Diazonium Salts and Aromatic Substitution (Brief)

Formation of arenediazonium salts

• Make nitrous acid in situ (NaNO₂ + strong acid, 0–5 °C) and add an aryl amine (Ar–NH₂) → Ar–N₂⁺ X⁻. Example: aniline → benzenediazonium chloride. Diazonium salts are reasonably stable cold, but decompose above ~5 °C, expelling N₂ (excellent leaving group).

Sandmeyer substitutions (Cu-assisted)

• Ar–Cl: CuCl/HCl.
• Ar–Br: CuBr/HBr.
• Ar–CN: CuCN.
  Mechanism likely involves N₂ loss and Cu capturing the aryl intermediate; enables Cl/Br/CN installs otherwise hard by direct EAS.

Other diazonium substitutions (non-Cu)

• Ar–I: KI (iodide directly displaces).
• Ar–OH: Warm H₂O or Cu₂O/Cu(NO₃)₂ (Gattermann) → phenols.
• Ar–F: Schiemann (HBF₄ → Ar–N₂⁺ BF₄⁻, then heat).
• Ar–H (deamination): H₃PO₂ (or EtOH) reduces diazonium to H, removing –NH₂.

Azo coupling (related)

• Diazonium acts as an electrophile toward activated rings (e.g., phenol, aniline) to give Ar–N=N–Ar′ azo dyes (typically para-coupling under mild basic conditions). Not a Sandmeyer replacement but an important diazonium pathway.

Uses and scope

• Converts aryl amines into –Cl, –Br, –I, –F, –CN, –OH, –H, expanding aromatic substitution options beyond classical EAS. Aliphatic diazoniums are too unstable; scope is mainly aryl/heteroaryl amines.