Introduction to Diazonium Salt: Diazonium compounds, also known as diazonium salts, are organic compounds with the generic chemical formula $\text{R}–{\text{N}}_2^{+}{\text{X}}^{–},$ where $\text{R}$ can be any alkyl or aryl group and $\text{X}$ can be halogens, hydrogen sulphate, or other organic compounds. In organic synthesis, aryl diazonium salts are often utilised as intermediates.

The diazonium group can be easily replaced by a variety of functional groups, allowing groups that can’t be directly substituted onto the aromatic ring, such $–\text{I},–\text{OH},–\text{F},–\text{CN}$ and $–\text{H},$ to be introduced. In addition, one can create substitution patterns that are completely contradictory to the norm (i.e., preparation of $1,3$-dihalosubstituted benzenes). The diazonium salt is not isolated in the majority of these replacements.

IUPAC Nomenclature

Arenediazonium compounds are salts. This is reflected in their name, “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, ionic compounds containing $\text{N}\equiv \text{N}$ are referred to as diazonium salts. They are identified by adding the word “Diazonium” to the name of the aromatic compound, followed by the name of the anion.

Preparation of Diazonium Salt

The reaction of aniline with nitrous acid at $273–278\text{K}$ produces benzene diazonium chloride. The reaction of sodium nitrite with hydrochloric acid produces nitrous acid in the reaction mixture. Diazotisation is the process of converting primary aromatic amines into diazonium ions. The diazonium salt is not commonly stored due to its volatility and is utilised shortly after manufacture.
$\text{NaN}{\text{O}}_2+\text{HCl}\to \text{HN}{\text{O}}_2+\text{NaCl}$
$\text{HN}{\text{O}}_2+\text{HN}{\text{O}}_2\to \stackrel{+}{\text{N}}=\text{O}+{\text{H}}_2\text{O}+\bar{\text{N}}{\text{O}}_2$

Physical Properties of Diazonium Salts

Diazonium salts are colourless crystalline substances that darken with air exposure. Many diazonium salts of nitrates and perchlorates explode when heated or struck when they are dry. As a result, these salts are not separated, and they are employed for additional synthetic preparations as soon as they are generated in situ. However, diazonium and zinc chloride double salts and diazonium and tetrafluoroborates double salts are stable at room temperature and have been employed as fast dye salts in the production of naphthol-AS dyestuffs.

Chemical Reactions of Diazonium Salts

The reactions of diazonium salts can be split into two categories: (A) reactions involving nitrogen displacement and (B) reactions involving diazo group retention.

Reactions Involving Displacement of Nitrogen

The best general technique to introduce $\text{F},\text{Cl},\text{Br},\text{I},\text{CN},\text{OH}$ and $\text{H}$ into an aromatic ring is to replace the diazonium group. Diazonium salts are useful in synthesis because they may be made from nearly all primary aromatic amines and react to generate a wide range of chemicals. Diazotization is hampered by the presence of a few groups in the molecule; diazonium salts differ from Grignard reagents in this regard. The amines used to make diazonium compounds are easily acquired from the nitro compounds made by direct nitration.

Replacement by Halogen (Sandmeyer Reaction)

$\text{Cl}$ or $\text{Br}$ replaces the diazonium group by mixing a freshly generated diazonium salt solution with cuprous chloride or cuprous bromide. Nitrogen is steadily developed at room temperature or occasionally at elevated temperatures, and the aryl chloride or aryl bromide can be separated from the reaction mixture after several hours. The Sandmeyer reaction is the name given to a method that uses cuprous halides.
${\text{ArN}}_2^{+}{\text{X}}^{–}\stackrel{\text{CuX}}{\to }\text{ArX}+{\text{N}}_2$

