There are two proposed mechanisms for Oxymercuration and demercuration.

The first mechanism is concerted (i.e. single transition state connecting reactants and products) characterized by an attack of the $\pi$-bond on the $\ce{Hg(OAc)2}$.

Why does mercuric acetate act as an electrophile in the first mechanism? It is in its most common 2+ oxidation state. Does this happen because mercury has empty $6d$ orbitals.

The second mechanism is a stepwise mechanism (i.e. contains at least one intermediate species along the reaction pathway between reactants and products) that is characterized by incorporating the first ionization of $\ce{Hg(OAc)2}$ followed by an attack of the nucleophilic $\pi$-bond.

$\ce{Hg(OAc)2}$ dissociates in water as follows: $$\ce{Hg(OAc)2 + H2O<=>Hg+OAc + OAc-}$$

Which one is the actual mechanism?


The mechanism you are looking for is probably not known and might well be never known. According to Brown et. al. there is a rapid and reversible equilibrium between associated and dissociated reactants.[1] They tested this for cyclohexene, norbonene and apobornylene, but I believe it is fair to generalise in this matter. $$\ce{R2C=CR2 + Hg(OOCCF3)2 <=> (F3CCOO)R2C=CR2Hg(OOCCF3)}$$

Due to this, even the formation of a mercurinium ion seems to be sometimes not observed. For more information please have a look at the mechanistic studies of Pasto and Gontarz,[2] where they focussed on the stereo selectivity of substituted cyclohexenes, and references within.

Whatever molecule you might assume acting as the electrophile, it is quite obvious, that the bond mercury oxygen bond is highly ionic, with most of the electron density located in the acetyl moieties, leaving a rather substantial positive charge at the mercury itself. The following structures were optimised at DF-BP86/def2-SVP and the charges displayed are natural charges from the Natural Bond Orbital analysis to illustrate the above point.

Hg+OAc charges
Hg(OAc)2 charges structure 1
Hg(OAc)2 charges structure 2

The lower structure of $\ce{Hg(OAc)2}$ is slightly more stable.

  1. Herbert Charles Brown, Min-Hon Rei, Kwang Ting Liu, J. Am. Chem. Soc., 1970, 92 (6), 1760–1761.
  2. Daniel J. Pasto, John A. Gontarz, J. Am. Chem. Soc., 1971, 93 (25), 6902–6908.

I don't know the answer (and I thought for a while that it was not knowable), but I can at least tell you how you would find out whether $\ce{Hg(OAc)2}$, $\ce{Hg(OAc)+},$ or (more likely) some combination is the active electrophile.

If $\ce{Hg(OAc)2}$ dissociates, then both possible electrophiles are in solution. However, if we can effectively remove one electrophile, and see if the reaction works, then we know something.

Easy: shift equilibrium to favor $\ce{Hg(OAc)2}$

We can shift the equilibrium to heavily favor $\ce{Hg(OAc)2}$ by adding a large excess of $\ce{OAc-}$ ions, for example as $\ce{NaOAc}$.

There are two possible outcomes:

  1. The reaction does not work (or goes very slowly). If this is the case, then $\ce{Hg(OAc)+}$ is the active electrophile.
  2. The reaction does work. We know that $\ce{Hg(OAc)2}$ can be the electrophile, but this does not rule out $\ce{Hg(OAc)+}$ as an electrophile.

Hard: Shift equilibrium to favor $\ce{Hg(OAc)+}$

I am not coming up wit ha good way to do this, but if we could, then we could test whether $\ce{Hg(OAc)+}$ was necessary.

  • $\begingroup$ It's very likely that nobody knows the answer to this, because nobody was interested enough to find out. +1 for the proposed experiments. $\endgroup$ – jerepierre Jun 1 '15 at 23:19
  • $\begingroup$ Perhaps one of the acetate ions can be protonated at a low pH, but not low enough that both acetate ions are removed? $\endgroup$ – Nicolau Saker Neto Jun 2 '15 at 1:32
  • $\begingroup$ In the solution both $\ce{HgOAc2} $ and $\ce{HgOAc+} $ exists.But why does $\ce{HgOAc2} $ acts as an electrophile here. Is it just because it has empty 6d orbitals. $\endgroup$ – yasir Jun 2 '15 at 4:48
  • $\begingroup$ When $\ce{Hg(OAc)2}$ acts as an electrophile, one acetate ion gets displace. I would not say that this displacement occurs by an associative or intermediate ligand substitution mechanism (because it is very challenging to tell the difference). If you had to identify some "orbitals" to be involved, why would you not consider the probably more accessible $6p$ orbitals? $\endgroup$ – Ben Norris Jun 2 '15 at 10:41
  • $\begingroup$ All i am asking is why would even HgOAc2 behave as an electrophile here.It is just in its stable state, and higher oxidation states of mercury are not known except with compounds of flouride leading to +4 oxidation state. $\endgroup$ – yasir Jun 2 '15 at 12:12

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