Chapter 2, Passage 4 - Question 25 (TBR Orgo 2012 - p.61)

[braxswag]

I understand the explanation and the logic behind their reasoning, but the way I approached the question was to use the concept of resonance. The nitrogen donates electrons more easily to the carbonyl oxygen (site c) than the oxygen in the OR group. But wouldn't this resonance prevent that oxygen from using those electrons to form a bond with hydrogen?

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[Czarcasm]

I understand the explanation and the logic behind their reasoning, but the way I approached the question was to use the concept of resonance. The nitrogen donates electrons more easily to the carbonyl oxygen (site c) than the oxygen in the OR group. But wouldn't this resonance prevent that oxygen from using those electrons to form a bond with hydrogen?

No; The electrons are delocalized through resonance, which is a stabilizing feature. Generally, we try to avoid forming new bonds that disrupt resonance (stability). I don't have this question in front of me, but based on your explanation I can see what you're asking. Both nitrogen and oxygen are engaged in resonance. In this scenario however, the oxygen is more electronegative than nitrogen and therefore holds its electrons a little bit more tightly than nitrogen would. This is an inductive effect. As a result, oxygen has a more partial negative charge than nitrogen does. Because of its more partial negative charge, it's more likely to act as a base and abstract a proton than nitrogen would. That's all there is to it. It's just a relative comparison between the two.

 

[braxswag]

Am I thinking too hard then? I thought that because the resonance effect was between oxygen and nitrogen, the electrons would be held more tightly, thus making that site not basic?

 

[Concrete Jungle]

This question is asking which is the most basic. Yes, the nitrogen makes choice C more basic than the oxygen makes choice B. You are right about resonance but this question involves resonance in all of your choices. So pick the most basic.

 

[braxswag]

I guess the first time I did the problem, I didn't really think that the RO group participated in resonance with the carbonyl oxygen on the other side of the compound. It does right?

 

[Concrete Jungle]

I mean this is a positive way but you might not understand resonance if you don't see this. Resonance is used to understand electron density in valence bond theory. In organic chemistry we use a qualitative explanation for something that is quantitative. The lewis dot structure is not a rigorous explanation of where electrons really are(really the wavefunction), so we use resonance to better approximate. A chemical species arranges its electrons so that it has the lowest energy. The -OR in question will not be a true sp3, instead it will have a different orbital that allows some of the electrons(really a probability distribution) in the pi bond to sort of smear across. Your Professor might have said something like it has some sp2 or pi character. The significance of this delocalization/increased stability/decrease in energy can be explained in the rules you learn about resonance. These rules are rooted in the actual math.

 

[braxswag]

Thanks everyone. I guess I was rushed with time and got caught up with resonance vs. induction when it would have helped to just think about the problem through a much simpler perspective.