JEE Main · 2023 · 06 Apr 2023 · Shift 1 · Haloalkanes and Haloarenes

This is a Finkelstein exchange, a bimolecular ${\mathrm{S}}_{\mathrm{N}}2$ substitution carried out in acetone. In the transition state the negative charge is spread over both the incoming iodide and the departing bromide, so the charge is dispersed and the transition state is less polar than a fully localised halide anion. Acetone, an aprotic solvent, solvates anions poorly and so favours the nucleophile, while a protic medium such as acetic acid would solvate the anion and hinder the reaction; moreover sodium bromide precipitates from acetone, removing bromide so it cannot compete. The correct statement is that the transition state is less polar than the localised anion.