Reactions of Diazonium Salts

Diazonium salts are the organic compounds consisting of triple bonds in between Nitrogen atoms as well as have either an alkyl or aryl (benzene ring) on the other side. The diazonium salts are the intermediate phase between the azo dyes (or compounds are known to be popular coloring agents). They are termed salts because of the double nitrogen (diazo), which is generally found in ionic salts where chloride molecules replace the nitrogen atom.  The overall process involved in the creation of diazonium salts is quite easy. The steps involved are:

Step I: Begin with either an alkyl or primary arylamine.

Step II: Once it gets in contact with sodium nitrite, in the presence of hydrochloric acid, it results in the formation of Nitrogen triple bonds and loss of H₂O.

Step III: Thanks to its gentle reaction process, it can occur at room temperature or below. 

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Diazonium Salt Chemical Reactions

Diazonium salt reactions are necessary because of their versatile ability to make the building blocks of other organic reactions. There are two types of such reactions, namely, Sandmeyer reactions and other mechanisms. 

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Type I: Sandmeyer Reaction

The Sandmeyer reaction is known as the radical-nucleophilic aromatic substitution, where the aryl diazonium salts can be further transformed into aryl halides. It is because of the Sandmeyer reaction, that helps Benzene to have numerous transformations like Benzene, halogenation, etc. This method facilitates the substitution of an aromatic amine by a nucleophile into copper(I) salts, where the nucleophile can be cyanide, thiols, or halide anions.

In such reactions, you can begin with taking the diazonium salts and introducing them to various copper compounds, for example, copper (I) chloride. Upon reacting, the copper (I) chloride transforms into aryl chloride, known as the Sandmeyer reaction. Discovered in the year 1884 by the Swiss chemist Traugott Sandmeyer, it's known for the instant loss of the Nitrogen in the diazonium salt by Chloride atoms, present via the copper (I) chloride reagent. 

The Process Involved in the Reaction are:

• For every aryl amine, the general precursor is its nitro compound, where it deactivates the benzene ring and undergoes electrophilic replacements across meta locations.

• When the nitro group present in an amine is reduced, the amine activates the aromatic ring to lead electrophilic substitution to ortho and para locations. 

• This is where the aryl amine gets converted into diazonium ions into different groups and can be used for the next-in-line reactions. 

Here are some of the examples:

CuCl, CuBr, CuCN, convert the aryl diazonium salts into aryl chlorides, bromides, and cyanides, respectively. 

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Type II - Other Reactions

Schiemann Reaction

The aryl fluorides can be formed when the substitution of the (X-) ion of the diazonium salt takes place with the tetrafluoroborate (BF4-) ion when treated with HB4. It is when exposed to heat, the fluorine present in the stable diazonium tetrafluoroborate salts may act as a nucleophile and lose its Nitrogen and yield aryl fluoride, with BF3 and N2 as the two other byproducts. This is called the Schiemann reaction, that is expressed in the chemical equation below:  

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Phenol Synthesis

Hydroxyl groups (OH) get added to the aryl diazonium salt when it is heated along with water and acid. These phenols are valuable as they are the constituent elements in pharmaceuticals and drug research. The chemical equation involved is: 

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Aryl Iodide

The aryl diazonium salts also react with potassium iodide and yield aryl iodide. The chemical reaction for the process is:

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Reduction of the Diazonium Salt From the Amino Group

The aryl diazonium salts can react with hypophosphorous acid (H₃PO₂) and get rid of its nitro (amino) group, and form C-H. It can be expressed via a chemical reaction as:

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Diazonium Salt Mechanism

To understand the process of diazonium ions formation, you must first understand the process involved in diazotization or the operation of diazonium salt synthesis. 

Once sodium nitrite (NaNO₂) gets treated with nitrous acid (HNO₂), it forms:

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Once the compound is then introduced to HCl, the NaNO₂ gets converted into HNO₂ by:

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However, since HCl is a strong acid, it then further reduces the HNO2 into its electrophile form NO+, also known as the nitrosonium ion that is the building block of the diazonium salt. 

Now that we have the nitrosonium ion, the diazonium ion is formed when it is treated with aromatic amine and a strong acid. 

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Diazonium Coupling Reactions

Because of the positive charge displacement on the nitrogen atoms, the electron-rich nucleophiles can only couple with that of the terminal nitrogen, instead of the inner nitrogen, yields azo dyes. It is represented with the help of the following reaction:

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The diazo coupling reactions are used across many fields, such as the formation of synthetic dying agents for colours like yellow, red, orange, etc. Here they are called azo dyes because of their colour-pertaining abilities and may exist in -cis and -transformations.