Diazotization Reaction Mechanism - Detailed Information with FAQs

Edited By Team Careers360 | Updated on Jul 02, 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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Diazotation is a Procedure of Generating Diazonium Compounds or Diazonium Salts.
Diazotization Mechanism:-
Reaction is illustrated as follows:-
Diazotization Titration:-

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.

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.

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

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

This is the final step in the diazotization reaction mechanism.

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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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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:

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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Frequently Asked Questions (FAQs)

1. What is Diazotization reaction?

Formation of diazonium salts through primary aromatic

amines is termed as diazotizaton reaction.

2. Why is Diazotization reaction requires low temperature?

Diazotization reaction and azo coupling reactions require

low temperature as these diazonium salts form other

compounds at elevated temperature and provide

phenol by reacting at increased temperature.

Aromatic diazonium compounds or salts are unstable

and form hydrogen chloride (HCl), nitrogen gas (N₂),

and chlorobenzene.

3. Why aromatic diazonium salt is more stable than aliphatic diazonium salt?

Absence of conjugation leads to the unstability of

aliphatic diazonium salts whereas a benzene ring is

present in aromatic diazonium salt which accounts

stability of the compound due to the presence of

conjugation.

4. State the uses of the diazonium salt.

Diazonium salts are used in the production of organic compounds.

5. What are the types of diazotization titration?

Three main types of diazotization titration are as

follows:

Direct method-

It involves the treatment of a drug containing amino group with acid.

Indirect method-

This method is mainly used for the titration of insoluble diazonium salts or compounds. Excess of HNO₂ is added to the titration species and is back titrated with the required titrant.

Other method-

This method involves the production of diazo oxide rather than diazo compound as diazo oxide is more stable.

6. What is diazotization?

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.

7. What is the general structure of a diazonium salt?

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-).

8. What is the Sandmeyer reaction, and how is it related to diazotization?

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.

9. How does electron-donating groups on the aromatic ring affect diazotization?

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.

10. How does the presence of halogen substituents on the aromatic ring affect diazotization?

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.

11. What is the first step in the mechanism of diazotization?

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.

12. What is the role of sodium nitrite in the diazotization reaction?

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.

13. Why is excess HCl used in the diazotization reaction?

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.

14. Can diazotization occur in basic conditions?

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.

15. How does the basicity of the amine affect the diazotization reaction?

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.

16. Can aliphatic amines undergo diazotization?

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.

17. Why is the temperature kept low during diazotization?

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.

18. What happens if the temperature rises above 5°C during diazotization?

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.

19. What is the significance of the diazonium group in organic synthesis?

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.

20. What is the role of ice in the diazotization reaction mixture?

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.

21. What is the difference between diazotization and diazo coupling?

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.

22. Can diazotization be performed on meta-substituted anilines?

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.

23. Can diazotization be performed on polyamines?

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.

24. How does the pH affect the stability of diazonium salts?

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.

25. What is the importance of stirring during the diazotization reaction?

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.

26. How does ring size affect the diazotization of heterocyclic amines?

Ring size in heterocyclic amines can affect diazotization. Five-membered heteroaromatic rings (e.g., pyrroles) are generally more reactive than six-membered rings (e.g., pyridines) due to increased electron density at the amino group.

27. How does the concept of hyperconjugation apply to the stability of diazonium ions?

Hyperconjugation can contribute to the stability of diazonium ions, particularly when alkyl groups are present on the aromatic ring. This electronic effect can influence the reactivity and lifetime of the diazonium species in solution.

28. What is the role of copper powder in some diazotization reactions?

Copper powder is sometimes added in diazotization reactions to remove excess nitrous acid. This prevents side reactions and improves the purity of the diazonium salt product.

29. Can diazotization be performed on aromatic compounds with strongly electron-withdrawing groups?

Yes, but it can be challenging. Strongly electron-withdrawing groups decrease the basicity of the amine, making it less reactive. In such cases, more vigorous conditions (e.g., higher concentrations of nitrous acid, longer reaction times) may be necessary.

30. What is the significance of the nitrosonium ion in the diazotization mechanism?

The nitrosonium ion (NO+) is the key electrophile in the diazotization mechanism. It attacks the amino group, initiating the formation of the N-N bond that eventually becomes the diazonium group.

31. How does the solubility of the amine affect the diazotization process?

The solubility of the amine in the acidic aqueous medium affects the efficiency of diazotization. Poorly soluble amines may require the addition of organic co-solvents or surfactants to improve dispersion and reactivity.

