The diazonium salts have the general formula R−N2+X−, where X is an organic or inorganic anion, and R is an alkyl or aryl group. They are organic compounds and contain two nitrogen atoms, one of which is positively charged. Diazonium salts are benzenediazonium chloride (C6H5N2+Cl–) and benzenediazonium hydrogen sulphate (C6H5N2+HSO4–).

Importance of Diazonium Salts

  • Used to produce dyed fabrics in the dye and pigment industries.
  • They help synthesise many organic compounds, particularly aryl derivatives.
  • For preparing aryl iodides and fluorides, direct halogenation is not suitable. The nucleophilic substitution of chlorine in chlorobenzene by a cyano group is impossible. But we can easily make cyanobenzene from diazonium salts.
  • Direct substitution in benzene cannot produce substituted aromatic compounds. We can make these compounds by replacing diazo groups in diazonium salts.
  • Used in document reproduction due to their property of breaking down near the UV light.

Chemical Reactions of Diazonium Salts

Reactions Involving Displacement of Nitrogen

Replace the diazonium group when adding Br, CN, I, F, Cl, and H to an aromatic ring. We can synthesise diazonium salts from every primary aromatic amine and react to produce a wide range of compounds.

 Because diazotisation is difficult due to some groups present in the molecule, diazonium salts are unique from Grignard reagents. We can use direct nitration nitro compounds to make the amines needed to make diazonium compounds.

  • Replacement by Halogen

We can use cuprous chloride or cuprous bromide to replace diazonium groups in a freshly formed diazonium salt solution. Nitrogen is slowly generated at room temperature or higher temperatures, and We can isolate aryl chloride or aryl bromide from the reaction mixture after several hours. A cuprous halide-based method is the Sandmeyer reaction.

Sometimes we use the Gattermann reaction to synthesise, using hydrogen halide and copper powder instead of cuprous halide. There is no need for cuprous halide or copper to replace the diazonium group with I; we mix potassium iodide and diazonium salt and enable it to react.

F replaces the diazonium group in a slightly different manner. The diazonium fluoroborate (ArN2+BF4–) precipitates when we add fluoboric acid (HBF4) to a diazonium salt solution, which we can then collect and dry on a filter. Diazonium fluoborates are among the few diazonium salts that are relatively stable chemicals. When we heat dry, diazonium fluoroborate decomposes into nitrogen, boron trifluoride, and aryl fluoride.

  • Replacement by CN (Synthesis of Carboxylic Acids)

CN replaces the diazonium group using cuprous cyanide to react with the diazonium salt. Before mixing the cuprous cyanide, we neutralise the diazonium solution with sodium carbonate to avoid cyanide loss as HCN.

C6H5N2+ + CuCN → C6H5CN +Cu+ + N2

When we hydrolyse nitriles, it forms carboxylic acids. As a result, producing nitrites from diazonium salts is an efficient method of converting nitro compounds to carboxylic acids.

  • Replacement by OH (Synthesis of Phenol)

Water and diazonium salts combine to form phenols. This reaction is slow in ice-cold diazonium salt solutions, so we use diazonium salts right after synthesis; however, it can become the diazonium salts’ major reaction at higher temperatures.

C6H5N2+ + H2O → C6H5OH +H+ + N2

  • Replacement by -H

We can use several reducing agents to replace the diazonium group with H, but hypophosphorous acid is the most useful. The loss of nitrogen and hypophosphorous acid to phosphorous acid oxidation occurs when we leave the diazonium salt in the presence of hypophosphorous acid.

Retention of diazo group reactions.

C6H5N2+Cl– + H3PO2 + H2O → C6H6 +H3PO3 + N2  + H2O

Diazonium salts easily react with naphthols, aromatic amines, and phenols to produce coloured azo compounds. Aromatic rings in the azo products connect with the –N=N– bond, which creates an extended conjugated system. We frequently colour and dye these substances. As benzene diazonium chloride reacts with phenol, we link the phenol molecule to the diazonium salt in the para position, producing the p-hydroxyazobenzene. We call this a coupling reaction. Similarly, aniline forms p-aminoazobenzene when it reacts with a diazonium salt. It is a working electrophilic substitution method.

As the pH increases from 5.0 to 8.0, the reaction rate accelerates—phenol functions as a phenoxide ion in mildly alkaline conditions, much more activating than phenol itself.

We prefer the para position of the hydroxyl group for coupling with benzene substrates. However, if we block this point, the coupling occurs orthogonally.

Conclusion

We commonly use Diazonium salts in synthetic organic chemistry. First, we used diazonium salts to dye fabrics by immersing them in an aqueous solution of the diazonium compound. An electron-rich ring is then immersed in a coupler solution and undergoes electrophilic substitution reactions. Diazonium salts play an essential role in the pigment and dye industries, and we use them to produce dyed fabrics.

We now widely use Diazonium salts in industry and synthetic organic chemistry to make azo dyes. We make the majority of azo dyes in two-step reactions, and the aromatic diazonium ion is first synthesised from an aniline derivative. We then couple the diazonium salt with an aromatic compound. Azo dyes come in shades of orange, brown, blue, yellow, and red.