Chapter 15: Alcohols, Diols and Thiols

Oxymercuration-Demercuration of Alkenes
(review of Chapter 14)

Reaction type:  Electrophilic Addition

Summary

• Overall transformation :  C=C to H-C-C-OH
• This is an alternative method for hydrating alkenes to give alcohols
• Typical reagents are mercury acetate, Hg(OAc)₂ in aqueous THF
• Unfortunatley, mercury compounds are generally quite toxic
• Regioselectivity predicted by Markovnikov's rule (most highly substituted alcohol)
• The reaction is not stereoselective
• Reaction proceeds via the formation of a cyclic mercurinium ion (compare with bromination of alkenes)
• The mercurinium is opened by the attack of water to complete the oxy-mercuration
• Demercuration is effected by a reduction using sodium borohydride, NaBH₄

• Hydration of Alkenes
• Hydroboration-Oxidation

MECHANISM FOR REACTION OF ALKENES WITH Hg(OAc)2 / H2O
Step 1: The C=C p electrons act as the nucleophile with the electrophilic Hg and loss of an acetate ion as a leaving group, forming the mercurinium ion.
Step 2: Water functions as a nucleophile and attacks one of the mercury substituted carbons resulting in cleavage of the C-Hg bond.
Step 3: The acetate ion functions as a base deprotonating the oxonium ion to give the alcohol. This completes the oxymercuration part of the reaction.
Step 4:(mechanism not shown) The hydride reduces the Hg off, creating a C-H bond while breaking the C-Hg bond. This is the demercuration part of the process.

© Dr. Ian Hunt, Department of Chemistry