Explain why the $alpha$

Question

Explain why the

α

-hydrogen of an N,N-disubstituted amide is less acidic (pK_a = 30) than the

α

-hydrogen of an ester (pK_a = 25).

Accepted Answer

All right. Hi, everyone. So, first question, let's discuss why the P K A for the alpha hydrogen of N N dimethyl aceto ace amide at 12.8 is higher than the P K A for the alpha hydrogen of ethyl aceto acetate at 10.6. So the first thing I'd like to bring to your attention, right is that the P K A for the alpha hydrogen of ethyl aceto acetate is lower than that of N N dimethyl aceto aide or specifically it's alpha hydrogen, right? So what that means is that the alpha hydrogen of ethyl aceto acetate is more acidic. So for this question, we have to go ahead and explain why that is right. Why is the alpha hydrogen of ethyl aceto acetate more acidic than that of N N dimethyl aceto ace? And there are actually two reasons the first of which has to do with the stability of the conjugate base in part because if you recall when the conjugate base is more stable, the molecule itself is more acidic. So in order to discuss that, right, we have to realize or recall that both mites and esthers can participate in resonance which to a certain extent is going to stabilize our conjugate base. Now, for both of these, the resonance is essentially the same, right, because both nitrogen and oxygen have low pairs of electrons. So for N N dimethyl aceto aceto, my lone pair of electrons on nitrogen is going to jump in between the nitrogen itself and the carbonyl carbon and simultaneously the pi electrons of my carbo are going to be shifted upwards towards oxygen. Now, what that creates that creates a double bond in between my carbonic carbon and nitrogen. Therefore, giving nitrogen a positive charge and my carbon oxygen is going to have a negative charge because it gained an extra pair of electrons from the pipe on of the car bottle. And so for ethyl aceto acetate, it's the same concept, right? Except this time, instead of nitrogen donating its lone pairs, we have oxygen doing the same thing. So oxygen donates a pair of electrons between itself and the carbonyl carbon, which therefore pushes the pi electrons of the carbon towards the carbon oxygen. And so what that creates is a double bond in between the carboy carbon and our oxygen from our ester, which therefore gives the oxygen of our ester a positive charge and our car bono carbon or oxygen, excuse me has a negative charge. So for this problem, right, or for these resonance structures that I've drawn here, which I almost forgot the second carbon. So that's my bed but anyways, right, notice how I've drawn the resonance without detonating my alpha hydrogens for either of these. Right. And that's important because this de localization, this resonance that I'm drawing here, it competes with the resonance that results in the deep protonation of our alpha hydrogen. So that competition is something that we have to take into consideration when considering the stability of the conjugate base and therefore the relative acidity of these molecules, right? Because recall that nitrogen is less electron negative than oxygen. So what that means is that nitrogen is less upset by the fact that it has a positive charge in the resonance structure. Whereas oxygen being more electron negative hates it more than nitrogen would, right. So because nitrogen can accommodate the positive charge a little bit better that load pair of electrons on the nitrogen is going to be more de localized. And so what that means is that this de localization of electrons by our nitrogen is going to compete more with the conjugate base that results from the detonation of our alpha hydrogen. So our first reason that we're discussing here is that the lone pair on nitrogen is more delois and that de localization is going to interfere with the carbon ion that results from the deep protonation of the alpha hydrogen. Right. So because that lone pair is more spread out, it's more likely to clash into those electrons from the carbon ion, which is not favorable because they have the same charge or partial charge. At least. Now, the second reason why EHO aceto acetate is slightly more acidic once again, circles back to the fact that oxygen is more electron negative than nitrogen, right? Because of oxygen is more electron negative than it is stronger or it is a stronger electron withdrawing atom, right? It has a stronger electron withdrawing effect. Therefore, right, what it does is it pulls away electron density from the rest of the molecule, including of course, our alpha hydrogen and our alpha carbon. So when you have a situation where you have this specific carbon or this specific proton, that is electron deficient due to this resonance or electron withdrawing effect, that specific proton becomes more acidic. So our second reason is that oxygen has a stronger electron withdrawing effect, which makes the alpha, it makes the alpha hydrogen of ethyl aceto acetate more reactive because it is electron deficient. Ok. So we got to the very end, right? If you took it this far. Thank you very much for watching. I appreciate it. And these two reasons are going to be your final answers. Number one, that the lone pair on nitrogen is more delos and number two, that oxygen has a stronger electron withdrawing effect than nitrogen. So once again, thank you very much for watching. And I hope you found this helpful.

Explain why the $α$ -hydrogen of an N,N-disubstituted amide is less acidic (pK_a = 30) than the $α$ -hydrogen of an ester (pK_a = 25).

Key Concepts

Acidity and pKa

Resonance Stabilization

Inductive Effect

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