chloride (BDC)

C6H5N2+BF4– – Benzene diazonium tetra fluro borate.

C6H5N2+HSO4– -Benzene diazonium hydrogen sulphate.

 

Benzene Diazonium Chloride structure can be written as

Preparation

Aniline reacts with Nitrous Acid (HNO2) at 273-278K and produces Benzene diazonium Chloride,

Nitrous acid is formed during the reaction   by chemical reaction of Sodium nitrite and Hydrochloric acid (NaNO2+ HCl) (in situ process) .

Primary aryl or aromatic amine converted to diazonium salt is called the Diazotisation reaction.

Physical Properties

BDC is a colorless crystalline Solid

It is soluble in water and stable at cold conditions but it reacts with water in hot conditions.

These salts are easily decomposed in a dry state, due to their instability, diazonium salts are not stored.so these are used immediately after their preparation.

Benzene Diazonium fluoroborate are not soluble in water they may be stable at room temperature.

Chemical Properties

Generally diazonium salts are shown two type of reactions

1. Reactions involve displacement of Nitrogen (N2)

2. Reactions involve retention of Nitrogen (diazo group).

1. Reactions involve displacement of Nitrogen (N2)

As the diazonium group is a best leaving group it can be easily replaced by Cl–, Br–, I– , OH -, CN– and  substitutedted on aromatic ring by the release of N2 gas.

a) Displacement by Cl-,Br-,CN-

The nucleophile Cl-, Br-, and CN- may be introduced in the aromatic ring the in presence of Cu (I) ion, such reaction is popularly the known as the Sandmeyer reaction

Chloro or bromo benzene can also be prepared by the reaction of the diazonium salt with corresponding halogen acid in presence of copper powder. But this reaction is known as the Gatterman reaction.

b) Displacement by I- (Iodide) ion

 

As the introduction of iodine in benzene is not easier, Iodo benzene cannot be prepared from the above method, but it can be prepared by the reaction diazonium salt with Potassium Iodide (KI).

c) Displacement by Fluoride ion.

When Benzene Diazonium chloride is reacted with fluroboric acid is formed, which on heating decomposed into Fluro benzene.

d) Displacement by H

 

Mild reducing agents like Phosphinic acid (hypophosphorous acid, H3PO3) or ethanol (C2H5OH) reduces the Benzene diazonium Chloride Benzene, but they are oxidized in to phosphorous acid and ethanol respectively. 

e) Displacement by -OH group (Hydroxyl Group)

 

When the aqueous solution of Benzene diazonium Chloride is heated or warmed upto 283K (50 0C.), it becomes hydrolyze in to Phenol.

f) Displacement by –NO2 group

 

Benzene diazonium fluoro borate is reacted with an aqueous solution of Sodium Nitrite (NaNO2) in the presence of Cupper, gives Nitro benzene, in such reaction diazonium group is displaced by the nitro group.

2. Reactions involve retention of Nitrogen (diazo group).

Coupling reaction

Benzene diazonium Chloride reacts with Phenol and forms p-hydroxy azo benzene an orange colour dye, in such reaction, the para position of the phenol molecule couple is coupled with a diazonium salt.

Similarly, Aniline reacts with Benzene diazonium chloride and forms a yellow color dye p-amino azo benzene.

This reaction is known as a coupling reaction.

These two reactions are useful for the identification of Phenol and aniline

 

Also, it is popularly known as an azo dye test due to the formation of colored compounds.

Why Diazonium salts are important?

 

Synthetic importance of Diazonum salts

As- F,-Cl,-Br,-I,-CN,-OH,-NO2 groups are directly introduced in aromatic rings by the different reactions of diazonium salts. So it can able prepare a wide variety of aromatic compounds and their intermediate.

Direct halogenation of aromatic compounds cannot be prepared from Aryl fluoride and iodide but it is easier to prepare from diazonium salts.

The Cyano benzene can be easily produced from diazonium salt (Benzene diazonium chloride) whereas, the cyano group introduction is difficult in the nucleophilic substitution reaction of chlorobenzene.

Hence, the displacement of the diazo group by another group can be a new route for preparing different substituted aromatic compounds.