Does 2-chloro-2-methylpentane react with NaI faster in acetone or water?
It  is simple this is the famous finkelstein reaction.


As you can see in the image NaCl is formed which is insoluble in acetone hence it precipitates and the reaction cannot go reverse.


But if we take water the reaction will shift in backward direction as NaCl is soluble in water.

 That is the reason we take acetone.