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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Notes: This reaction follows Markovnkov’s rule. Only a catalytic amount of mercury is required in this reaction. Acid is also important. Often mercuric sulfate (HgSO4) is used in conjunction with sulfuric acid (H2SO4), although this exact combination is not crucial.

Examples:

Notes:  Note for that the third example a mixture of ketones is obtained because no clear preference can be established using Markovnikoff’s rule (i.e. both carbons are equally substituted)

Mechanism: For the purposes of Org 1/Org 2 this mechanism is generally not considered “important” due to the weird “de-mercuration” step. Mercury adds to the alkyne (Step 1, arrow A) giving the mercuronium ion. This is then attacked by water at the more substituted carbon (“Markovnikoff selectivity”) (Step 2, arrows B and C) to give the mercurated “enol” after deprotonation (Step 3, arrows D and E). Removal of the mercury happens in two steps: first, protonation of the enol at carbon (Step 4, arrows F, G, H) gives the protonated carbonyl; attack at mercury by solvent (or other nucleophile) regenerates the enol (Step 5, arrows I, J K). Then, the enol is converted to the more stable ketone through tautomerization (Step 6, arrows L and M).

Notes: 

  • The ketone is formed because it is more stable than the enol. This process is called keto-enol tautomerism.
  • Note that there’s actually quite a few ways this mechanism could be written and this is just one suggestion. The key steps to really be concerned with are steps 1, 2, and 6.

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{ 8 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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