Chapter 6, Problem 62P

Interpretation Introduction

Interpretation:The reason behind slower reaction rates of halocyclopropane and halocyclobutane substrates relative to analogous acyclic secondary alkyl halides 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 that occurs in a concerted manner 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.