Sandmeyer reaction
/ SAND-my-er /
The Sandmeyer reaction answers a stubborn problem: how do you put a chlorine, bromine, or cyano group onto a benzene ring at a specific spot? You cannot reliably nitrate or directly substitute your way there, and ordinary aromatic halogenation goes where the existing groups direct it, not where you want. The Sandmeyer reaction lets you place the group exactly where an amino group used to be.
It works on an aryl diazonium salt (made from an aniline). You treat Ar-N2+ with a copper(I) salt — copper(I) chloride to install Cl, copper(I) bromide for Br, or copper(I) cyanide for the nitrile — and the copper catalyzes replacement of the -N2+ group, which leaves as nitrogen gas, by the new group at the same carbon. The mechanism is not a simple ionic substitution; it runs through aryl radicals generated by single-electron transfer from copper, which is why a plain halide salt alone does not do the job and the copper is essential. (Iodide and fluoride are installed by related but slightly different procedures — KI works without copper, and fluoride uses the Balz-Schiemann route via the diazonium tetrafluoroborate.)
Strategically, the Sandmeyer reaction is gold because it gives you a precise way to convert C-NH2 into C-Cl, C-Br, or C-CN on an aromatic ring. Since the amino group is itself a strong ortho/para director that can be installed by nitration-then-reduction, you can use it to steer where other groups go, then convert it through the diazonium salt into a halide or nitrile. That makes -NH2 a removable, redirectable handle — one of the most powerful tactics in planning substituted-benzene syntheses.
Benzenediazonium chloride + CuCN gives benzonitrile (C6H5-CN), installing a nitrile exactly where the original aniline's nitrogen sat — a transformation hard to achieve any other way on an aromatic ring.
Turning C-NH2 into C-CN at a chosen ring position — the Sandmeyer payoff.
The copper(I) salt is essential and the reaction goes through aryl radicals, not a simple ionic substitution — a plain halide salt alone will not replace the diazonium group.