Chapter 12, Problem 12.39P

Interpretation Introduction

Interpretation:

The mechanism for the given reaction that accounts for the observed stereochemistry is to be explained.

Concept introduction:

The electrophilic addition of $RO-Br$ molecule across the $\text{C=C}$ bond can take place similar to the addition of $HO-Br$. In the first step, the electron-rich $\text{C=C}$ bond attacks the bromine atom and the lone pair of the same bromine atom attacks the other carbon of $\text{C=C}$ bond forming cyclic bromonium ion intermediate and simultaneously breaks the $\text{O}-\text{Br}$ bond. The bromonium ion is a three-membered ring with one positively charged bromine atom. The alkoxide ion produced in the first step acts as a nucleophile, and the positively charged bromine of bromonium ion intermediate is the leaving group. Thus, the reactions proceed through ${\text{S}}_{\text{N}}\text{2}$ where the nucleophilic alkoxide ion attacks one of the carbon of the three-membered intermediate of bromonium ion from the opposite side of positively charged bromine atom resulting in the breaking of one $\text{C}-\text{Br}$ bond. As the nucleophile approaches from the opposite side of the positively charged bromine atom, the stereochemistry of groups $Br$ and $OR$ is trans.