The Gattermann reaction, in which copper powder and hydrogen halide are employed instead of cuprous halide, is sometimes utilised to carry out the synthesis. It is not necessary to utilise a cuprous halide or copper to replace the diazonium group with I; the diazonium salt and potassium iodide are simply mixed together and allowed to react.
${\text{ArN}}_2^{+}{\text{X}}^{–}+{\text{I}}^{–}\to \text{ArI}+{\text{N}}_2+{\text{X}}^{–}$

The diazonium group is replaced by $\text{F}$ in a somewhat different manner. When fluoboric acid, $\text{HB}{\text{F}}_4,$ is added to a diazonium salt solution, the diazonium fluoborate, ${\text{ArN}}_2^{+}{\text{BF}}_4^{–},$ precipitates, which can be collected on a filter cleaned, and dried. Among diazonium salts, diazonium fluoborates are uncommon in that they are relatively stable compounds. When dry diazonium iluoborate is heated, it decomposes into aryl fluoride, boron trifluoride, and nitrogen.
${\text{ArN}}_2^{+}{\text{X}}^{–}\stackrel{\text{HB}{\text{F}}_4}{\to }{\text{ArN}}_2^{+}{\text{BF}}_4^{–}\stackrel{\text{heat}}{\to }\text{ArF + B}{\text{F}}_3+{\text{N}}_2$

Replacement by CN (Synthesis of Carboxylic Acids)

Allowing the diazonium salt to react with cuprous cyanide results in the diazonium group being replaced by $\text{CN}.$ The diazonium solution is neutralised with sodium carbonate before combining with the cuprous cyanide to prevent cyanide loss as $\text{HCN}.$
${\text{ArN}}_2^{+}{\text{X}}^{–}\stackrel{\text{CaCN}}{\to }\text{ArCN}+{\text{N}}_2$
Carboxylic acids are formed when nitriles are hydrolysed. Thus, the manufacture of nitrites from diazonium salts provides a good pathway from nitro compounds to carboxylic acids.

Replacement by OH (Synthesis of Phenol)

Diazonium salts react with water to yield phenols.

${\text{ArN}}_2^{+}{\text{X}}^{–}+{\text{H}}_2\text{O}\to \text{ArOH}+{\text{N}}_2+{\text{H}}^{+}$

This reaction occurs slowly in ice-cold diazonium salt solutions, which is why diazonium salts are utilised right away after synthesis; at higher temperatures, it can be made the primary reaction of diazonium salts.

Replacement by -H

A variety of reducing agents can be used to replace the diazonium group with $\text{H},$ with hypophosphorous acid being one of the most helpful. Simply allowing the diazonium salt to stay in the presence of hypophosphorous acid causes nitrogen to be lost and hypophosphorous acid to be oxidised to phosphorous acid.
${\text{ArN}}_2^{+}{\text{X}}^{–}+{\text{H}}_3\text{P}{\text{O}}_2+{\text{H}}_2\text{O}\to \text{ArH}+{\text{N}}_2+{\text{H}}_3\text{P}{\text{O}}_3+\text{HX}$

Reactions Involving Retention of Diazo Group: Coupling Reactions

Diazonium salts easily react with phenols, naphthols, and aromatic amines to produce brightly coloured azo compounds. Both aromatic rings are connected through the $–\text{N}=\text{N}–$ bond in the azo products, resulting in an extended conjugated system. These compounds are frequently dyed and coloured. When benzene diazonium chloride interacts with phenol, the para position of the phenol molecule is associated with the diazonium salt, forming $\text{p}$-hydroxyazobenzene. Coupling Reaction is the name for this type of reaction. Similarly, diazonium salt reacts with aniline to produce $\text{p}$-aminoazobenzene. This is an electrophilic substitution process in operation.

As the $\text{pH}$ rises from $5$ to $8,$ the pace of reaction increases. Phenol acts as a phenoxide ion under weakly alkaline conditions, which is far more activating than phenol itself.

