Chapter 9, Problem 9.26P

Interpretation Introduction

Interpretation:

It is to be determined in which of the two solvents, ethanol or acetone, will the given reaction proceed faster via an ${\text{S}}_{\text{N}}\text{2}$ mechanism.

Concept introduction:

An ${\text{S}}_{\text{N}}\text{2}$ mechanism is a single-step mechanism, in which both the substrate and the nucleophile are involved. Therefore, any effect that hampers the approach of the nucleophile at the site of reaction will tend to slow down the reaction. Since the nucleophile must start forming a bond with the reaction center before the leaving group breaks off, a strong, often negatively charged nucleophile is required for an ${\text{S}}_{\text{N}}\text{2}$ reaction. A protic solvent will strongly solvate a negatively charged nucleophile, preventing its close approach to the reaction center. Therefore, a protic solvent considerably reduces the rate of an ${\text{S}}_{\text{N}}\text{2}$ reaction. An aprotic solvent interacts much less strongly with the negative charge of the nucleophile. Therefore, an aprotic solvent is preferred for an ${\text{S}}_{\text{N}}\text{2}$ reaction.