Exhaustive Guide: Diazonium Salts & Reactions | Class 12 Chemistry

Diazonium Salts & Applications

Module 3 | CBSE Class 12 Chemistry | Organic Chemistry

Table of Contents

• 1. Introduction & Structure
• 2. Preparation: Diazotization
• 3. Reactions involving Displacement of Nitrogen
  • 3.1 Sandmeyer & Gattermann Reactions (-Cl, -Br, -CN)
  • 3.2 Replacement by Iodide & Fluoride (-I, -F)
  • 3.3 Replacement by Hydrogen (-H)
  • 3.4 Replacement by Hydroxyl Group (-OH)
• 4. Reactions involving Retention of Diazo Group
  • 4.1 Coupling Reactions (Azo Dyes)
• 5. Importance in Synthetic Organic Chemistry
• 6. NCERT Solved Examples
• 7. Previous Year Questions (PYQs)

1. Introduction & Structure

Diazonium salts have the general formula R-N₂⁺X^- where R stands for an aryl group and X^- ion may be Cl^-, Br^-, HSO₄^-, BF₄^-, etc. They are named by suffixing diazonium to the name of the parent hydrocarbon from which they are formed.

Example: C₆H₅N₂⁺Cl^- is named Benzene diazonium chloride.

Stability Anomaly (Aliphatic vs. Aromatic):
Primary aliphatic amines form highly unstable alkyl diazonium salts which immediately decompose to yield alcohols and nitrogen gas.
Primary aromatic amines form arene diazonium salts which are stable for a short time at low temperatures (273-278 K). This stability is due to the resonance dispersal of the positive charge over the benzene ring.

2. Preparation: Diazotization

Benzene diazonium chloride is prepared by the reaction of aniline with nitrous acid at incredibly low temperatures. Since nitrous acid (HNO₂) is highly unstable, it is produced in situ (inside the reaction flask) by reacting sodium nitrite with concentrated hydrochloric acid.

Diazotization: The conversion of a primary aromatic amine into a diazonium salt is known as diazotization. The reaction must be carried out in an ice bath at 0 to 5°C (273-278 K).

C₆H₅NH₂ + NaNO₂ + 2HCl →(273-278 K) C₆H₅N₂⁺Cl^- + NaCl + 2H₂O

Because they are unstable and decompose easily, diazonium salts are generally not stored; they are used immediately after their preparation.

3. Reactions involving Displacement of Nitrogen

The diazonium group is an exceptionally good leaving group because it leaves as highly stable, neutral nitrogen gas (N₂). These reactions are vital for introducing groups like -F, -Cl, -Br, -I, -CN, and -OH into an aromatic ring.

3.1 Sandmeyer & Gattermann Reactions (-Cl, -Br, -CN)

A. Sandmeyer Reaction: The Cl^-, Br^-, and CN^- nucleophiles can easily be introduced into the benzene ring in the presence of Cu(I) ion (Cuprous salts).

Ar-N₂⁺X^- + Cu₂Cl₂/HCl → Ar-Cl + N₂
Ar-N₂⁺X^- + Cu₂Br₂/HBr → Ar-Br + N₂
Ar-N₂⁺X^- + CuCN/KCN → Ar-CN + N₂

B. Gattermann Reaction: Alternatively, chlorine or bromine can be introduced by treating the diazonium salt solution with corresponding halogen acid in the presence of Copper powder.

Ar-N₂⁺X^- + Cu / HCl → Ar-Cl + N₂ + CuX
Ar-N₂⁺X^- + Cu / HBr → Ar-Br + N₂ + CuX

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

3.2 Replacement by Iodide & Fluoride (-I, -F)

A. Replacement by Iodide: Iodine is not easily introduced into the benzene ring directly. However, when a diazonium salt solution is simply warmed with Potassium Iodide (KI), iodobenzene is formed. (No copper catalyst is required).

Ar-N₂⁺Cl^- + KI → Ar-I (Iodobenzene) + KCl + N₂

B. Replacement by Fluoride (Balz-Schiemann Reaction): Direct fluorination is highly explosive. When arene diazonium chloride is treated with fluoroboric acid (HBF₄), arene diazonium fluoroborate is precipitated. On heating, it decomposes to yield fluorobenzene.

Ar-N₂⁺Cl^- + HBF₄ → Ar-N₂⁺BF₄^- →(Δ) Ar-F (Fluorobenzene) + BF₃ + N₂

3.3 Replacement by Hydrogen (Removal of NH₂ group)

Crucial Synthesis Tool: Sometimes we use the -NH₂ group only to direct an electrophile (like Bromine) to ortho/para positions, and then we want to remove the -NH₂ group entirely. This is done by converting it to a diazonium salt and then replacing the N₂ group with Hydrogen.

Certain mild reducing agents like Hypophosphorous acid (H₃PO₂) or Ethanol (C₂H₅OH) reduce diazonium salts to benzene and themselves get oxidized to phosphorous acid (H₃PO₃) and ethanal respectively.

Ar-N₂⁺Cl^- + H₃PO₂ + H₂O → Ar-H (Benzene) + N₂ + H₃PO₃ + HCl

Ar-N₂⁺Cl^- + CH₃CH₂OH → Ar-H (Benzene) + N₂ + CH₃CHO + HCl

3.4 Replacement by Hydroxyl Group (-OH)

If the temperature of the diazonium salt solution is allowed to rise up to 283 K (or if it is gently warmed with water), the salt gets hydrolyzed to form phenol.

