In a lab, a common experiment is to perform SN2 reactions involving a primary alkyl bromide and sodium iodide in acetone (polar aprotic solvent).
The result is that the iodide ion is a good nucleophile, so the iodide will displace the bromide from the primary alkyl bromide, and sodium bromide will precipitate from the solution.
However, without the presence of the sodium cations, would the reaction still take place?
I'm somewhat confused, because iodide is both a better leaving group and a better nucleophile than bromide, so what is the determining factor in the reaction?
Is the strength of the nucleophile more important or the ability of the leaving group be displaced?