R−CH2−Br+I−⟶ Acetone R−CH2−I+Br−
As mentioned the reaction follows SN2 reaction mechanism. The Finkelstein reaction classically involves the conversion of Alkyl bromides into Alkyl iodides by the treatment with a solution of Sodium iodide in Acetone. NaBr is not readily soluble in acetone and hence reaction shifts in forward reaction. For SN2 reactions, transition state formed is less polar than the localised anion.