Diazonium Salts

What are Diazonium Salts?

The diazonium salts (di refers to ‘two’, azo is indicative of ‘nitrogen’ and ium implies that it is cationic in nature), or diazonium compounds,  are

the class of organic compounds with general formula R−N2+X where X is an organic or inorganic anion (for example, Cl, Br, BF4, etc.) and R is an alkyl or aryl group.

Diazonium salts

Benzenediazonium Chloride – A Diazonium Salt

Hence, they have two nitrogen atoms with one being charged. Benzenediazonium chloride (C6H5N2+Cl), benzene diazonium hydrogen sulfate (C6H5N2+HSO4), etc. are some of the examples of the diazonium salt.

  • Diazonium salts are one of the most versatile combinations of organic and inorganic components. Its general way of representation is R−N+2X. The R is an organic group, generally an aryl group while X represents ion.
  • Generally, diazonium salts have Cl, Br, BF−4, as X. The name of these salts is based on the presence of the N+2 group or the diazonium group.
  • The naming of these salts is done by adding the suffix diazonium to the parent hydrocarbon from which they are derived and then it is followed by the anion X such as bromide.

The importance of these compounds lies in their application and uses.

Diazonium Salts Preparation

  • The process of converting an organic compound, generally primary aromatic amines, into diazonium salts is referred to as diazotization or dissociation.
  • It has been seen that the diazonium group is highly unstable under normal conditions and hence it is not generally stored, most of the time it is used just after its preparation.
  • One of the most common methods of preparation of diazonium salt is by reacting nitrous acid with aromatic amines. The reaction of aniline (aromatic amine) with nitrous acid results in the diazonium salt formation that is benzene diazonium chloride.
  • Nitrous acid is a highly toxic gas and hence it is generally prepared during the reaction itself (in situ) by reacting NaNO2 with a mineral acid.
  • Another important parameter that governs the product formation during the preparation of these salts is temperature; most diazonium salts are stable below the temperature of 5o C.
  • It is, therefore, important to keep the reaction conditions below 5oC or else the diazonium group will decompose to give N2 as soon as it is formed.

The reaction of aniline with NaNO2 and hydrochloric acid is given below, the product of the reaction is benzene diazonium chloride.

\( \require{AMScd} \begin{CD} C_6H_5NH_2 + NaNO_2+ 2HCl @> Temperature 0 – 5^\circ C >> C_6H_5N^+- Cl + NaCl + 2H_2O\end{CD} \)

The diazonium group finds application in many ways. From dyes and pigment industry to synthesis of various organic compounds, this combination of organic and inorganic components has been a boon for the scientists.

Properties of Diazonium Salts

  1. They are ionic in nature.
  2. They are water-soluble.
  3. Aryl diazonium salts are colourless crystalline solids.
  4. Benzenediazonium chloride is soluble in water but reacts with it only when warmed.
  5. Benzenediazonium fluoroborate is not soluble in water. It is pretty stable at room temperature.

Importance of Diazonium Salts

  • They find application in the dye and pigment industries and are used to produce dyed fabrics.
  • Due to their property of breaking down near the ultraviolet light, they are used in document reproduction
  • They are useful in the synthesis of a large variety of organic compounds, especially aryl derivatives.
  • Direct halogenation is not a suitable method for preparing aryl iodides and fluorides. Nucleophilic substitution of chlorine in chlorobenzene by a cyano group is not possible. However, diazonium salts can easily be used to produce cyanobenzene.
  • It is not possible to prepare substituted aromatic compounds by direct substitution in benzene. For these compounds, we use the replacement of the diazo group in diazonium salts.
  • They are used as intermediates for introducing –F, –Br, –Cl, –I, –NO2, –OH and –CN groups into the aromatic ring.

To learn more about diazonium salts, their nomenclature, methods of preparation, physical and chemical properties and their importance in the synthesis of aromatic compounds by watching vibrant video lectures, download BYJU’S The Learning App.

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