How to determine order of nucleophilicity among commonly used nucleophiles? For example, in Finklestein reaction, $$\ce{NaI}$$ (reagent) - taken in acetone (solvent) - displaces $$\ce{SH-}$$ ion from alkyl group via SN2 mechanism to form an alkyl iodide. But in a separate question we may be asked to write the product formed when $$\ce{NaSH}$$ reacts with an 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.