32. What is the purpose of adding sodium nitrite solution slowly during diazotization?

Sodium nitrite solution is added slowly to control the rate of nitrous acid formation. This helps prevent an excess of nitrous acid, which could lead to side reactions or decomposition of the diazonium salt.

33. Can diazotization be performed on aromatic diamines?

Yes, aromatic diamines can undergo diazotization. Depending on the reaction conditions and stoichiometry, either one or both amino groups can be converted to diazonium salts. Selective mono-diazotization is often possible with careful control of reagents.

34. What is the importance of the counterion in diazonium salts?

The counterion (usually Cl- or HSO4-) plays a crucial role in stabilizing the positively charged diazonium group. It affects the solubility, reactivity, and stability of the diazonium salt, influencing subsequent reactions and handling.

35. What is the role of sulfamic acid in some diazotization procedures?

Sulfamic acid is sometimes used to destroy excess nitrous acid after diazotization is complete. This prevents further reactions of the nitrous acid with the diazonium salt or other components, improving the purity and yield of the product.

36. How does the presence of ortho-substituents affect diazotization?

Ortho-substituents can sterically hinder the approach of the nitrosonium ion to the amino group, potentially slowing down the diazotization reaction. This effect is more pronounced with bulky substituents.

37. What is the significance of the electrophilic aromatic substitution mechanism in diazotization?

While diazotization primarily involves the amino group, the overall process can be viewed as an electrophilic aromatic substitution where the diazonium group replaces a hydrogen on the aromatic ring. This perspective helps in understanding the reactivity and orientation of the products.

38. How does the concept of resonance apply to diazonium salts?

Resonance stabilization in diazonium salts involves the delocalization of the positive charge onto the aromatic ring. This resonance contributes to the stability of the diazonium ion and influences its reactivity in subsequent transformations.

39. What is the importance of the leaving group ability of the diazonium group?

The diazonium group (N2+) is an excellent leaving group due to the stability of the nitrogen molecule (N2) that is eliminated. This property makes diazonium salts versatile intermediates in various substitution and coupling reactions.

40. How does the presence of a para-substituent affect the stability of a diazonium salt?

Para-substituents can significantly affect diazonium salt stability. Electron-donating groups in the para position can increase stability by resonance, while electron-withdrawing groups can decrease stability by enhancing the electrophilicity of the diazonium group.

41. What is the role of urea in some diazotization procedures?

Urea is sometimes added to diazotization reactions to react with excess nitrous acid, preventing side reactions. It forms harmless nitrogen gas and carbon dioxide, helping to maintain the purity of the diazonium salt product.

42. How does the concept of kinetic vs. thermodynamic control apply to diazotization?

Diazotization is typically under kinetic control due to the low temperatures used. This favors the formation of the diazonium salt over thermodynamically more stable products (like phenols) that would form if the reaction were allowed to reach equilibrium at higher temperatures.

43. What is the significance of the pKa of the amine in diazotization?

The pKa of the amine influences its basicity and thus its reactivity in diazotization. Amines with lower pKa values (more acidic) are generally harder to diazotize as they are less nucleophilic, often requiring more forceful conditions.

44. How does the concept of microscopic reversibility apply to the diazotization mechanism?

Microscopic reversibility in diazotization implies that each step in the forward reaction mechanism has a corresponding reverse step. This concept is important in understanding the equilibria involved, especially in the initial protonation and nitrosation steps.

45. What is the role of sodium acetate in some diazotization procedures?

Sodium acetate is sometimes added near the end of diazotization to neutralize excess acid. This can help stabilize the diazonium salt and prevent unwanted side reactions, especially if the product is to be used in subsequent coupling reactions.

46. How does the principle of green chemistry apply to modern diazotization techniques?

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).

47. What is the significance of the inductive effect in diazotization reactions?

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.

48. How does the concept of Hammond's postulate apply to the diazotization mechanism?

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.

49. What is the role of surfactants in some diazotization procedures?

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.

50. How does the concept of frontier molecular orbitals apply to diazotization?

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.

51. What is the significance of the isotope effect in studying the diazotization mechanism?

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.

52. How does the concept of hard and soft acids and bases (HSAB) apply to diazotization?

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.

53. What is the role of phase-transfer catalysts in some diazotization reactions?

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.

54. How does the principle of Le Chatelier apply to the equilibria involved in diazotization?

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.

55. What is the significance of the Japp-Klingemann reaction in relation to diazotization?

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.