Finkelstein reaction is used for the synthesis of

Alkyl iodides are often prepared by the reaction of alkyl chlorides/bromides with $\mathrm{NaI}$ in dry acetone. This reaction is known as Finkelstein reaction.

$\mathrm{R}-\mathrm{X} + \mathrm{NaI} \stackrel{\mathrm{Acetone}}{\to }\mathrm{R}-\mathrm{I} + \mathrm{NaX} \left(\mathrm{X}=\mathrm{Cl}, \mathrm{Br}\right)$

$\mathrm{NaCl} \mathrm{or} \mathrm{NaBr}$ thus formed is precipitated in dry acetone. It facilitates the forward reaction according to Le Chatelier’s Principle.