Consider an organic substitution reaction that proceeds via a concerted mechanism. The transition state formed during the reaction is less polar than the initial localised nucleophilic anion. Furthermore, the reaction rate is maximised in a solvent that selectively solvates cations but leaves the anions unsolvated. Which of the following reactions best matches this mechanistic description?

Step-by-step solution

The description states the reaction follows a concerted mechanism, which implies an S _ N 2 pathway. In an S _ N 2 reaction between a neutral substrate and an anionic nucleophile, the negative charge is dispersed over the incoming nucleophile and the leaving group in the transition state. This dispersal of charge makes the transition state less polar than the initial localised nucleophilic anion. The rate of an S _ N 2 reaction is maximised in a polar aprotic solvent (like acetone), which solvates cations (like Na