Chapter 6, Problem 66P

Interpretation Introduction

Interpretation: The observed value of rate for chloromethane and hydroxide reac ${\text{S}}_{\text{N}}2$ tion should be chosen.

Concept introduction: Bimolecular substitution or proceeds via single-step mechanism. Thus it is well known as concerted mechanism. Nucleophile approaches carbon while the leaving group still departs from the rear side (opposite to leaving group). The transition state only illustrates the geometric orientation of the substrates and reagents as they pass through the maxima in the single-step mechanism.

Bimolecular substitution reaction depends concentration of both the alkyl halide and nucleophile. Hence the name ${\text{S}}_{\text{N}}2$ is abbreviated to indicate second order dependence of this substitution.

A general ${\text{S}}_{\text{N}}2$ reaction mechanistic pathway is illustrated below:

  

${\text{S}}_{\text{N}}2$ pathway as it is a stereospecific reaction. This essentially means the R stereoisomer can only lead to inverted S stereoisomer and vice versa. Thus the outcome is the rear side displacement of leaving group.

Polar-aprotic solvents accelerate the rate of ${\text{S}}_{\text{N}}2$ reactions. These solvents possess partial positive and negative charges also. However, they do not liberate any proton to the substrate unlike water, methanol solvents. Acetone, DMSO, THF or DMF are categorized as polar aprotic solvent.