Finkelstein reaction for the preparation of alkyl iodide is based upon the fact that
Sodium iodide is soluble in methanol, while NaCl and NaBr are insoluble in methanol
R−X+NaISoluble in(CH3OH,Me2CO)acetone−−−−→R−I+NaX↓Insoluble in(CH3OH.Me2CO)
(where X=Cl or Br)