$\mathrm{R}-\mathrm{Cl}+\mathrm{NaI}\stackrel{\text{acetone}}{\to }\mathrm{R}-\mathrm{I}+\mathrm{NaCl}$ This reaction is

$\mathrm{R}-\mathrm{Cl}+\mathrm{NaI}\stackrel{\text{acetone}}{\to }\mathrm{R}-\mathrm{I}+\mathrm{NaCl}$

This reaction is known as Finkelstein reaction 

NaCl and NaBr are insoluble in acetone and reaction is favoured in forward direction