Isn't there greater resonance stabilization on the right molecule via charge delocalization and thus it should be a stronger acid - lower pka and higher Ka? Also, I remember that molecules with greater number of oxygens tend to be stronger acids.
Organic chemistry problem
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Isn't there greater resonance stabilization on the right molecule via charge delocalization and thus it should be a stronger acid - lower pka and higher Ka? Also, I remember that molecules with greater number of oxygens tend to be stronger acids.
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I have 2 years of orgo TAing under my belt, so hopefully I can shed some light on this. Typically, ketones are more acidic than esters (ketone = acetone, ester = methyl acetate). The reason for this is destabilization of the conjugate base of the ester. When you draw the contributing resonance structures for the ester, you'll find some crazy charge distributions (the major contributor has two charges: a negative on the ketone-based oxygen and a positive on the oxygen of the OCH3 group). The conjugate base of the ketone has reduced charges (its major contributor only has one negative charge on the oxygen- which is exactly where it wants to be). In comparing conjugate bases, you always want to reduce the number of charges- especially if they're different charges on the same element (+ and - on two separate oxygens is highly destabilizing).
Does that make sense? I wish I could attach a drawing of my resonance structures, but I'm assuming you drew them correctly!
Does that make sense? I wish I could attach a drawing of my resonance structures, but I'm assuming you drew them correctly!
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To respond to this- this would be correct if you were comparing two similar compounds that have the same functional groups. In this case, you're comparing two different functional groups (ketone vs. ester) and they have significantly different pKa values.
Correct, but that resonance structure you drew for the ester, when applied to the conjugate base of the ester, does actually contribute a non-trivial amount because that's the very reason why the alpha carbon is not acidic in the ester. Put another way, the electron-pushing nature of the methoxy group makes the carbonyl less electrophilic, with the effect of withdrawing less electron density from the alpha carbon. Thus, the hydrogen on the alpha carbon becomes less like a proton as compared to the same hydrogen on the ketone.
This is the very reason why nature uses acetyl-CoA instead of the corresponding ester. Thioesters are worse at reducing the electrophilicity of the carbonyl because the sulfur p orbitals are higher in energy than the carbon and oxygen p orbitals so it donates into the pi system less - with the end result of making the carbonyl in a thioester much more like a ketone in terms of pKa.
