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For eg. in Finklestein reaction, $$\ce{NaI}$$ (reagent) taken in acetone(solvent) displaces $$\ce{SH-}$$ ion from alkyl group via SN2 mechanism to form alkyl iodide. But in a separate question we may be asked to write the product formed when $$\ce{NaSH}$$ reacts with alkyl iodide in the presence of acetone. Since I do not know which of the two is a stronger nucleophile, I will displace $$\ce{I-}$$ and form $$\ce{alkyl-SH}$$. But definitely only one of the reactions shall occur. So my question is that is there an order of strength of nucleophilicity (like a metal reactivity series)? I am preparing for JEE Advanced 2015 and am expecting answers which within this scope of knowledge.