Chapter 12, Problem 12.40P

Interpretation Introduction

Interpretation:

The mechanism for the given reaction forming cyclic ether on addition of bromine to the double bond in $\text{pent-4-en-1-ol}$ is to be explained.

Concept introduction:

The substrate having hydroxyl group, along with $\text{C=C}$ bond on the electrophilic addition of bromine in ${\text{CCl}}_{\text{4}}$, can form cyclic ether because of intramolecular $\text{S}\text{2}$ reaction where the hydroxyl group can attack the bromonium ion intermediate as a nucleophile instead of a bromide ion. The reaction depends on the position of the hydroxyl group with respect to the $\text{C=C}$ bond. The first step is the same as the formation of the bromonium ion intermediate. In the first step, the electron-rich $\text{C=C}$ bond attacks one of the bromine atoms, and the lone pair of the same bromine atom attacks the other carbon of the $\text{C=C}$ bond forming a cyclic bromonium ion intermediate, and simultaneously breaking the $\text{Br}-\text{Br}$ bond. The bromonium ion is a three-membered ring with one positively charged bromine atom.