Organic Chemistry, "Change": Part 11, Alkene Addition Reaction via a Concerted Mechanism; Oxymercuration-Deoxymercuration

 Greetings,

This post covers one example of alkene hydration reactions that are driven by the use of particular reagents which can bond to both carbon atoms, across a double bond, simultaneously. This results in a concerted type of reaction mechanism, which has the following advantages:

• Reaction is carried out under milder conditions
• Control over the stereochemistry of the product
• Alkene hydrations are not susceptible to rearrangement or polymerization

The particular reagent discussed in this post is aqueous Mercury(II) Acetate, which is in equilibrium with its partially-dissociated form, Mercury(II) Acetate cation. Here is the equilibrium equation:

Hg(OAc)2 (aq) <==> [HgOAc]+  + OAc-

The Mercury(II) Acetate cation is the electrophile which adds across the double bond forming the concerted reaction intermediate.Water, acting as a nucleophile, adds to the opposite side, attacking the more substituted carbon. A proton then leaves the water molecule. Finally, Sodium Borohydride provides a hydride ion, which replaces Mercury Acetate from the less substituted carbon atom. The chart, below, shows the mechanism.

I have provided number labels in the lower chart portion so that you can more easily understand the processes taking place. In reality, the numbered sequence of events occurs simultaneously.

That's all for this post. As always, thank you for reading!