Replacing the diazonium group

Diazonium reactions fall into two groups: those that displace nitrogen and those that keep the diazo group. The diazonium group is an excellent leaving group; the N₂ it forms escapes as a gas.

Sandmeyer reaction: with Cu(I) salts, Cl⁻, Br⁻ and CN⁻ replace the diazonium group: Cu₂Cl₂/HCl gives ArCl, Cu₂Br₂/HBr gives ArBr, CuCN/KCN gives ArCN.

Gatterman reaction (NCERT's spelling): the diazonium salt with the halogen acid and copper powder gives ArCl or ArBr. The Sandmeyer reaction gives the better yield.

Iodide: warming the diazonium salt solution with potassium iodide gives iodobenzene; iodine is hard to put on the ring directly.

Fluoride: fluoroboric acid precipitates the arenediazonium fluoroborate, which on heating gives the aryl fluoride, BF₃ and N₂.

Replacement by H: mild reducing agents, hypophosphorous acid (phosphinic acid) or ethanol, reduce the salt to the arene and are themselves oxidised to phosphorous acid and ethanal.

Replacement by –OH: if the diazonium solution warms up to 283 K, it hydrolyses to phenol, N₂ and HCl.

Replacement by –NO₂: the diazonium fluoroborate heated with aqueous NaNO₂ and copper gives the nitroarene.