Physical Properties of Diazonium Salts

Diazonium salts, also known as diazonium compounds, are salts of diazonium (Zn) metals. Compounds with the generic formula RN2+X-, where R is an alkyl or aryl group containing an organic/inorganic anion such as BF4- or Cl- or Br. A charged nitrogen atom is therefore present on both nitrogen atoms. Many salts of benzene diazonium can be found, such as benzene diazonium chloride, benzene diazonium hydrogen sulphate, and others.

Cl-, Br- and BF4- are the most common X atoms found in diazonium salts. Their names are derived from the presence of N2+or diazonium groups in the salts. The suffix diazonium is added to the parent hydrocarbon before the anion X to name these salts, such as bromide.

Physical Properties of Diazonium Salts

The physical properties of diazonium salts are as follows: 

• One of the most important physical properties of diazonium salts is that they are ionic in composition.

• This salt is soluble in water in general. The chloride of benzene diazonium salt is water-soluble and only reacts with water once it has been warmed. However, diazonium salt, like Benzenediazonium Fluoroborate (BF), is insoluble in water.

• Diazonium salts are colourless and odourless.

Preparation of Diazonium Salts 

• When an organic molecule such as primary aromatic amines is converted into diazonium salts, it is called diazotisation or dissociation.

• The diazonium ion, an organic moiety with two connected nitrogen atoms at the end, is rarely stored and is almost always used immediately after production as it is highly unstable.

• Temperature is another factor that affects the manufacture of diazonium salts; however, most salts are stable below 5°C. The reaction conditions must be kept below 5 °C to prevent the diazonium group from decomposing into N2.

• When nitrous acid and aromatic amines are mixed, diazonium salt can be produced. In the presence of nitrous acid, aniline (an aromatic amine) forms benzene diazonium chloride, a diazonium salt. 

• Aniline reacts with NaNO2 and HCl to produce benzene diazonium chloride, as shown below:

    C6H5NH2 + NaNO2 +2HCl C6H5N2+Cl– +Na Cl + 2H2O                   

• Creating nitrous acid in situ (on the spot) is common by reacting NaNO2 with a mineral acid.

Chemical Reactions of Diazonium Salts

There are two types of reactions that diazonium salts go through:

• Substitutes for nitrogen in chemical reactions.

It focuses on nitrogen-substituting reactions. Substitutes for the aromatic ring’s nitrogen include groups such as Cl-, Br-, CN- and OH-. This nitrogen gas is released from the reaction mixture as a byproduct of the chemical reaction.

• Reactions in which the diazo group is retained.

Reactions involving retention of the Diazo group are coupling reactions. Arenediazonium salts react with highly reactive aromatic compounds such as phenols and amines to form brightly coloured azo compounds, Ar-N=N-Ar. Mainly such compounds are used in the manufacture of dyes, as these compounds are coloured in nature.

1. Replacement with halide or cyanide ion

Nucleophiles like Cl-, Br-, and CN- can easily be incorporated into the benzene ring when Cu (I) ion is present. The Sandmeyer reaction is the popular name for this reaction.

2. Iodide ion replacement

It is difficult to incorporate iodine into the benzene ring directly; hence, iodide ion replacement is used. On the other hand, iodobenzene is generated when potassium iodide is added to a diazonium salt solution.

3. Hydroxyl group replacement

Salt hydrolyses to phenol at 283 K, replacing the diazonium ion with a hydroxyl group in solution.

4. The -NO2 group replacement

The –NO2 group is substituted for the diazonium group when diazonium fluoroborate is heated in the presence of an aqueous sodium nitrite solution.

Importance of Diazonium Salts

• They are utilised to make dyed fabrics in the dye and pigment industries.

• They are employed to make copies of the source documents (document reproduction or photocopy) because of their ability to degrade when exposed to ultraviolet light.

• A wide range of chemical molecules can be synthesised using diazonium salts, particularly aryl derivatives.

• The direct halogenation of aryl iodides and fluorides is not a suitable approach. Nucleophilic substitution is not an option to replace chlorine in chlorobenzene with a cyano group. However, cyanobenzene can be made from diazonium salts.

• Substituted aromatic compounds cannot be made by substituting benzene directly. The diazo group is replaced in diazonium salts for these compounds.

• They are water-soluble and cold-stable but begin to react with warm water.

• When completely dry, diazonium salts are prone to exploding.

Conclusion

The properties of diazonium salt are fundamental to organic chemistry and must be studied thoroughly. Diazonium salts are salts of diazonium (Zn) metals. When heated or struck dry, diazonium salts of nitrates and perchlorates explode. Consequently, these salts are not separated and are promptly utilised in other synthetic compositions. Double salts of diazonium-zinc chloride and diazonium and tetrafluoroborate salts are stable at room temperature. They have been used as rapid dye salts in the synthesis of naphthol-AS.