(R)-2-Bromobutane is allowed to react with NaI in acetone. The product formed is :
I⊖ is a capable nucleophile. The given substrate - a secondary alkyl bromide - has a good enough leaving group. The solvent is polar aprotic and overall everything is conducive for an SN2. Now let us explore the stereochemical implications; in SN2 reactions, at the carbon where substitution happens, inversion of configuration occurs because of the concerted nature of the mechanism. Therefore (R)-2-Bromobutane on treatment with NaI in acetone gives (S)-2-iodobutane.