Chapter 12, Problem 12.41P

Interpretation Introduction

Interpretation:

The mechanism for the given reaction of electrophilic addition of mixed halogen $\text{I}-\text{Cl}$ across the double bond to predict the product with appropriate regiochemistry and stereochemistry is to be drawn.

Concept introduction:

In the halogen molecule, as both atoms are same, any halogen atom acts as an electrophile and gets attacked by the $\text{C=C}$ bond. In case of mixed halogen molecule, the less electronegative atom acts as an electrophile in the first step to form a halonium ion and more electronegative atom acts as a nucleophile in the second step.

The electrophilic addition of halogen molecule across the $\text{C=C}$ bond forms a vicinal dihalide product. In the first step, the electron-rich $\text{C=C}$ bond attacks one of the halogen atom and the lone pair of the same halogen atom attacks the other carbon of the $\text{C=C}$ bond forming a halonium ion intermediate and simultaneously breaking the bond between two halogen atoms. The halonium ion is the three-membered ring with one positively charged halogen atom. The halide ion produced in the first step acts as a nucleophile, and the positively charged halogen of the intermediate is the leaving group. Thus, reactions proceed through ${\text{S}}_{\text{N}}\text{2}$ where the nucleophilic halide ion attacks the most substituted carbon of the three-membered intermediate from the opposite side of the positively charged halogen atom which results in breaking of one $\text{C}-\text{X}$ bond to forma vicinal dihalide product. As the halide ion approaches from the opposite side of positively charged halogen atom, the stereochemistry of both halogen atoms with respect to each other in the product is trans.