Treatment of a primary alkyl halide or pseudohalide with an alkali metal halide (e.g. KF, KI) leads to replacement of the halogen via an SN2 Reaction.
Mechanism of the Finkelstein Reaction
The equilibrium position of the reaction depends on the nucleophilicity of the anion, whether a good leaving group is present, and whether one anion is better stabilized than the other in a given solvent (see Nucleophilic Substitution). For example, reactions with KF will thus lead cleanly to fluoroalkanes, because fluoride is such a poor leaving group due to the stability of the C-F bond.
In general, the reaction is run with an excess of the metal halide. The use of metal salts that have a high lattice energy require the addition of a crown ether (compare the reaction times for KF and CsF in the example above).
The equilibrium position of the reaction also depends on the solubility of the metal salt in the solvent used. Thus, the substitution of bromo- and chloroalkanes with KI in acetone leads cleanly to the desired iodoalkane products, since KCl and KBr are insoluble in acetone and are consequently removed from the equilibrium: