Oxymercuration of Alkynes

by James

Description: Alkynes treated with mercury (usually HgSO4) and water will be hydrated to give ketones, via an enol intermediate.
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{ 14 comments… read them below or add one }

Andrew Ross

If I remember organic I correctly HgSO4 is only required for terminal alkynes so that the reaction is carried out under more mild acidic conditions, and that for internal alkynes it isn’t necessary but often is carried out in this manner as well (the HgSO4 component, that is)?

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james

With HgSO4 the reaction is milder than with straight acid (e.g. H3O+) although in truth more advanced methods these days avoid mercury and use late metal cations such as Au and Ag

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Seah

Given that the cyclic mercuronium ion formed in step 2 has lone pairs of electrons on it and that resonance is possible, shouldnt the ion be anti aromatic and unstable? Is it possible that the intermediate in step 2 is a vinylic carbocation?

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James Ashenhurst

The C-Hg bonds are very long and orbital overlap between any empty p orbital on Hg with the carbon p orbitals is likely very poor. Antiaromaticity isn’t really a concern here.

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Carl Silverstein

Will HgSO4 have an effect on a double carbon bond?

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James Ashenhurst

Yes – it will work just as well as Hg(OAc)2 . For alkynes we generally use HgSO4 because the reaction is done in concert with H2SO4, which helps to remove the mercury from the substrate.

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priya

Can we use Hg(NO3)2 in oxymercuration?

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James

Not sure I’ve seen that reagent in an oxymercuration, but I don’t see why not. Should work well.

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Nan Hsien Kuo

if we use Br2 and H2O to react with terminal Alkyne,it will yield ketone. How it works?

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James

Forms an enol, and then tautomerization to give the ketone.

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Neeraj Shrivastav

we can use Hg(NO3)2

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James

Yes, that would also work.

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Neeraj Shrivastav

we can use HgSO4 with dil.H2SO4

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James

Yes, we can.

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