Why doesn't acetone have high basicity?
The C−O bond is polar, O becomes partially negatively charged and it should attract protons, becoming a strong base.
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1$\begingroup$ Related: Are hydrogen atoms in acetone molecule acidic?; Why is acetone less acidic than cyclohexanol?; Why is the pKa of a ketone higher than of water? $\endgroup$ – andselisk♦ Sep 5 '19 at 16:29
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$\begingroup$ Andelisk im not asking about the acidity of hydrogens in acetone , just why the oxygen cant become a proton acceptor. $\endgroup$ – user82418 Sep 5 '19 at 17:04
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1$\begingroup$ Inspired by (I think) comments to this question: chemistry.stackexchange.com/questions/119921/… $\endgroup$ – Karsten Theis Sep 5 '19 at 17:10
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$\begingroup$ Do you have a good explanation why acetone isnt a base? $\endgroup$ – user82418 Sep 5 '19 at 17:12
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I don't know the reason, but I can describe the trend (source: http://cactus.dixie.edu/smblack/chem2310/summary_pages/pka_chart.pdf):
Protonated ketones have a pKa of about -7. Protonated ethers have a pKa of about -4 and protonated primary alcohols of about -2. Comparison to hydronium in water is difficult, but in DMSO, hydronium is more acidic by about 2 pKa units (source: comment by DavePhD below https://chemistry.stackexchange.com/a/27278/72973).
So it seems water is a little bit basic. If you replace one of the bonds to hydrogen with a bond to carbon, it becomes less basic (alcohol). If you replace the other bond to hydrogen with a bond to carbon as well, it becomes even less basic (ether and ketone).
A hand-waiving explanation would be to say the electronegativity difference between hydrogen and oxygen is higher than that between carbon and oxygen. I am not sure what to say about the difference between ethers and ketones. I think of ketones as the better hydrogen bond acceptors, and as more soluble in water (all other things being equal), so I don't have a good rationale why it would be easier for an ether to accept a proton.
answered Sep 5 '19 at 17:39
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$\begingroup$ And what do you mean by" protonated ketones"? $\endgroup$ – user82418 Sep 5 '19 at 18:15
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1$\begingroup$ I would consider the geometry of a ketone vs an ether to perhaps explain the answer. I am pretty sure the real reason lies somewhere in the MO of the two groups (i.e. the HOMO of a ketone sits lower than an ether). However, I am not sure if it would be too rigorous to dive into that though. $\endgroup$ – mothgod Sep 5 '19 at 18:21
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2$\begingroup$ @SoftwarePlayer A protonated ketone is the product of a ketone acting as a base, when a proton (or H+) attaches at the oxygen. $\endgroup$ – Karsten Theis Sep 5 '19 at 18:37
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1$\begingroup$ @SoftwarePlayer "extra oxygen atom"? Alcohols, ethers and ketones all have oxygen atoms - there is no extra oxygen. $\endgroup$ – Karsten Theis Sep 5 '19 at 18:38
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$\begingroup$ yes you are correct i thought you meant something else $\endgroup$ – user82418 Sep 5 '19 at 19:00
