Chapter 25, Problem 25.56P

Interpretation Introduction

Interpretation:

A detailed mechanism is to be proposed for conversion of $\text{cis-1, 2-diphenylethene}$ to $\text{trans-1, 2-diphenylethene}$ on addition of ${\text{I}}_{\text{2}}$ and irradiation by UV light.

Concept introduction:

Iodine dissociates homolytically when irradiated with UV light, generating a pair of radicals. The iodine radical adds to the double bond of an alkene. The second carbon of the initial double bond is converted to a radical in the process and the double bond is converted temporarily to a single bond. Since the rotation of groups about a single bond is possible, this allows for a rearrangement if the alkene is unsymmetrically substituted, particularly from a sterically hindered cis conformation to a less hindered trans conformation.

Subsequent elimination of iodine, made possible because the $\text{C}-\text{I}$ is weak, converts it back to the alkene, with possible isomerization.