What is diazonium salt?

Diazonium salts (‘di’ implies ‘two’, ‘azo’ indicates the presence of nitrogen and ‘ium’ refers to the cationic nature of the compound) are the category of compounds that are organic and generally has the chemical formula R-N2 +X where, X represents an anion (organic or inorganic like Br , Cl, BF4, etc.) and R denotes either an aryl or alkyl group.

Another definition of diazonium salts is that they are a class of chemical compounds that all have the same R-NH2+X function group. In R-NH2+X, R is an organic group. For instance, the alkyl or aryl groups. Diazonium salts can also be used to make azo compounds by reacting with other aromatic compounds.

When hydrochloric acid is present, aryl or alkyl primary amine is taken and added to the solution along with sodium nitrite to form diazonium salts. For example, a reaction where diazonium salts are involved is the Sandmeyer reaction: Copper (I) chloride is used in the production of an aryl chloride.

Diazonium salts, therefore, possesses two nitrogen atoms in which one of them is charged. Some examples of diazonium salts are C6H5N2+HSO4– (benzene diazonium hydrogen sulfate), C6H5N2+Cl (benzene diazonium chloride), C6H5N2+HSO4 (benzene diazonium hydrogen sulfate) etc.

General description of diazonium salts:

  • Usually, the ‘X’ in diazonium salt chemical formula represents Br, Cl, BF4. The presence of the diazonium group or the N2+ is what determines the name of the salts.
  • The parent hydrocarbon from which these salts are formed is suffixed with the suffix diazonium, which is then followed by the anion X, such as bromide.
  • Diazonium salts have a very versatile combination of inorganic and organic components. The common way of representing this compound is R−N2+X. The ‘X’ indicates an ion while the ‘R’ represents an organic aryl group.

How are diazonium salts prepared?

  • Dissociation or diazotization is the process in which organic compounds like aromatic amines are converted into diazonium salts.
  • Under room conditions, diazonium salts are observed to be highly unstable and hence it is prepared only when it is needed to be used so that storing may not be necessary.
  • The common procedure of preparing diazonium salt is by letting aromatic amines react with nitrous acid. Diazonium salt forms when nitrous acid reacts with aromatic amine (aniline) and this formed salt is called benzene diazonium chloride.
  • Due to the high toxicity of gas emitted by nitrous acid, it is mostly prepared when the procedure is performed, that is, in situ and this is done by allowing mineral acid to react with sodium nitrite (NaNO2).
  • Temperature is another important factor that keeps the formation of the product from disrupting. Generally, 5° C or below is the temperature that keeps the diazonium salt stable.
  • If the required temperature is not maintained, the diazonium compound will surely decompose and emit N2 just as the compound is formed.
  • The chemical reaction of NaNO2 with aniline along with HCl (Hydrochloric acid) which results in the formation of benzene diazonium chloride, is shown below:

2HCL + NaNO2 + C6H5NH2 NaCl + C6H5N2+Cl + 2H2O

The diazonium group has a wide range of applications. This mix of organic and inorganic components has been a godsend for scientists in a variety of fields, from dyes and pigments to the production of different organic molecules. Diazonium salts are extremely useful in many fields.

Characteristics and properties of diazonium salts or compounds

  • Diazonium salts are soluble in water.
  • The nature of diazonium salts is ionic.
  • Aryl diazonium salts are colourless crystalline solids.
  • Though benzene diazonium chloride is water-soluble, it can react when the solution is heated.
  • Water does not dissolve benzene diazonium fluoroborate. At room temperature, it is fairly stable.

Importance and uses of diazonium salts

Diazonium salts break down when exposed to violet or ultraviolet light implying that the compound is extremely sensitive to light. This behaviour gave way to these compounds being popularly used in copy papers. Diazonium compounds are common reagents in the synthesis of chemical molecules, particularly aryl derivatives.

  • Diazonium compounds are useful for dyeing making them abundantly used in the pigment industry for dyeing fabrics.
  • They may be used to make a wide range of organic compounds, particularly aryl derivatives.
  • They are very useful in the reproduction of documents due to their nature of breaking down when exposed to ultraviolet and violet light.
  • For the preparation of aryl iodides and fluorides, direct halogenation is not a suitable option. It is impossible to substitute a cyano group for the nucleophilic substitution of chlorine in chlorobenzene. Diazonium salts, on the other hand, may readily be used to make cyanobenzene.
  • They’re utilised to introduce, –NO2, –Br, –Cl, –F, –I, –CN, –OH, and groups into the aromatic ring as intermediates.
  • Preparation of substituted aromatic compounds by the method of direct substitution is not possible in benzene. For these types of compounds, the diazo group is replaced.

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