NH₂ → -N₂⁺** (diazonium) at 0 °C with HNO₂; diazonium is a versatile platform. - **Sandmeyer reactions**: Ar-N₂⁺ + CuX → Ar-X (X = Cl, Br, CN). - **-N₂⁺ → -F** (Schiemann reaction, with BF₄⁻). - **-N₂⁺ → -OH** (aqueous, hot). - **-N₂⁺ → -H** (H₃PO₂; for removing the amine after using it for directio