Illustrated Glossary of Organic Chemistry

Finkelstein reaction: An SN2 reaction in which one halogen atom (the leaving group) is replaced by another halogen atom (the nucleophile).

In this example of the Finkelstein reaction, 1-chloro-2-phenylethane (a primary alkyl halide) is treated with sodium iodide (the nucleophile) to produce 1-iodo-2-phenylethane. Acetone is used as the solvent because NaI is soluble in acetone whereas NaCl is not, thus (via Le Chatelier's principle) shifting the equilibrium towards the right as NaCl precipitates out of the reaction mixture.