Diazonium Salts-Definition, Preparation, Properties, Importance, Important Question Answers, FAQs

Diazonium salts are a class of organic compounds containing the diazonium functional group (-N₂⁺X^–), where X is an anion such as chloride (Cl^–), tetrafluoroborate (BF₄^–), or sulfate (SO₄^2-). These compounds are characterized by the presence of a positively charged nitrogen atom (N⁺) bound to two other atoms or groups. Diazonium salts are used in the synthesis of azo dyes and pigments. Azo dyes are extensively used in the textile, printing, and coloring industries due to their vibrant colors.

In this article, we will learn in detail about diazonium salts, its nomenclature, preparation, properties and importance.

What are Diazonium Salts?

Diazonium salts are organic compounds with the chemical formula R–N₂⁺X^– where R can be any alkyl or aryl group and X can be halogens, hydrogen sulphate, or other organic compounds. Aryl diazonium salts are often used as intermediates in chemical synthesis.

The diazonium group may be easily replaced by a number of functional groups, including –I, –OH, –F, –CN and –H, which cannot be directly substituted into the aromatic ring. Furthermore, one may generate replacement patterns that are diametrically opposed to the norm (i.e., preparation of 1,3-dihalo substituted benzenes). In the majority of these replacements, the diazonium salt is not separated.

IUPAC Nomenclature of Diazonium Salt

Arenediazonium salts are a kind of arene diazonium chemical. In “Diazonium salts” the word di refers to two, aza stands for nitrogen, and the last term onium suggests the ionic nature of the compound. As a result, diazonium salts refer to ionic compounds containing N≡N.

Preparation of Diazonium Salt

At 273–278K, aniline reacts with nitrous acid to generate benzene diazonium chloride. When sodium nitrite reacts with hydrochloric acid, nitrous acid is formed in the reaction mixture. The process of diazotisation is the transformation of primary aromatic amines into diazonium ions. Due to its volatility, the diazonium salt is seldom kept and is used immediately after production.

NaNO₂ + HCl → HNO₂ + NaCl

HNO₂ + HNO₂ → N⁺=O + H₂O + NO₂^–

Physical Properties of Diazonium Salts

The properties of diazonium salt are mentioned below:

• Diazonium salts are crystalline compounds that are colourless but darken when exposed to air. 
• When heated or hit when dry, several diazonium salts of nitrates and perchlorates explode. As a result, these salts are not separated and are used in other synthetic preparations as soon as they are produced in situ. 
• At room temperature, however, diazonium and zinc chloride double salts, as well as diazonium and tetrafluoroborates double salts, are stable and have been used as fast dye salts in the manufacture of naphthol-AS dyestuffs.
• Diazonium salts derived from aromatic amines retain the aromatic character of the parent amine.
• Diazonium salts are commonly used in coupling reactions to form azo compounds.
• Diazonium salts often exhibit vibrant colors, particularly when coupled with aromatic compounds.
• Some diazonium salts can undergo decomposition upon exposure to light.

Chemical Reactions of Diazonium Salts

The reactions of diazonium salts can be divided into two categories: 

• Nitrogen displacement reactions.
• Diazo group retention reactions.

Reactions Involving Displacement of Nitrogen

To add F, Cl, Br, I, CN, OH, and H into an aromatic ring, the best general strategy is to replace the diazonium group. Diazonium salts are helpful in synthesis because they may be generated from almost any primary aromatic amine and can react to produce a variety of compounds. The presence of a few groups in the molecule makes diazotization difficult, diazonium salts vary from Grignard reagents. The amines needed to generate diazonium compounds may be easily obtained from direct nitration nitro compounds.

• Replacement by Halogen (Sandmeyer Reaction)

By combining a freshly created diazonium salt solution with cuprous chloride or cuprous bromide, Cl or Br replaces the diazonium group. At ambient temperature or occasionally at higher temperatures, nitrogen is slowly generated, and the aryl chloride or aryl bromide can be isolated from the reaction mixture after several hours. The Sandmeyer reaction is the term given to a cuprous halide-based technique.

ArN₂⁺X^–→ ArX+N₂

The Gattermann reaction, which uses copper powder and hydrogen halide instead of cuprous halide to carry out the synthesis, is sometimes used. To replace the diazonium group with I, no cuprous halide or copper is required; the diazonium salt and potassium iodide are simply mixed together and allowed to react.

ArN₂⁺X^–+I^–→ArI+N₂+X^–

F replaces the diazonium group in a slightly different way. When fluoboric acid (HBF₄) is added to a diazonium salt solution, the diazonium fluoborate (ArN₂⁺BF₄^–) precipitates, which may then be collected and dried on a filter. Diazonium fluoborates are unusual among diazonium salts in that they are relatively stable chemicals. Heat decomposes dry diazonium fluoroborate into aryl fluoride, boron trifluoride, and nitrogen.

ArN₂⁺X^– → ArN₂⁺BF₄⁻ → ArF + BF₃ + N₂

• Replacement by CN (Synthesis of Carboxylic Acids)

When you react to the diazonium salt with cuprous cyanide, the diazonium group is replaced by CN. To avoid cyanide loss as HCN, the diazonium solution is neutralised with sodium carbonate before mixing with the cuprous cyanide.

