Cyanides, Isocyanides and Diazonium Salts — Part 2: Reactions of diazonium salts: visual lesson, ISC Class 12 Chemistry

Lesson

The diazonium toolkit

Part 2 — replacing –N₂⁺ by other groups, and coupling to make azo dyes.

In this lesson you will

Write equations for converting benzenediazonium chloride into halobenzenes, phenol, benzene and nitrobenzene
Write equations for azo coupling with phenol and aniline and identify the coloured products
Plan syntheses that use a diazonium salt intermediate

Concept

Sandmeyer and Gattermann reactions

Sandmeyer: copper(I) salts — CuCl/HCl → ArCl; CuBr/HBr → ArBr; CuCN/KCN → ArCN. Gattermann: freshly prepared copper powder with HCl or HBr → ArCl or ArBr (lower yields).

\ce{C6H5N2Cl ->[CuCl/HCl] C6H5Cl + N2}

\ce{C6H5N2Cl + CuCN -> C6H5CN + N2 + CuCl}

\ce{C6H5N2Cl + HBr ->[Cu] C6H5Br + N2 + HCl}

Table of Sandmeyer and Gattermann reactions
Reagent	Product	Name
CuCl/HCl	Chlorobenzene	Sandmeyer
CuBr/HBr	Bromobenzene	Sandmeyer
CuCN/KCN	Benzonitrile	Sandmeyer
Cu + HCl	Chlorobenzene	Gattermann
Cu + HBr	Bromobenzene	Gattermann

Concept

Iodide, fluoride, hydrogen, hydroxyl and nitro groups

KI: warm → iodobenzene. Balz–Schiemann: HBF₄ → ArN₂⁺BF₄⁻ (stable solid), heat → fluorobenzene + BF₃ + N₂. –H: hypophosphorous acid (H₃PO₂) or ethanol → benzene. –OH: warm with water → phenol. –NO₂: ArN₂⁺BF₄⁻ + NaNO₂, Cu, heat → nitrobenzene.

\ce{C6H5N2Cl + KI -> C6H5I + KCl + N2}

\ce{C6H5N2BF4 ->[\Delta] C6H5F + BF3 + N2}

\ce{C6H5N2Cl + H3PO2 + H2O -> C6H6 + N2 + H3PO3 + HCl}

\ce{C6H5N2BF4 + NaNO2 ->[Cu][\Delta] C6H5NO2 + N2 + NaBF4}

Table of other replacement reactions
Reagent	Product
KI (warm)	Iodobenzene
HBF₄, then heat	Fluorobenzene
H₃PO₂ or CH₃CH₂OH	Benzene
H₂O, warm	Phenol
NaNO₂/Cu on ArN₂BF₄	Nitrobenzene

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