The para position of the hydroxyl group is preferred for coupling with benzene substrates. However, if this location is blocked, the coupling takes place in the ortho position. $\text{p}$–cresol, for example, produces $\text{o}$–azo compound. $1$– and $2$–naphthols in alkaline solution couple with a diazonium salt in the $4$– and $1$–position, respectively.

Importance of Diazonium Salts in Synthesis of Aromatic Compounds

Let’s have a look at some of the ways diazonium salts can be used in organic synthesis. To begin, we can look at some very simple molecules, such as the three isomeric bromotoluenes. Diazotization is used in the optimal synthesis of each, but not for the same reason in all three situations. The corresponding $\text{o}$- and $\text{m}$-nitrotoluenes are used to make the $\text{o}$- and $\text{p}$-bromotoluenes. The advantage of these multi-step syntheses over direct bromination is that a pure product is obtained, as we have seen. It is not possible to separate the $\text{o}$- and $\text{p}$-bromotoluenes produced by direct bromination.

The reactions that produce m-bromotoluene are as follows:

Summary

1. Diazonium compounds, also known as diazonium salts, are organic compounds with the generic chemical formula $\text{R}–{\text{N}}_2^{+}{\text{X}}^{–},$ where $\text{R}$ can be any alkyl or aryl group and $\text{X}$ can be halogens hydrogen sulphate, or other organic compounds.
2. The reaction of aniline with nitrous acid at $273–278\text{K}$ produces benzene diazonium chloride. The reaction of sodium nitrite with hydrochloric acid produces nitrous acid in the reaction mixture.
3. Diazonium salts are colourless crystalline substances that darken with air exposure.
4. The reactions of diazonium salts can be split into two categories: (A) reactions involving nitrogen displacement and (B) reactions involving diazo group retention.

FAQs on Diazonium Salt

Q.1. How are diazonium salts prepared?
Ans: There are various methods to prepare diazonium salts. The reaction of aniline with nitrous acid at $273–278\text{K}$ produces benzene diazonium chloride. The reaction of sodium nitrite with hydrochloric acid produces nitrous acid in the reaction mixture. Diazotisation is the process of converting primary aromatic amines into diazonium ions. The diazonium salt is not commonly stored due to its volatility and is utilised shortly after manufacture.

Q.2. What are the physical properties of Diazonium salt?
Ans: Diazonium salts are colourless crystalline substances that darken with air exposure. Many diazonium salts of nitrates and perchlorates explode when heated or struck when they are dry. As a result, these salts are not separated, and they are employed for additional synthetic preparations as soon as they are generated in situ. However, diazonium and zinc chloride double salts, as well as diazonium and tetrafluoroborates double salts, are stable at room temperature.

Q.3. What type of reactions are shown by diazonium salt?
Ans: The reactions of diazonium salts can be split into two categories: (A) reactions involving nitrogen displacement and (B) reactions involving diazo group retention.

Q.4. Discuss the reactions of diazonium salts involving the displacement of nitrogen.
Ans: The best general technique to introduce $\text{F},\text{Cl},\text{Br},\text{I},\text{CN},\text{OH}$ and $\text{H}$ into an aromatic ring is to replace the diazonium group. Diazonium salts are useful in synthesis because they may be made from nearly all primary aromatic amines and react to generate a wide range of chemicals. Diazotization is hampered by the presence of a few groups in the molecule; diazonium salts differ from Grignard reagents in this regard. The amines that are used to make diazonium compounds are easily acquired from the nitro compounds that are made by direct nitration.

Q.5. Discuss the synthesis of phenol from diazonium salt.
Ans: Diazonium salts react with water to yield phenols.
${\text{ArN}}_2^{+}{\text{X}}^{–}+{\text{H}}_2\text{O}\to \text{ArOH + }{\text{N}}_2+{\text{H}}^{+}$
This reaction occurs slowly in ice-cold diazonium salt solutions, which is why diazonium salts are utilised right away after synthesis; at higher temperatures, it can be made the primary reaction of diazonium salts.

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