ArN₂⁺X^– → ArCN + N₂

When nitriles are hydrolyzed, carboxylic acids are produced. As a result, making nitrites from diazonium salts is an efficient approach to get from nitro compounds to carboxylic acids.

• Replacement by OH (Synthesis of Phenol)

Phenols are formed when diazonium salts combine with water.

ArN₂⁺X^– + H₂O → ArOH + N₂ + H⁺

This reaction is sluggish in ice-cold diazonium salt solutions, which is why diazonium salts are used straight soon after synthesis; it may be made the major reaction of diazonium salts at higher temperatures.

• Replacement by -H

To replace the diazonium group with H, a number of reducing agents can be utilised, with hypophosphorous acid being one of the most useful. Allowing the diazonium salt to remain in the presence of hypophosphorous acid results in the loss of nitrogen and the oxidation of hypophosphorous acid to phosphorous acid.

ArN₂⁺X^– + H₃PO₂ + H₂O → ArH + N₂ + H₃PO₃ + HX

Reactions Involving Retention of Diazo Group: Coupling Reactions

Diazonium salts react readily with phenols, naphthols, and aromatic amines to generate colourful azo compounds. In the azo products, the –N=N– bond connects both aromatic rings, resulting in an extended conjugated system. These substances are routinely coloured and dyed. When benzene diazonium chloride reacts with phenol, the phenol molecule’s para position is linked to the diazonium salt, resulting in p-hydroxyazobenzene. This sort of event is known as a coupling reaction. Similarly, aniline interacts with a diazonium salt to form p-aminoazobenzene. This is a working electrophilic substitution method.

The rate of reaction accelerates when the pH rises from 5 to 8. Under mildly alkaline circumstances, phenol functions as a phenoxide ion, which is significantly more activating than phenol itself.

For coupling with benzene substrates, the para position of the hydroxyl group is favoured. If this point is blocked, however, the coupling occurs in the ortho position. For example, p–cresol yields o–azo compound.

In alkaline solution, 1– and 2–naphthols pair with diazonium salts in the 4– and 1–positions, respectively.

Importance of Diazonium Salt

Diazonium salts are important intermediates in organic synthesis and find diverse applications. The importance of diazonium salts are mentioned below:

• Diazonium salts serve as versatile intermediates in organic synthesis, enabling the formation of a wide range of organic compounds.
• Diazonium salts are valuable precursors for the introduction of nitrogen atoms into organic molecules.
• One of the most significant applications of diazonium salts is in the synthesis of azo dyes and pigments. Azo dyes are extensively used in the textile, printing, and coloring industries
• Certain diazonium salts can undergo decomposition upon exposure to light. This property is exploited in photolithography processes used in microfabrication and semiconductor device manufacturing.
• Diazonium salts have potential applications in biomedicine and drug delivery systems. They can be functionalized with biologically active molecules or targeting ligands and used as building blocks for the synthesis of drug conjugates.
• Diazonium salts are employed as coupling agents in colorimetric and spectrophotometric methods for the quantification of phenolic compounds, aromatic amines, and other analytes.

FAQs on Diazonium Salts

How are diazonium salts prepared?

Diazonium salts can be made in a variety of ways. At 273–278K, aniline reacts with nitrous acid to generate benzene diazonium chloride. When sodium nitrite reacts with hydrochloric acid, nitrous acid is formed in the reaction mixture. The process of diazotisation is the transformation of primary aromatic amines into diazonium ions. Due to its volatility, diazonium salt is seldom kept and is used immediately after production.

What are the physical properties of Diazonium salt?

Diazonium salts are crystalline compounds that are colourless but darken when exposed to air. When heated or hit when dry, several diazonium salts of nitrates and perchlorates explode. As a result, these salts are not separated and are used in other synthetic preparations as soon as they are produced in situ. Double salts of diazonium and zinc chloride, as well as double salts of diazonium and tetrafluoroborates, are stable at room temperature.

Diazonium salt exhibits what kind of reactions?

The reactions of diazonium salts can be split into two categories:

1. Reactions involving nitrogen displacement.
2. Reactions involving diazo group retention.

What is the synthesis of phenol from diazonium salt?

 Diazonium salts react with water to yield phenols.

ArN₂⁺X^– + H₂O → ArOH + N₂ + H⁺

This reaction is sluggish in ice-cold diazonium salt solutions, which is why diazonium salts are used straight soon after synthesis; it may be made the major reaction of diazonium salts at higher temperatures.

What is Sandmeyer Reaction?

Cl or Br replaces the diazonium group when a freshly prepared diazonium salt solution is combined with cuprous chloride or cuprous bromide. Nitrogen is slowly created at room temperature or sometimes at higher temperatures, and the aryl chloride or aryl bromide can be separated from the reaction mixture after several hours. A cuprous halide-based approach is known as the Sandmeyer reaction.

ArN₂⁺X^– → ArX + N₂

What is Gattermann Reaction?

The Gattermann reaction, which employs copper powder and hydrogen halide to carry out the synthesis instead of cuprous halide, is occasionally utilised. No cuprous halide or copper is required to replace the diazonium group with I; the diazonium salt and potassium iodide are simply mixed together and allowed to react.

ArN₂⁺X^–+I^–→ArI+N₂+X^–