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Diazotization Reaction Mechanism - Detailed Information with FAQs

Diazotization Reaction Mechanism - Detailed Information with FAQs

Team Careers360Updated on 02 Jul 2025, 04:52 PM IST

Diazotation is a Procedure of Generating Diazonium Compounds or Diazonium Salts.

This reaction was first illustrated by Peter Griess. Hence, the diazotization technique yields salts of aromatic compounds through aromatic amines.

Diazotization Reaction:-

Diazotization Reaction involves the formation of diazonium salts when aromatic amines are made to react with nitrous acid in presence of mineral acid. Water is obtained as a by-product or side product.

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The reaction can be clarified as follows:-

Aromatic Amine + Nitrous Acid + Mineral Acid gives Diazonium Salt + Water

Aromatic amine is usually a derivative of aromatic hydrocarbon where the hydrogen of the aromatic ring is replaced by or substituted by an amino group. This amine group is nucleophilic and directly bonded to the aromatic ring compound. Nitrous acid is generally a nitrogen oxoacid a conjugate acid of nitrite. It is a monoprotic weak acid used in the gaseous phase. The structure of nitrous acid is as follows-

Nitrous acid

Mineral acids or inorganic acids are derived from one or more inorganic chemical compounds. Mineral acids when dissolved in water tend to produce hydrogen ions and conjugate base.

General diazotization reaction is as follows:-

ArNH2 + HNO2 + HX ---------- ArN2+X- + H2O

Diazotization of an Amine:

Diazotization of amines is a process where primary amines are converted to diazonium salts using nitrous acid and mineral acid respectively.

Diazotization of an Amine

This reaction involves losing nitrogen from the aromatic amine generating a carbon cation further leading to the formation of a diazonium salt.

Commonly Asked Questions

Q: What is diazotization?
A:
Diazotization is a chemical reaction where a primary aromatic amine reacts with nitrous acid to form a diazonium salt. This process is crucial in organic synthesis, particularly for creating azo dyes and other aromatic compounds.
Q: What is the general structure of a diazonium salt?
A:
The general structure of a diazonium salt is Ar-N≡N+ X-, where Ar represents an aromatic ring, N≡N+ is the diazonium group, and X- is the counterion (usually Cl- or HSO4-).
Q: What is the Sandmeyer reaction, and how is it related to diazotization?
A:
The Sandmeyer reaction is a method to replace the diazonium group with a halogen (Cl, Br, or CN). It involves treating the diazonium salt (formed through diazotization) with a copper(I) salt of the desired halide, allowing for the synthesis of various aromatic halides.
Q: How does electron-donating groups on the aromatic ring affect diazotization?
A:
Electron-donating groups on the aromatic ring increase the basicity of the amine, making it more reactive towards diazotization. This can lead to faster reaction rates and potentially higher yields of the diazonium salt.
Q: How does the presence of halogen substituents on the aromatic ring affect diazotization?
A:
Halogen substituents on the aromatic ring generally have a mild electron-withdrawing effect, slightly decreasing the basicity of the amine. This can make diazotization slightly more challenging, potentially requiring longer reaction times or more concentrated reagents.

Diazotization Mechanism:-

Now, let us have a look at the mechanism followed by aromatic primary amine to form diazonium salt.

  • Formation of nitrosonium ion

Diazotization Mechanism

The main reagent used in this reaction is either sodium

nitrite (NaNO2) or Nitrous acid (HNO2).

Sodium nitrite salt can be easily handled while Nitrous acid is an unstable liquid to be handled cautiously.

Secondly, mineral acid plays an important role in converting NaNO2 to HNO2 provided if NaNO2 is used in the reaction.

The mineral acid generally used is HCl.

HCl converts NaNO2 to HNO2 further converting it into nitrosonium ion (NO+) which is a strong electrophile.

This electrophile is responsible for the formation of a diazonium salt. The formation of nitrosonium ion takes place by protonation of a hydroxyl group (OH-) resulting in loss of a water molecule.

  • Formation of diazonium ion

Reactions of Diazonium Salts: Sandmeyer and Related Reactions

The reaction between amine and nitrosonium ion

accompanied by mineral acid is the next step.

  1. The reaction between nitrosonium ion and aromatic amine leads to the formation of a new N-N bond.

  2. Two proton transfers take place from nitrogen to oxygen followed by rearrangement of pi bonds.

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i.e. N-N a pi bond is formed and an N-O bond is broken.

  1. N-N triple bond is formed with the elimination of water molecule.

This is the final step in the diazotization reaction mechanism.

Also Read:

Diazonium Coupling Reaction:-

We all know that dyes form an important part of our day to day lives. Dyes are nothing but diazobenzene derivatives. The structure of diazobenzene or azobenzene is such that the two benzene molecules are attached by the nitrogen-nitrogen double bond. The most important characteristic of diazonium salts is that they undergo coupling reactions. The azo group is the N-N bond present in the compound.

