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Reactions of Arene Diazonium Salts

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Estimated time: 5 minutes
Maharashtra State Board: Class 12

Definition: Diazo Group

The general formula for diazonium salts is ArN₂⁺X⁻, where Ar refers to an aryl group while X⁻ ion may be Cl⁻, Br⁻, HSO₄⁻, BF₄⁻, etc.; the group N₂⁺ or [—N≡N] is called the diazo group.

Maharashtra State Board: Class 12

Definition: Coupling Reaction

Benzene diazonium chloride reacts with phenol in which phenol molecule at its para-position is coupled with diazonium salt is give the product p-hydroxy azobenzene. Hence, the reaction is known as coupling reaction.

Maharashtra State Board: Class 12

Key Points: Reactions of Arene Diazonium Salts

  • Preparation: Primary aromatic amines (aniline) react with HNO₂ (NaNO₂ + 2HCl) at 273–278 K (0–5 °C) to form diazonium chloride + NaCl + 2H₂O.
  • Sandmeyer reactions: C₆H₅N₂⁺Cl⁻ with CuCl/HCl, CuBr/HBr, CuCN/KCN give C₆H₅Cl, C₆H₅Br, C₆H₅CN, respectively (with N₂↑).
  • Other displacement reactions: Gattermann (Cu/HCl) → C₆H₅Cl; KI → C₆H₅I; H₃PO₂/H₂O → C₆H₆; H₂O/283 K → C₆H₅OH; HBF₄/Δ → C₆H₅F (Balz–Schiemann reaction).
  • Coupling with phenol: Benzene diazonium chloride + phenol (OH⁻) → p-hydroxy azobenzene (orange dye) + HCl.
  • Coupling with aniline: Benzene diazonium chloride + aniline (mild alkaline medium) → p-aminoazobenzene (yellow dye) + HCl; azo products are coloured and used as dyes.
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