Chapter 12, Problem 12.5YT

Interpretation Introduction

Interpretation:

The mechanism for the reaction in Equation 12-17 is as follows, but the curved arrows have been omitted. The mechanism by drawing in the curved arrows is to be completed and the name of each elementary step is to be written. Also, label the chloronium ion intermediate.

Concept introduction:

Cl₂ does not appear to possess an electron–poor atom. Br₂ behave as an electrophile once before in the S_N2 chlorination of enols and enolate anions. As the electron-rich π bond approaches Cl₂, the Cl₂ molecule becomes polarized, temporarily making one of the Cl atoms electron-poor. The mechanism proceeds through a chromium ion intermediate. The curved arrow from the π bond to Cl represents the flow of electrons from an electron-rich site to an electron-poor site. A lone pair of electrons on Cl forms a bond back to one of the alkene C atoms to avoid breaking the C atom’s octet, resulting in a three-membered ring. The weak Cl-Cl bond breaks and one of the atoms leaves as Cl^-.