Chapter 6, Problem 63P

Interpretation Introduction

Interpretation:The reason behind slower reaction rates of halocyclohexanesubstrate 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.

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 $120°$ . In case of cyclopropane and cyclobutanes, the bond angle is $60°$ and $90°$ respectively. Hence, such large deviation from ideal bond angle results in considerable amount of angle strain this is highly unfavorable, and thus,slower reaction rates are observed with of halocyclopropane and halocyclobutane substrates unlike acyclic secondary alkyl halides that cause no angle strain.