Diazonium Salts
Preparation, stability and use in the synthesis of substituted aromatics and azo dyes. (Chemistry › Amines and Diazonium Salts, NEET UG syllabus.)
What is Diazonium Salts?
Organic compounds containing the functional group -N₂⁺X⁻, where N₂⁺ is the diazonium group and X⁻ is an anion. Aromatic diazonium salts are important synthetic intermediates.
Key formula / rule: Diazotisation Reaction
Key points
- Describe the preparation of aromatic diazonium salts (diazotisation).
- Explain the stability factors of aromatic diazonium salts.
- List and explain various replacement reactions of diazonium salts (Sandmeyer, Gattermann, Balz-Schiemann, hydrolysis, reduction).
- Describe coupling reactions of diazonium salts with phenols and anilines to form azo dyes.
Common exam trap
Not maintaining the low temperature (0-5°C) during diazotisation, leading to decomposition.
Definitions
- Term
Diazonium Salts
- Meaning
Organic compounds containing the functional group -N₂⁺X⁻, where N₂⁺ is the diazonium group and X⁻ is an anion. Aromatic diazonium salts are important synthetic intermediates.
- Term
Diazotisation
- Meaning
The chemical process of converting a primary aromatic amine into an aromatic diazonium salt by reacting it with nitrous acid (generated from NaNO₂ and a mineral acid) at 0-5°C.
- Term
Azo Dyes
- Meaning
A class of synthetic organic dyes characterized by the presence of one or more azo groups (-N=N-) linking two aromatic or heteroaromatic systems. They are formed by coupling reactions of diazonium salts with phenols or amines.
Learning objectives
Describe the preparation of aromatic diazonium salts (diazotisation).
Explain the stability factors of aromatic diazonium salts.
List and explain various replacement reactions of diazonium salts (Sandmeyer, Gattermann, Balz-Schiemann, hydrolysis, reduction).
Describe coupling reactions of diazonium salts with phenols and anilines to form azo dyes.
Identify the reagents and conditions for specific reactions involving diazonium salts.
Predict the products of reactions involving diazonium salts.
Formulae
- Name
Diazotisation Reaction
- Note
Formation of aromatic diazonium salt from primary aromatic amine.
- Expression
Ar-NH₂ + NaNO₂ + 2HX (0-5°C) → Ar-N₂⁺X⁻ + NaX + 2H₂O
- Name
Sandmeyer Reaction (Chlorine)
- Note
Replacement of diazonium group by chlorine.
- Expression
Ar-N₂⁺Cl⁻ + Cu₂Cl₂/HCl → Ar-Cl + N₂
- Name
Sandmeyer Reaction (Bromine)
- Note
Replacement of diazonium group by bromine.
- Expression
Ar-N₂⁺Cl⁻ + Cu₂Br₂/HBr → Ar-Br + N₂
- Name
Sandmeyer Reaction (Cyanide)
- Note
Replacement of diazonium group by cyanide.
- Expression
Ar-N₂⁺Cl⁻ + CuCN/KCN → Ar-CN + N₂
- Name
Gattermann Reaction (Chlorine)
- Note
Replacement of diazonium group by chlorine using copper powder.
- Expression
Ar-N₂⁺Cl⁻ + Cu powder/HCl → Ar-Cl + N₂
- Name
Gattermann Reaction (Bromine)
- Note
Replacement of diazonium group by bromine using copper powder.
- Expression
Ar-N₂⁺Cl⁻ + Cu powder/HBr → Ar-Br + N₂
- Name
Balz-Schiemann Reaction (Fluorine)
- Note
Introduction of fluorine atom.
- Expression
Ar-N₂⁺Cl⁻ + HBF₄ → Ar-N₂⁺BF₄⁻ (intermediate) --(heat)-→ Ar-F + BF₃ + N₂
- Name
Replacement by Iodine
- Note
Replacement of diazonium group by iodine.
- Expression
Ar-N₂⁺Cl⁻ + KI → Ar-I + N₂ + KCl
- Name
Replacement by Hydroxyl Group (Hydrolysis)
- Note
Formation of phenol.
- Expression
Ar-N₂⁺Cl⁻ + H₂O (warm) → Ar-OH + N₂ + HCl
- Name
Replacement by Hydrogen (Reduction)
- Note
Removal of the diazonium group.
- Expression
Ar-N₂⁺Cl⁻ + H₃PO₂/H₂O → Ar-H + N₂ + H₃PO₃ + HCl
- Name
Coupling Reaction (with Phenol)
- Note
Formation of an azo dye (e.g., p-hydroxyazobenzene).
- Expression
Ar-N₂⁺Cl⁻ + C₆H₅OH (alkaline medium) → Ar-N=N-C₆H₄-OH + HCl
- Name
Coupling Reaction (with Aniline)
- Note
Formation of an azo dye (e.g., p-aminoazobenzene).
- Expression
Ar-N₂⁺Cl⁻ + C₆H₅NH₂ (acidic medium) → Ar-N=N-C₆H₄-NH₂ + HCl
Prerequisites
Basic knowledge of amines (primary, secondary, tertiary).
Understanding of aromatic compounds and their reactivity.
Concepts of electrophilic aromatic substitution.
Knowledge of resonance and its effect on stability.
Basic organic reaction mechanisms.
Common mistakes
Not maintaining the low temperature (0-5°C) during diazotisation, leading to decomposition.
Confusing reagents for Sandmeyer (Cu(I) salts) and Gattermann (Cu powder).
Incorrectly identifying the products of various replacement reactions.
Forgetting that the diazonium group can be replaced by -H (reduction).
Not understanding the acidic/alkaline conditions required for coupling reactions with anilines/phenols respectively.
Keywords
Diazonium
Diazotisation
Azo Dyes
Sandmeyer
Gattermann
Balz-Schiemann
Coupling Reaction
Aromatic Amines
Nitrous Acid
Leaving Group
Practice preview
What is the product formed when benzenediazonium chloride reacts with phenol in a weakly alkaline medium?…
medium
Which of the following reagents can be used to convert benzenediazonium chloride to benzene?…
medium
Which of the following statements about the Gattermann reaction is INCORRECT?…
hard