Gattermann Reaction

Replacing a diazonium group with a halogen can be done by two closely related routes. The Sandmeyer reaction uses a cuprous halide such as CuBr or CuCl, whereas the Gattermann reaction achieves the same overall substitution using copper powder together with the corresponding halogen acid. When benzenediazonium chloride is treated with copper powder and hydrobromic acid, the copper metal mediates replacement of the –N2+ group by bromine, evolving nitrogen gas and giving bromobenzene; this is the Gattermann reaction. The Sandmeyer option is incorrect because that specifically requires a preformed cuprous halide salt, not copper powder with HBr, and it names chlorobenzene rather than the bromo product. The Balz-Schiemann reaction installs fluorine via a diazonium tetrafluoroborate decomposed thermally, which is unrelated to copper and HBr. The coupling option is wrong because coupling needs an electron-rich aromatic partner, not copper, and yields no halide. This distinction between Gattermann and Sandmeyer conditions is a precise JEE Advanced point, with the Gattermann variant typically giving slightly lower yields but avoiding the need to prepare a pure cuprous halide beforehand. As a plausibility check, the evolution of nitrogen and a single bromine substituting the diazonium position confirm a clean halodediazoniation to bromobenzene rather than any coupling or hydrolysis side reaction.