Chapter 6, Problem 51P

Interpretation Introduction

Interpretation:Bromoalkanes that most closely resemble ${\text{S}}_{\text{N}}2$ reactivity analogous to bromocyclopentane should be chosen and reason behind the choice should be explained.

Concept introduction: Bimolecular substitution or ${\text{S}}_{\text{N}}2$ 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.

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.

The three-dimensional orbital description of ${\text{S}}_{\text{N}}2$ is illustrated as follows:

  

The $s{p}^2$ hybridization of transition state is facilitated if the bond angles are not deviated from nearly $120°$ . Slower reaction rate of halocyclopropane and halocyclobutane substrates relative to analogous acyclic secondary alkyl halides is primarily because of ring strain.