Aromatic diazo compounds are stable and crystalline chemical species. They can be synthesized by azo cooling reaction following electrophilic substitution reaction. The reaction takes place between the aryl diazonium salt and an aryl ring. The aryl ring attacks the diazonium cation forming azo compounds.

ArN2+ + Ar'H ------------ ArN=NAr' +H+

The diazo coupling chemical reactions take place at or near 0°C as the diazonium compounds or diazonium salts are unstable at the above-mentioned temperature.

The azo compound formation also takes place via the oxidation of hydrazines (R-NH-NH-R).

Condensation of nitro aromatic compounds with aniline result in the formation of azoxy intermediate which is further reduced to the azo compound.

Ar(NO2) + ArNH2 ArN(O)=NAr + H2O

ArN(O)=NAr + C6H12O6 ArN=NAr +

C6H12O6 + H2O

As a result of π-delocalisation of electrons, Aromatic diazo compounds express themselves in various colours, mostly red, orange and yellow. Hence, they are usually used as dyes and so are called azo dyes.

Diazotization Of Aniline:-

Aniline or Arenediazonium salt reacts with a highly reactive compound like phenol and or amines leading to the formation of coloured compounds called azo compounds. This reaction is termed a coupling reaction. This chemical reaction is used in the preparation of red azo dye. Preparation of aromatic diazonium salt generally takes place by the addition of cold aqueous sodium nitrite to primary aromatic amine in presence of an acid. The temperature required for this reaction to take place is 273 – 278K.

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Commonly Asked Questions

Q: What is the first step in the mechanism of diazotization?
A:
The first step in the diazotization mechanism is the protonation of nitrous acid (HONO) to form the nitrosonium ion (NO+), which is the active electrophile in the reaction.
Q: What is the role of sodium nitrite in the diazotization reaction?
A:
Sodium nitrite (NaNO2) serves as the source of nitrous acid when it reacts with hydrochloric acid. This in-situ generated nitrous acid is the actual reagent that reacts with the primary aromatic amine to form the diazonium salt.
Q: Why is excess HCl used in the diazotization reaction?
A:
Excess HCl is used to ensure complete protonation of the amine, forming the more reactive anilinium ion. It also helps maintain the acidic conditions necessary for the stability of the diazonium salt.
Q: Can diazotization occur in basic conditions?
A:
No, diazotization requires acidic conditions. In basic conditions, the nitrous acid would be converted to nitrite ions, which are not reactive towards the amine. Additionally, the amine would not be protonated, reducing its reactivity.
Q: How does the basicity of the amine affect the diazotization reaction?
A:
The basicity of the amine affects the ease of diazotization. Less basic amines (e.g., those with electron-withdrawing groups) are harder to diazotize and may require stronger acidic conditions or longer reaction times.

Reaction is illustrated as follows:-

Reaction is illustrated

Aliphatic Azo or Diazo compounds are less used than aromatic diazo compounds. Aliphatic azo or diazo compounds are used as radical initiators. Diethyldiazine, an aliphatic azo compound cleaves the C-N bond at increased temperature leading to the elimination of nitrogen gas generating radicals.

The aliphatic azo chemical should be handled with great care as they can explode due to their instability.

NCERT Chemistry Notes:

Commonly Asked Questions

Q: What is the difference between diazotization and diazo coupling?
A:
Diazotization is the process of forming a diazonium salt from a primary aromatic amine, while diazo coupling is a subsequent reaction where the diazonium salt reacts with an aromatic compound (usually phenols or aromatic amines) to form azo compounds.
Q: Can diazotization be performed on meta-substituted anilines?
A:
Yes, diazotization can be performed on meta-substituted anilines. The position of the substituent doesn't significantly affect the reaction, as the diazonium group is formed on the nitrogen atom attached directly to the aromatic ring.
Q: Can diazotization be performed on polyamines?
A:
Yes, diazotization can be performed on polyamines (compounds with multiple amino groups). However, careful control of reaction conditions and stoichiometry is necessary to achieve selective diazotization of specific amino groups.
Q: How does the pH affect the stability of diazonium salts?
A:
Diazonium salts are most stable in acidic conditions (pH < 3). As the pH increases, the stability decreases, and above pH 7, they rapidly decompose. Maintaining the correct pH is crucial for successful diazotization and subsequent reactions.
Q: What is the importance of stirring during the diazotization reaction?
A:
Stirring is crucial during diazotization to ensure uniform temperature distribution, prevent localized heating, and promote efficient mixing of reagents. This helps maintain the stability of the diazonium salt and improves the overall yield of the reaction.

Diazotization Titration:-

Converting primary amine the diazonium salt or compound is called diazotization titration.

A primary amine is made to react with sodium nitrite and mineral acid like HCl to obtain a diazonium salt compound.

Nitrous acid is formed when sodium nitrite and HCl react with each other.