Ar-N₂⁺Cl^- + H₂O →(Δ) Ar-OH (Phenol) + N₂ + HCl

4. Reactions involving Retention of Diazo Group

4.1 Coupling Reactions (Azo Dyes)

The diazonium ion acts as a weak electrophile. It can undergo electrophilic substitution reactions with highly electron-rich aromatic rings, such as phenols and aromatic amines, to form brightly colored compounds known as azo dyes. The nitrogen atoms are retained in the product, linking the two rings (-N=N-).

Coupling Reaction: The reaction of diazonium salts with phenols or aromatic amines to form colored azo compounds is called coupling. It generally occurs at the para position of the phenol or amine.

A. Coupling with Phenol (Orange Dye):
Occurs in a mildly basic medium (pH 9-10).
C₆H₅N₂⁺Cl^- + Phenol →(OH^-) p-Hydroxyazobenzene (Orange Dye) + Cl^- + H₂O

B. Coupling with Aniline (Yellow Dye):
Occurs in a mildly acidic medium (pH 4-5).
C₆H₅N₂⁺Cl^- + Aniline →(H⁺) p-Aminoazobenzene (Yellow Dye) + Cl^- + H⁺

5. Importance in Synthetic Organic Chemistry

From the above reactions, it is clear that the diazonium salts are excellent intermediates for the introduction of -F, -Cl, -Br, -I, -CN, -OH, -NO₂ groups into the aromatic ring.

Why is this so important?
Aryl fluorides and iodides cannot be prepared by direct halogenation. The cyano group cannot be introduced by nucleophilic substitution of chlorobenzene. Thus, the diazonium salt route provides the only practical way to synthesize these vital compounds.

6. NCERT Solved Examples (Step-by-Step)

NCERT Example 13.6: How will you convert 4-nitrotoluene to 2-bromobenzoic acid?

Solution:
This is a multi-step synthesis utilizing the directing effects and diazonium salts.
Step 1 (Bromination): 4-Nitrotoluene is brominated with Br₂/Fe. The methyl group is activating and o/p directing, while -NO₂ is deactivating and m-directing. Both direct the incoming bromine to the position ortho to the methyl group. Product: 2-Bromo-4-nitrotoluene.
Step 2 (Reduction): Reduce the -NO₂ group to -NH₂ using Sn/HCl. Product: 4-Amino-2-bromotoluene.
Step 3 (Diazotization): Treat with NaNO₂ + HCl at 0-5°C to convert -NH₂ to -N₂⁺Cl^-.
Step 4 (Replacement by H): Treat the diazonium salt with H₃PO₂ and H₂O to remove the diazo group entirely. Product: 2-Bromotoluene.
Step 5 (Oxidation): Finally, oxidize the methyl group using alkaline KMnO₄ followed by acidification (H₃O⁺) to yield 2-Bromobenzoic acid.

7. Previous Year Questions (PYQs) & Exhaustive Question Bank

Part A: Conceptual (1-2 Marks)

[CBSE 2018, 2021]

Q1. Why must diazotization be carried out at low temperatures (0 to 5°C)?

Answer: Arene diazonium salts are highly unstable and decompose easily to form phenol and nitrogen gas if the temperature rises above 5°C (278 K). The low temperature prevents their decomposition, allowing them to be utilized as synthetic intermediates.

[CBSE 2017, 2020]

Q2. Why is benzene diazonium chloride not stored and is used immediately after its preparation?

Answer: Benzene diazonium chloride is very unstable, even at room temperature. The diazonium group (-N₂⁺) is an excellent leaving group because it leaves as highly stable nitrogen gas (N₂). If stored, it will spontaneously decompose or react with moisture in the air to form phenol. Hence, it is prepared in situ and used immediately.

Part B: Assertion-Reason Type (1 Mark)

[CBSE Sample Paper 2023]

Q3. Assertion (A): Aromatic primary amines can be converted into diazonium salts, but aliphatic primary amines cannot form stable diazonium salts.
Reason (R): Aromatic diazonium salts are resonance stabilized.

Answer: Both Assertion and Reason are correct, and Reason is the correct explanation for Assertion. In arene diazonium salts, the positive charge on the nitrogen atom is dispersed over the benzene ring through resonance, granting it slight stability at low temperatures. Alkyl diazonium salts lack this resonance stabilization and decompose instantly.

Part C: Synthesis and Conversions (3 Marks)

[CBSE 2016, 2019, 2022]

Q4. How will you carry out the following conversions?
(a) Aniline to Fluorobenzene
(b) Aniline to Benzene
(c) Aniline to p-Aminoazobenzene

Answer:
(a) Aniline to Fluorobenzene (Balz-Schiemann Reaction):
Step 1: Convert aniline to benzene diazonium chloride using NaNO₂ + HCl at 0-5°C.
Step 2: Treat with Fluoroboric acid (HBF₄) to form the fluoroborate precipitate, then heat (Δ) to get Fluorobenzene.

(b) Aniline to Benzene:
Step 1: Convert aniline to benzene diazonium chloride (NaNO₂ + HCl, 0-5°C).
Step 2: Treat the diazonium salt with a mild reducing agent like Hypophosphorous acid (H₃PO₂) and water, or Ethanol (CH₃CH₂OH) to yield Benzene.

(c) Aniline to p-Aminoazobenzene (Coupling Reaction):
Step 1: Convert a portion of aniline to benzene diazonium chloride.
Step 2: React the diazonium salt with a fresh molecule of aniline in a mildly acidic medium (pH 4-5). The electrophile attacks the para position of aniline to form the yellow azo dye, p-Aminoazobenzene.

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This concludes the exhaustive series on the Amines Chapter for CBSE Class 12.

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