A detailed mechanism is to be proposed for conversion of cis-1, 2-diphenylethene to trans-1, 2-diphenylethene on addition of I 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 C − I is weak, converts it back to the alkene, with possible isomerization.

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Chapter 25, Problem 25.56P

Interpretation Introduction

Interpretation:

A detailed mechanism is to be proposed for conversion of cis-1, 2-diphenylethene to trans-1, 2-diphenylethene on addition of I2 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 C−I is weak, converts it back to the alkene, with possible isomerization.

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