Oxymercuration: Alcohols from alkenes using Hg(OAc)2 and Water

by James

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.

Examples:

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.

Video Walkthrough

 

{ 5 comments… read them below or add one }

Dave Blackburn

Nice. Thanks for the radical mech in reduction step, though should it be Hg(l) instead of Hg(s)?
I point out to my students that the environmental impact of Hg(OAc)2 must be balanced against improved yield and avoidance of C+ rearrangements.

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james

Ha!. Yes, I guess it should be Hg(liquid). The times I’ve done it, it formed this beautiful little grey dispersion, it must have stuck in my mind as a solid & I forgot it’s actually a liquid.

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md abu zafar al munsur sarker

it is very helpful for student.thanks to you

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Md2be

Kudos dude. Needed this earlier in the course but sleep is overrated anyway.

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Jeffery Seah

Sorry, but can I know why sometimes THF is used in this reaction? Is it purely a polar solvent which can dissolve the organic compound and Mercuric Acetate? Or it has got to do with the stabilising ability of the oxygen atom on the Hg atom? (Something similiar to why Grignard reactions are performed in ether and THF)

Thanks

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