Diazonium salt - Physical and Chemical Properties: Sandmeyer Reaction, Gattermann Reaction, Coupling Reaction and Mechanism, Practice Problems and FAQs:

There’s a function going to happen in your family and you want to look smart by wearing that favorite outfit. As all the dry cleaning shops are closed you decided to wash it by your own.

What you think, what would have happened?

Yes, unfortunately the shinning color of the outfit just faded away. This will be worst nightmare for anyone.

It’s not for the first time that you have washed your clothes but why it didn’t happen before?

Well, to understand the above question first we have to understand which is the most important chemical that is used for dying clothes?

In most of the textile industry, diazonium salts are used to dye fabrics. It is an organic compound and we know that every compound has a different mode of reaction with water and other chemicals. Hence, it is important to understand the physical and chemical properties of diazonium salt.

Table of Content

• Diazonium Salt
• Physical Properties
• Chemical Properties
• Reactions Involving Displacement of Nitrogen
• Reactions Involving Retention of Diazo Group
• Importance of Diazonium Salt
• Practice Problems
• Frequently Asked Questions

Diazonium Salt

• Diazonium salts are ionic salts.
• The general formula for diazonium salt is $Ar{N_2}^{+} X^{-}$

$Ar=Aryl group$

$X^{-}=For any ions, for example : N{O_3}^{-}, HS{O_4}^{-}, F^{-}, C{l}^{-}and B{r}^{-}$

The word ‘Diazo’ indicates the presence of two nitrogen atoms linked to each other.

• During the nomenclature of diazonium salt, first we write the name of hydrocarbon, followed bhy the suffix diazonium and then the respective anion. To understand it better here is an example
• $C_6{H}_5{N_2}^{+}C{l}^{-}$ (Benzene diazonium chloride)

The ${N_2}^{+ }i.e., -N^{+}\equiv N$ is called diazo group.

Diazonium salts serve as useful synthetic intermediates for the synthesis of a variety of aromatic compounds and azo dyes.

Physical properties

The majority of arenediazonium salts are colourless, crystalline solids that are very soluble in water. In the dry condition, many diazonium salts, particularly nitrates, are explosively deadly. As a result, they are never isolated; instead, they are typically made on-site and consumed right away.

Some diazonium salts, like fluoroborates, are comparatively insoluble in water, yet they are stable enough to be dried and kept. Some diazonium salts can also combine with metallic salts like rine chloride, such as (ArN), ZnCl₂, to produce complexes. In general, these compounds are insoluble.

Chemical properties

Arenediazonium salts are highly reactive compounds. Their high reactivity arises due to excellent leaving ability of the diazo group as nitrogen gas, N₂. They are, therefore, extremely useful synthetic reagents in organic chemistry. Diazonium salt reactions can be roughly divided into two types.:

• Reactions in which monovalent atoms or groups displace the diazo group.
• Reactions where the diazo group is retained.

Reactions where nitrogen is displaced

Some important reactions of this category along with their synthetic applications are discussed below

:

Sandmeyer Reaction: Replacement by -CN

CN can be introduced by treating benzenediazonium chloride with CuCN dissolved in aqueous KCN. This procedure using cuprous salts is called Sandmeyer reaction.

Replacement by -F

Diazotisation of aryl amines to produce aryl fluorides, followed by thermal decomposition of the resulting tetrafluoroborates or hexafluorophosphates. Photochemically induced decomposition is also possible. This reaction is called Baltz-Schiemann reaction

Gattermann Reaction

The gattermann reaction is a technique for formylating compounds with aromatic rings. Gattermann formylation and Gattermann salicylaldehyde synthesis are other names for the same reaction. The Friedel-Crafts reaction and the Gattermann reaction are comparable.

By treating benzenediazonium chloride with Cu/HCl or Cu/HBr, Gattermann reaction is used to produce chlorobenzene or bromobenzene respectively.

The yield in Sandmeyer reaction is found to be better than Gattermann reaction.

Replacement by -I

Nitrogen gas is released and oily droplets of iodobenzene are formed when potassium iodide solution is added to benzenediazonium chloride solution. The diazonium ions and the potassium iodide solution's iodide ions undergo a simple reaction.

Replacement by Hydrogen-Deamination

The replacement of the diazo group by hydrogen or the reduction of diazonium salts to arenes (hydrocarbons) can be carried out by using sodium borohydride (NaBH) or by treating a diazonium salt with hypophosphorous (H^₂PO^₂) or phosphinic acid at room temperature preferably in presence of Cu (I) salts as a catalyst.

$Ar{N_2}^{+}{X}^{-}+H_{3}P{O}_2+H_{2}O\to Ar-H+H_{3}P{O}_3+HX+N_2\uparrow$

This reaction can also be carried out by treating amines with sodium nitrite and hypophosphorous acid as diazotising reagent.

This whole process involving the diazotization of an amine followed by reduction of diazonium salt or replacement of the diazo group by hydrogen is called deamination.

Replacement by -OH

If diazonium salt solution is allowed to reach a temperature higher than 278 K, the salt is hydrolyzed to produce phenol. But phenols thus produced couple with unreacted diazonium salts to form azo dyes. Further, more acidic the solution, more slowly the coupling reaction occurs. Therefore, to minimize coupling during synthesis of phenols, the diazonium salt solution is added slowly to a large volume of boiling dil. H₂SO₄. This reaction is especially important since only a few methods are available for the direct introduction of an hydroxyl group into the benzene nucleus.

Replacement by -NO₂

Nitro compounds are generally prepared by treating an aryl diazonium fluoroborate with an aqueous solution of sodium nitrite in presence of copper powder.

