datQvault question, more oxygen means more or less acidic?

[dodobird2]

I just did datQvault organic chemistry test #7 and I'm confused on the answers for questions #27.

#27 asked to rank the following by increasing pka (so from most to least acidic).
acetone, acetic acid, F3C-COOH, and ethyl acetate.

I know that F3C-COOH is the most acidic, then acetic acid. But why is acetone more acidic than ethyl acetate? I see that OCH2CH3 is an EDG so it makes ethyl acetate less acidic. But I remember Chad from Chad gen chem videos saying that more oxygen means more acidic in oxo acids.

Thanks for clarifying!

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[Dental1717]

this is how i look at it : when it comes to electron donating groups ( and EWG) not all have equal activating strength; acetone is more acidic because it contains -ch3 which is a moderate activator of ortho para vs the -OR in ethyl acetate which is a strong activator of Ortho para. I wouldnt recommend the oxoacid trends here since we are dealing with different functional groups.

 

[dodobird2]

Thank you Dental1717! But what counts as an oxoacid? When can I use the "more oxygen=more acidic" rule?

 

[Dental1717]

there are 2 situations when you can use the Oxoacid trends; 1) when the number of oxygen is the same for all the compounds while the central atom differs and 2) the number of oxygen varies. I'll give you examples :
1- when comparing the strength of HClO3 vs HBrO3 .... the number of O is the same but Cl is more electronegative than Br, as a result , HClo3 is a stronger acid.
2) when comparing HClO3 vs HClO4.... HClO4 wins since it has more O ...
and if you see anything that doesn't fit this trend, ( like the question you had) , use the EDG vs EWG to figure out the strength .

 

[100%permwinner]

In acetone, when (either) alpha proton gets removed, the carbonyl oxygen can the accept (electron donation through resonance) negative charge only from the alpha carbon. In ethyl acetate, again the alpha proton is removed. But the carbonyl oxygen can accept both the negative charge from the alpha carbon and the lone pair of electrons from the oxygen. Because of the competing factor, the alpha carbon of the ester is less likely to donate all of its negative charge to carbonyl oxygen. The negatively charged alpha carbon of EtOAc is less stable than that of acetone. A less stable conjugate base means the acid is weaker, proton less likely to come off alpha carbon, equilibrium less likely to favor deprotonation, and higher pKa. Hope I explain everything clearly.