The Finkelstein reaction, named after the German chemist Hans Finkelstein, is an S_N2 reaction (Substitution Nucleophilic Bimolecular reaction) that involves the exchange of one halogen atom for another. Halide exchange is an equilibrium reaction, but the reaction can be driven to completion by exploiting the differential solubility of halide salts, or by using a large excess of the halide salt.