Reactions where the diazo group is retained

• Aryldiazonium salts generally produces bright color azo compounds. This happens when Aryldiazonium salts reacts with amines (-NH₂) and Phenol. This reaction is commonly called as coupling reaction. This is a very popular reaction in textile industries.
• Coupling reaction with amines takes place in moderate acidic medium that is pH ranging from 4-5.
• Coupling reaction with phenol takes place in basic medium that is pH ranging from 9-10.
• In a coupling reaction, the diazonium cation with the positive charge on the terminal nitrogen functions as the electrophile and compounds containing a lot of electrons, including phenols and amines, as the nucleophiles.

Coupling often takes place at the p-position with respect to the -OH or -NH₂ group. If the p-position is blocked, however, it happens at the o-position. For instance,

The double bonds of both the arene rings are extendedly conjugated through the -N = N- double bond to give azo compounds their colour.

When the diazonium salt of sulphanilic acid and N, N-dimethylaniline are combined, the result is the famous indicator that is methyl orange, which is frequently employed in acid-base titrations.

Mechanism

The coupling reaction between benzenediazonium chloride and phenols or amines is an example of electrophilic substitution reaction. In the reaction, diazonium ion with a positive nitrogen iona cts as an electrophile while electron rich phenol or amine acts as the nucleophile.

(a) Coupling with phenols. Since benzenediazonium ions are only weakly electrophilic, therefore, strong nucleophiles are needed for the reaction to occur. Further, since phenol is weakly nucleophilic but phenoxide ion is strongly nucleophilic, therefore, coupling reactions of phenols are usually carried out in

alkaline medium.

However, in the alkaline medium, the diazonium salt first forms diazonium hydroxide which immediately rearranges to diazohydroxide which further reacts with alkali to form the corresponding salt called the diazotate. All these three species, in fact, exist in equilibrium.

Since at high concentration of OH-ions, the equilibrium will shift in favour of diazohydroxide and diazotate both of which do not couple, therefore, for the desired coupling reaction to occur, the pH must be kept as low as possible in the alkaline medium. However, a too low pH would suppress the dissociation of phenol to phenoxide ion.

Therefore, to carry out the coupling reaction, an optimum pH is needed. In actual practice, the optimum pH for such coupling reactions is between 9-10.

(b) Coupling with amines. In highly acidic solutions, the lone pair of electrons on the N-atom accepts a proton. As a result, the amine exists predominantly in the protonated form.

Due to the absence of a lone pair of electrons on the N-atom, the amine is no longer nucleophilic and hence the coupling reaction will not occur in highly acidic (low pH) solutions. Further, as explained above, too high alkalinity (high pH) will also not favour coupling reaction by forming unreactive diazohydroxide or diazotate salt in significant amounts. Therefore, an optimum pH of 4-5 is employed for such coupling reactions.

Advantages of diazonium salt

• The diazonium salts direct halogenation on aryl ring is not possible. So are the replacement with cyanide and nitro group. But the diazonium salts are very useful to introduce groups of - F, - C1, - Br, - I, - CN, - OH and - NO₂ in the aromatic ring.

• Likewise -OH group cannot be directly introduced into the benzene ring. However, these can be easily prepared through diazonium salts. We can therefore prepare those substituted aromatic compounds that otherwise cannot be prepared either by direct substitution in benzene or the substituted benzene by replacing the diazo group with a number of other monovalent groups.

Practice problems

Q1. Which of the following reagent is used in the Balz-Scheimann reaction to yield flourobenzene?

A. HBF₄
B. CN
C. CuCN
D. CuCl₂

Answer: (A)

Solution:
When Benzenediazonium chloride reacts with HBF₄ it gives fluorobenzene. This reaction is commonly called as Balz-Scheimann reaction.

Q2. When benzene diazonium chloride reacts with cuprous powder and HBr it yields

A. Chlorobenzene
B. Bromobenzene
C. Fluorobenzene
D. Iodobenzene

Answer:(B)

Solution: When benzene diazonium chloride reacts with $\frac{Cu}{HBr}$ it gives bromobenzene. This reaction is called sandmeyer's reaction.

Q3. The colour of the azo compound is due to

A. Nature of the reactant
B. Extended conjugation
C. Both A & B
D. None of the above

Answer: (B)

Solution: The double bonds of both the arene rings are extendedly conjugated through the -N = N- double bond to give azo compounds their colour.

Q4. Which of the following compound is obtained during the coupling of diazonium salt of sulphanilic acid with N, N-dimethylaniline.

A. Phenolphthalein
B. Chlorobenzene
C. Aniline
D. Methyl orange

Answer: (D)

Solution: When the diazonium salt of sulphanilic acid and N, N-dimethylaniline are combined, the result is the famous indicator that is methyl orange, which is frequently employed in acid-base titrations.

Frequently asked questions (FAQs)

Q.1 Why it is difficult to store diazonium salt?
Answer: Because it is very unstable and dissociates to liberate nitrogen even with a slight increase in temperature, benzene diazonium chloride cannot be stored and must be used immediately after preparation.

Q.2 What will be the reaction of water with diazonium salt?
Answer: Byproducts of heating benzene diazonium chloride with water include phenol, nitrogen gas, and hydrochloric acid.

Q.3 Can we dissolve benzene diazonium chloride in water?
Answer: Benzenediazonium chloride is water soluble but only reacts with it when heated. Water does not dissolve benzenediazonium fluoroborate. At room temperature, it is fairly stable.

Q.4 Is the salt of diazonium a nucleophile?
Answer: Since nitrogen has a positive charge, the diazonium ion, which is an electron-deficient compound, functions as an electrophile.

Related Topics