Types of Reactions of Diazonium Salts with Mechanism and Examples
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.
FAQs on Reactions of Diazonium Salts in Organic Chemistry
1. What are diazonium salts in organic chemistry?
Diazonium salts are organic compounds containing the diazonium group (–N2+) attached to an aromatic ring, usually written as Ar–N2+ X-. They are commonly formed from primary aromatic amines and are highly reactive intermediates in organic synthesis. For example, benzenediazonium chloride (C6H5N2+Cl-) is a typical diazonium salt. These compounds are widely used in substitution and coupling reactions, especially in the preparation of dyes and substituted aromatic compounds.
2. How are diazonium salts prepared from aniline?
Diazonium salts are prepared from aniline by diazotization with nitrous acid (HNO2) at 0–5°C. Nitrous acid is generated in situ by reacting sodium nitrite with hydrochloric acid. The balanced reaction is:
C6H5NH2(aq) + NaNO2(aq) + 2HCl(aq) → C6H5N2+Cl-(aq) + NaCl(aq) + 2H2O(l)
Key conditions:
- Temperature maintained at 0–5°C
- Use of dilute mineral acid (HCl or H2SO4)
- Primary aromatic amine as starting material
3. Why are diazonium salts prepared at low temperature?
Diazonium salts are prepared at 0–5°C because they are unstable at higher temperatures and may decompose explosively. Above 5°C, aromatic diazonium salts can break down to form phenols or release nitrogen gas uncontrollably. Low temperature:
- Prevents decomposition
- Controls the reaction rate
- Improves yield and safety
4. What happens when diazonium salts react with water?
When heated with water, diazonium salts undergo hydrolysis to form phenols with the evolution of nitrogen gas. For example:
C6H5N2+Cl-(aq) + H2O(l) → C6H5OH(aq) + N2(g) + HCl(aq)
This reaction replaces the diazonium group (–N2+) with –OH and is an important method for preparing phenol from aniline.
5. What is the Sandmeyer reaction of diazonium salts?
The Sandmeyer reaction is the replacement of the diazonium group by Cl, Br, or CN using cuprous salts. In this reaction, nitrogen gas is released and a substituted aromatic compound is formed. Example:
C6H5N2+Cl-(aq) + CuCl(aq) → C6H5Cl(aq) + N2(g)
Common reagents:
- CuCl for chlorobenzene
- CuBr for bromobenzene
- CuCN for benzonitrile
6. How do diazonium salts react with potassium iodide?
Diazonium salts react with potassium iodide to form iodobenzene with the release of nitrogen gas. The reaction does not require a copper catalyst. Example:
C6H5N2+Cl-(aq) + KI(aq) → C6H5I(aq) + KCl(aq) + N2(g)
This reaction is useful for introducing an iodine substituent into the aromatic ring via the diazonium intermediate.
7. What is the azo coupling reaction of diazonium salts?
Azo coupling is the reaction of a diazonium salt with an activated aromatic compound to form an azo compound (–N=N–). It usually occurs with phenols or aromatic amines in mildly alkaline conditions. Example:
C6H5N2+Cl-(aq) + C6H5OH(aq) → C6H5–N=N–C6H4OH(aq) + HCl(aq)
The product is an azo dye, characterized by the –N=N– linkage and bright colors. This reaction is important in dye and pigment manufacture.
8. What happens when diazonium salts are reduced?
When reduced, diazonium salts form aromatic hydrocarbons with the evolution of nitrogen gas. For example, reduction with hypophosphorous acid (H3PO2) gives:
C6H5N2+Cl-(aq) + H3PO2(aq) + H2O(l) → C6H6(aq) + N2(g) + H3PO3(aq) + HCl(aq)
This reaction replaces the diazonium group with hydrogen and is useful for removing an –NH2 group from an aromatic ring.
9. What is the Gattermann reaction of diazonium salts?
The Gattermann reaction is the replacement of the diazonium group by Cl or Br using copper powder and hydrochloric or hydrobromic acid. Example:
C6H5N2+Cl-(aq) + HCl(aq) + Cu(s) → C6H5Cl(aq) + N2(g)
It is similar to the Sandmeyer reaction but uses copper metal instead of cuprous salts. This method is applied for preparing aryl chlorides and bromides.
10. What are the important reactions of diazonium salts?
The important reactions of diazonium salts include substitution, hydrolysis, reduction, and azo coupling reactions. Major reactions are:
- Hydrolysis: Formation of phenol
- Sandmeyer reaction: Formation of aryl halides or nitriles
- Reaction with KI: Formation of iodobenzene
- Reduction: Formation of benzene
- Azo coupling: Formation of azo dyes (–N=N–)