NaNO2 + HCl →NaCl + H2O

The nitrous acid forms react with primary amine forming diazonium salt. The addition of ammonium sulphamate solution is added to remove excess nitrous acid formed.

R-NH2 + HNO2→ R-N=NH + H2O

Starch iodide paper is used in the detection of the endpoint. The end point is detected by formation of blue colour.

The starch iodide paper preparation involves immersion of filter paper in starch and potassium iodide solution.

The following reactions depict the end point raections:-

KI + HCl→ KCl + H2O

2HI + 2HNO2 →I2 + 2NO=2H2O

I2 + starch mucilage →blue colour

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Commonly Asked Questions

Q: Can aliphatic amines undergo diazotization?
A:
No, aliphatic amines cannot undergo diazotization in the same way as aromatic amines. The resulting diazonium compounds from aliphatic amines are extremely unstable and decompose rapidly, making them impractical for further reactions.
Q: Why is the temperature kept low during diazotization?
A:
The temperature is kept low (0-5°C) during diazotization to prevent the decomposition of the unstable diazonium salt. At higher temperatures, the diazonium salt can break down, releasing nitrogen gas and forming unwanted byproducts.
Q: What happens if the temperature rises above 5°C during diazotization?
A:
If the temperature rises above 5°C, the diazonium salt may start to decompose, releasing nitrogen gas and forming phenols or other unwanted byproducts. This can significantly reduce the yield and purity of the desired product.
Q: What is the significance of the diazonium group in organic synthesis?
A:
The diazonium group acts as an excellent leaving group in various reactions. It can be replaced by numerous nucleophiles, allowing for the synthesis of a wide range of aromatic compounds from a single diazonium salt precursor.
Q: What is the role of ice in the diazotization reaction mixture?
A:
Ice is added to the reaction mixture to maintain the low temperature (0-5°C) required for the stability of the diazonium salt. It helps absorb the heat generated during the exothermic reaction and prevents decomposition of the product.

Frequently Asked Questions (FAQs)

Q: What is the significance of the Japp-Klingemann reaction in relation to diazotization?
A:
The Japp-Klingemann reaction is a rearrangement that can occur with certain diazonium salts, leading to the formation of hydrazones. Understanding this reaction is important for predicting and controlling the outcome of diazotization reactions with specific substrates.
Q: How does the principle of Le Chatelier apply to the equilibria involved in diazotization?
A:
Le Chatelier's principle is relevant in understanding how changes in concentration, temperature, and pH affect the various equilibria in diazotization. For example, excess acid shifts the equilibrium towards the formation of the nitrosonium ion, promoting the reaction.
Q: What is the role of phase-transfer catalysts in some diazotization reactions?
A:
Phase-transfer catalysts can facilitate diazotization of poorly water-soluble amines by promoting the transfer of reagents between the aqueous and organic phases. This can improve reaction rates and yields for certain substrates.
Q: How does the concept of hard and soft acids and bases (HSAB) apply to diazotization?
A:
The HSAB concept can explain the preference of the soft nitrosonium ion (NO+) for the relatively soft amino group over harder nucleophiles like water. This selectivity is crucial for the success of the diazotization reaction.
Q: What is the significance of the isotope effect in studying the diazotization mechanism?
A:
Isotope effects, particularly kinetic isotope effects using deuterated compounds, can provide insights into the rate-determining step of diazotization. This helps in elucidating the detailed mechanism and identifying key intermediates.
Q: How does the concept of frontier molecular orbitals apply to diazotization?
A:
In diazotization, the interaction between the HOMO (Highest Occupied Molecular Orbital) of the amine and the LUMO (Lowest Unoccupied Molecular Orbital) of the nitrosonium ion is crucial. This orbital interaction governs the reactivity and selectivity of the process.
Q: What is the role of surfactants in some diazotization procedures?
A:
Surfactants can be added to improve the solubility and dispersion of poorly water-soluble amines in the aqueous reaction medium. This can enhance the efficiency of diazotization, especially for hydrophobic aromatic amines.
Q: How does the concept of Hammond's postulate apply to the diazotization mechanism?
A:
Hammond's postulate suggests that the transition state in diazotization resembles the less stable species (either reactants or products) more closely. This helps in predicting the effect of substituents and reaction conditions on the rate and yield of diazotization.
Q: What is the significance of the inductive effect in diazotization reactions?
A:
The inductive effect of substituents on the aromatic ring can influence the electron density at the amino group, affecting its basicity and reactivity. Electron-withdrawing groups decrease reactivity, while electron-donating groups increase it.
Q: How does the principle of green chemistry apply to modern diazotization techniques?
A:
Green chemistry principles in diazotization focus on using safer reagents (e.g., polymer-supported nitrite sources), minimizing waste (e.g., through in-situ generation and use of diazonium salts), and improving energy efficiency (e.g., through flow chemistry techniques).
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