My chemistry book says that acetone is aprotic, but I think that, since the conjugate base of acetone has possible resonating structures, the hydrogen atom must be free to move out as $\ce{H+}$ (as the resulting structure is more stable than the previous one). Where am I going wrong?
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What is the conjugate base of acetone? – DHMO Oct 24 '16 at 14:24
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What is the pKa of a hydrogen $\mathrm{\alpha}$ to a ketone? – Zhe Oct 24 '16 at 14:25
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@DHMO conjugate base of hydrogen is the molecule we get on removing a H+ unit from the molecule, as per my knowledge . – Pyro Recorcinol Oct 24 '16 at 14:26
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@zhe sorry, I haven't yet studied the concept of pKa . – Pyro Recorcinol Oct 24 '16 at 14:27
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You literally said that "the conjugate base of acetone has possible resonating structures". I'm asking you what "the conjugate base of acetone" is. – DHMO Oct 24 '16 at 14:27
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3There are levels upon levels of wholly different meanings to the word "acidic". Yes, acetone is acidic (in a way), but not like ordinary acids. And yes, it is aprotic. – Ivan Neretin Oct 24 '16 at 14:28
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1Generally, we refer to protic as meaning "can hydrogen bond." This may be a bit of an oversimplification, but it might help you understand what the problem is here. – Zhe Oct 24 '16 at 14:29
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@Ivan neretin, will u please explain what aprotic really means and/or why this compound is aprotic ? – Pyro Recorcinol Oct 24 '16 at 14:29
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@Dhmo, it's CH3COCH2- – Pyro Recorcinol Oct 24 '16 at 14:31
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You can't remove a hydrogen like this... – DHMO Oct 24 '16 at 14:32
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1Why, you can, but it is pretty hard. – Ivan Neretin Oct 24 '16 at 14:33
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@Dhmo , that's what the question is . Why can't we remove the H when the resultant molecule is resonance stabilized . We can't remove it means it's aprotic – Pyro Recorcinol Oct 24 '16 at 14:33
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1@ Ivan neretin , what makes it hard? – Pyro Recorcinol Oct 24 '16 at 14:34
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@PyroRecorcinol You'd better wait till you get familiar with the concept of pKa. Before that, think of it this way: there are strong acids, like H2SO4 and HCl, and there are weak acids, like H2S. Then there are even weaker acids, which are so weak that they don't work as acids in water. And then there are other acids, which are many orders of magnitude weaker than that. – Ivan Neretin Oct 24 '16 at 14:46
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The point is that after removing one proton, the resulting structure is in fact not resonance stabilised. The terminal double bond of the enolate is not very favourable. If you have a tertiary C-atom there, then the enolate is sterically less hindered and has an advantage. If you have another double bond at the beta C-Atom, then you get resonance stabililsation. – Karl Oct 24 '16 at 15:23
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When the alpha hydrogen moves out, it's the first step of tautomerism.. given the small percentage of enol form present, you can safely say it's not much acidic – Black Jack 21 Oct 24 '16 at 16:44
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With respect to a large number of other molecules, deprotonating acetone is remarkably simple. Also, I reject Ivan’s opinion that acetone be aprotic; it can very well protonate given appropriate bases. – Jan Oct 24 '16 at 19:29
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You’re not going wrong anywhere. Acetone can indeed be deprotonated. The resulting structure is called an enolate.
$$\ce{H3C-C(=O)-CH3 <<=> H3C-C(-O^-)=CH2}$$
When compared with truly acidic compounds such as $\ce{HCl}$ and even $\ce{NH4+}$, acetone is a very, very weak acid, though. That does not mean it should be considered aprotic. Given a strong enough base — e.g. $\ce{HMDS}$ ($\ce{(Me3Si)2N-}$) — it can be deprotonated. Weaker bases will allow for partial (but probably hardly verifiable) deprotonations.
Jan
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What's the threshold for something being aprotic, then? (Where does, for example, toluene lie?) – orthocresol Oct 24 '16 at 22:28
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@orthocresol Very good question! Somewhere between acetone and DMSO would be my guess so a $\mathrm{p}K_\mathrm{a}$ value of $25$~$35$. Toluene has ~$40$ so it’s definitely out. – Jan Oct 25 '16 at 13:16