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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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  • $\begingroup$ What is the conjugate base of acetone? $\endgroup$ – DHMO Oct 24 '16 at 14:24
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    $\begingroup$ There 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. $\endgroup$ – Ivan Neretin Oct 24 '16 at 14:28
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    $\begingroup$ Generally, 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. $\endgroup$ – Zhe Oct 24 '16 at 14:29
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    $\begingroup$ Why, you can, but it is pretty hard. $\endgroup$ – Ivan Neretin Oct 24 '16 at 14:33
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    $\begingroup$ @ Ivan neretin , what makes it hard? $\endgroup$ – Pyro Recorcinol Oct 24 '16 at 14:34
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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.

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  • $\begingroup$ What's the threshold for something being aprotic, then? (Where does, for example, toluene lie?) $\endgroup$ – orthocresol Oct 24 '16 at 22:28
  • $\begingroup$ @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. $\endgroup$ – Jan Oct 25 '16 at 13:16

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