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Is hydronium an electrophile or a nucleophile? I expected $\ce{H3O+}$ to be a nucleophile due to the presence of electrons in the $\ce{2p}$ orbitals of $\ce{O}$. But the answer key of the test states that its none of them.

I can rationalise it this way: If $\ce{O}$ donates its 2 electrons then there would be 2 positive charges on $\ce{O}$ leading to instability.

But I am not too sure of my reasoning either. Isn't hydronium serving as a nucleophile in its reaction with $\ce{OH-}$ ? Why is it then neither an electrophile nor a nucleophile?

Google books results show that its mentioned as an electrophile in some books. While there's disagreement on other sides on the internet? What's the true classification?

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    $\begingroup$ $\ce{H^{+}}$ (or equivalently $\ce{H3O^{+}}$) is an electrophile. If you react $\ce{C6H6}$ with $\ce{D3O^{+}}$ you will get $\ce{C6H5D}$ as the initial product, formed by the standard electrophilic aromatic substitution mechanism. $\endgroup$
    – ron
    Mar 22, 2018 at 1:07
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    $\begingroup$ $\ce{H^{+}}$ is shorthand for $\ce{H3O^{+}}$ which is shorthand for the Zundel and Eigen cations. See here. $\endgroup$
    – ron
    Mar 22, 2018 at 3:11
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    $\begingroup$ @GaurangTandon Like I mentioned above, $\ce{H3O^{+}}$ doesn't really exist either. You actually just have a solvated proton that still needs 2 electrons. $\endgroup$
    – ron
    Mar 22, 2018 at 13:25
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    $\begingroup$ @GaurangTandon I see no contradiction. $\endgroup$
    – ron
    Mar 22, 2018 at 13:42
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    $\begingroup$ As I mentioned earlier H3O+ can be further protonated in strongest superacidic solutions, so it can still act as nucleophile. Almost anything can be one or another depending on reaction. $\endgroup$
    – Mithoron
    Mar 22, 2018 at 23:45

1 Answer 1

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Firstly, I would like to point out that, when we talk about electrophiles (E) and nucleophiles (Nu), we may be calling the entire molecule an electrophile or a nucleophile, but in reality we are referring to a single atom or functional group that serves as the E/Nu. For example, when we call hydroxide (OH-) a nucleophile, we really mean that the oxygen anion is nucleophilic.

Now for hydronium. It should be noted that there are no 2p orbitals on the oxygen atom; it is sp3 hybridized. Given the positive charge on oxygen, it is already electron-deficient, meaning that it is highly unfavorable to lose more electron density by sharing it with another atom through a bond (as it would do if the oxygen acted as a nucleophile).

Instead, one off the hydrogen atoms acts as an electrophile. Once again, the positively-charged oxygen atom is electron-hungry, weakening the H-O bond and making the σ* orbital easier to donate electron density into. As a result, one of the hydrogen atoms of hydronium is quite easy to strip off, giving it its strong acidity (pKa~ -1.74). Since no other parts of hydronium react with other species, the only reactive component is the hydrogen atom, which serves as an electrophile and gives hydronium its categorization as an electrophile. Below is an arrow-pushing mechanism depicting the deprotonation of hydronium.

enter image description here

*Note that the oxygen of α-Methylbenzyl alcohol is acting as the nucleophile to attack a hydrogen atom of hydronium, the electrophile.

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