Chapter 12, Problem 12.59P

Interpretation Introduction

Interpretation:

The mechanism for the given oxymercuration reduction reaction in a sequence is to be drawn.

Concept introduction:

The oxymercuration-reduction reaction is used to add a water molecule across the double bond of an alkene. The reaction is a Markovnikov addition of an H and an OH to the carbons of the initial double bond. No rearrangement is observed as the mechanism involves a cyclic mercurinium ion rather than a carbocation.

The mercury atom in $\text{mercury}\left(\text{II}\right)\text{ acetate}$ is electron deficient, so it acts as an electrophile. At the same time, the lone pair on it gives is a nucleophilic character. So when adding to across a double bond, it simultaneously forms two bonds with both carbons of the initial double bond. One bond is formed by the $\text{π}$ electron pair and the other by the mercury lone pair. This leads to the formation of a three-membered ring with a positive charge. The positive charge is more concentrated on the more substituted carbon because that would have formed a more stable carbocation. Therefore, in the next step, the oxygen from a water molecule uses a lone pair to form a bond with this carbon, opening up the mercurinium ring. Deprotonation of this group leads to the addition of an OH group to the more substituted carbon. Sodium borohydride then reduces the other carbon, replacing the mercury group with a hydrogen atom and completing the addition reaction.