Chapter 7.3, Problem 7.3E

Interpretation Introduction

Interpretation: The reason behind loss of optical activity of $\text{3-Bromo-3-methylhexane}$ when dissolved in nitromethane should be explained.

Concept introduction: Haloalkane solvolysis is a typical example of unimolecuar substitution. It proceeds via two-step mechanism. The first slow step that determines rate is the removal of leaving group from the substrate haloalkane and generates a carbocation. Since the rate is only governed by substrate alone and no other nucleophile or solvent it is termed as unimolecuar substitution. The final step is attack of nucleophile on carbocation generated and formation of racemic products. The most important outcome of unimolecuar substitution is conversion of an optically active substrate to racemic mixture of products.

Tertiary or secondary halides undergo fastest unimolecuar substitution as they can readily form the tertiary carbocation followed by secondary and least reactive are primary.

The haloalkanes react with various solvents such as water, ethanol to give substituted products that have halide replaced by nucleophilic part of solvent. Such reactions are termed solvolysis. The experimental order of reactive for three types of halides towards solvolysis is as follows:

  $\text{Tertiary}>>\text{ secondary}>\text{primary}$