Sandmeyer reaction: ArN2+ + CuX/HX -> ArX + N2. Works for X = Cl, Br, CN. Catalyst is Cu(I) salt (CuCl, CuBr, CuCN). Examples: C6H5N2+Cl- + CuCl -> C6H5Cl + N2; + CuBr -> C6H5Br; + CuCN -> C6H5CN. Gattermann reaction: Similar but uses Cu powder (not Cu salt) as catalyst: ArN2+ + Cu/HCl -> ArCl + N2. Slightly different mechanism but same products as Sandmeyer. Key distinction: Sandmeyer = Cu(I) salt. Gattermann = Cu powder. For iodine: ArN2+ + KI -> ArI directly (no Cu catalyst needed). For fluorine: Balz-Schiemann (ArN2+BF4- -> ArF + BF3 + N2). These reactions make diazonium salts incredibly versatile synthetic intermediates.