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Does the bonding pair in $\ce{FeCl4^-}$ attack a proton or does a lone pair on the chlorine attack the proton?

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My prof has an argument for the first one; the first one involves fewer steps, so it is probably more favorable. My proposed mechanism has three steps of electron pushing versus two, so okay.

However, I can see an argument for the second mechanism - the first one involves a bonding pair acting as a nucleophile rather than a lone pair on chlorine. The bonding pair is stabilized by the chlorine and the iron atom, while the lone pair is stabilized by only the chlorine.

So, which one wins? I'm guessing both mechanisms are operative here.

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I don't know the answer to your question, but we can speculate. A lone pair is more sterically exposed than a bonding pair of electrons and therefor more accessible. Also a lone pair is generally more nucleophilic because the electrons are concentrated in a smaller region since they are not being shared. These two arguments would favor the second mechanism. Finally, when we consider the SN2 reaction where, for example, water is the attacking nucleophile; I've never seen the mechanism drawn where the attacking electrons are from the O-H bond, but rather they always seem to show involvement of the lone pair electrons. All of these arguments favor the second mechanism.

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    $\begingroup$ The example involves an analogous choice of which electron reservoir to tap. I didn't think of it as an SN2, but isn't it really just a bimolecular, nucleophilic displacement? It just has H at the center of things rather than carbon. $\endgroup$ – ron Sep 26 '14 at 17:28
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    $\begingroup$ Generally, when you're guessing about something, then yes. But here, especially with electrons that move so quickly across very short distances, I think such an argument would have less significance - just my opinion. $\endgroup$ – ron Sep 27 '14 at 2:51
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    $\begingroup$ The differences between the 2 mechanisms are pretty minor. I don't think there is an experiment with high enough precision that could be done. Perhaps a computational study. $\endgroup$ – ron Sep 27 '14 at 2:55
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    $\begingroup$ organic chemist, was a professor, moved to industry; I suspect your professors know more, but would rather spend time on their research than talk with students $\endgroup$ – ron Sep 27 '14 at 2:57
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    $\begingroup$ I haven't compared introductory organic texts in a while, but I like "Organic Chemistry" by M. Jones, Jr. $\endgroup$ – ron Sep 27 '14 at 3:04

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