Chapter 10.6, Problem 16E

Interpretation Introduction

Interpretation:The formation of both $\text{meso-stilbene}$ dibromide and $\text{dl-stilbene}$ dibromide should be demonstrated with the help of carbocation intermediate formed upon addition of bromine to $\left(\text{E}\right)\text{-stilbene}$ .

Concept introduction:Alkenes are considered electron-rich and undergo bromination addition reaction with ${\text{Br}}_2$ in presence of ${\text{CCl}}_{\text{4}}$ by electrophilic addition reaction.

The mechanism of ${\text{Br}}_2$ addition occurs in two steps. In the first step, nucleophilic olefin abstracts the electrophilic bromine atom from ${\text{Br}}_2$ and this result in attack of ${\text{Br}}^{+}$ on the electron-rich olefinic bond that forms a sigma bond to each end of the olefinic bond. Such interaction leads to a three-membered bromonium ion intermediate.

In the second step, the strained bromonium ion intermediate opens while ${\text{Br}}^{-}$ attack from behind the electron-deficient carbon analogous to ${\text{S}}_{\text{N}}\text{2}$ attack.