Chapter 9, Problem 9.21P

Interpretation Introduction

Interpretation: The stereochemistry of the alkyl bromide formed from the reaction of $\left(\text{R}\right)\text{-butan-2-ol}$ with PBr₃ is to be stated.

Concept introduction: Alkyl bromides are obtained by the reaction of ${\text{1}}^{\text{ο}}\text{or}{\text{2}}^{\text{ο}}$ alcohols with phosphorous tribromide $\left({\text{PBr}}_{\text{3}}\right)$. The reaction yields ${\text{HOPBr}}_{\text{2}}$ as by-product. The mechanism of the reaction involves two steps. The first step is conversion of the $\text{OH}$ group into a good leaving group. The second step is nucleophilic attack by ${\text{Br}}^{-}$ through an ${\text{S}}_{\text{N}}2$ reaction. This ${\text{S}}_{\text{N}}2$ reaction results inversion of stereochemistry at stereogenic center.