Oxymercuration: Alcohols from alkenes using Hg(OAc)2 and Water
Description: Alkenes treated with mercuric acetate [ Hg(OAc)2 ] and water will be hydrated to form alcohols, after addition of NaBH4.
Notes: This reaction follows Markovnikov’s rule – the oxygen adds to the more substituted carbon. The purpose of the second step (NaBH4) is to remove the mercury.
Notes: See that the oxygen is attached to the more substituted carbon of the alkene. Also, there is no clear preference for syn or anti addition. In the third example notice that here Markovnikoff’s rule gives us no clear preference for which carbon to add the oxygen to, so we obtain a mixture.
Mechanism: The alkene adds to Hg(OAc)2, displacing acetate and forming a mercuronium ion (Step 1, arrow A and B). Next, the more substituted carbon is attacked by water (Step 2, arrows C and D). The resulting protonated alcohol is then deprotonated by acetate (Step 3, arrows E and F). Removal of mercury is done with NaBH4. The mechanism of this step is generally not considered to be important for Org 1 / Org 2. However it is shown separately below.
Note that attack of alcohol (Step 2) occurs trans to the mercury, although that is not depicted here. Removal of mercury leads to the formation of a free radical (Step 6) so the reaction is not stereoselective.
Here’s the mechanism of the reduction.