Rank the following compounds from highest wavenumber to lowest...

Question

Rank the following compounds from highest wavenumber to lowest wavenumber for their C-O absorption band:

Accepted Answer

All right. Hi, everyone. So first question, we must rank the compounds given below in order of decreasing wave number for the highlighted carbon oxygen bond absorption peak in the I R spectrum. And here we have three which for simplicity's sake, I'm going to go ahead and label 12 and three, going from left to right. So before we begin, let's go ahead and talk about what it means, right? When it comes to this decreasing wave number that the question is mentioning, right? So this question wants us to focus on the bond between our carbo neo carbon and our oxygen in our right. So recall really quickly that the greater single bond character you have the lower frequency that bond is going to stretch at, right. So the more single bond character, the lower frequency you're going to see it at therefore lower wave number. Now, on the other hand, right, the less single bond character you have this bond is going to absorb at a higher wave number, right? So at a higher frequency and why is that? Well, the reason for that is because recall it double bonds are stronger than single bonds, right? So, because double bonds are stronger um functional groups that have more double bond character are going to absorb at a higher energy and therefore, they show up at a higher wave number. So in order to understand the single bond character for each of these functional groups, we have to be able to understand their resonance structures, right? So what I'd like to bring to your attention is that only esthers two and three can exhibit a resonance, right? Because notice how they each have all keys, they each have pi bonds that can go ahead and move around to create a resonance structure. So the first thing I'd like to do is focus on the third, that's to the very right side. So what I'm going to do here is I'm going to go ahead and redraw it on the bottom. So I have my and this is molecule number three, like I mentioned before. So what I want to bring to your attention is that two unique resonance structures are possible, right? Because the lone pairs of oxy, sorry, the lone pairs of electrons on oxygen, they can move around in two different ways, right? So the loan pair of electrons on our single bonded oxygen here, they're de localized or rather they can move around either in between itself and the carbonyl carbon or itself. And this S P two carbon in our all key, right? So in our first step, which I'm going to draw in blue, the lone pair of electrons can go ahead and shift in between the oxygen and the carbonyl carbon. Now that creates a resonance structure because the pi electrons on our carbon are going to shift upwards towards oxygen. And so that creates a resonance structure that I'm going to go ahead and draw in blue. So it's the same molecule but because I'm drawing the resonance arrows in different colors, I want to show the resonance structure in the resulting color, right? So what happens is that now we have a double bond in between this other oxygen, which therefore creates a positive charge. And the original oxygen that was in our car bottle now has a negative charge. Now, on the other hand, right, a lone pair of electrons on our oxygen can shift in between itself and the S P two carbon of our key, right. So what that does is that creates a carbon ion because these pi bonds in our alkene are forced to move towards this terminal carbon over here to the very right side of the screen. So I'm going to draw the corresponding resonance structure in red. And here we're going to have a positively charged oxygen once again and a negatively charged carbon. And this time our car bono stayed untouched, right? So what I wanna uh draw to your attention here is that the low pair of electrons is shared in between two atoms, right. So that means that this particular carbon oxygen bond has a higher double bond character. Why? Because more double bonds can form via resonance. So therefore, right molecule three is expected to appear at a higher wave number or specifically the carbon oxygen bond of this molecule should show up at a higher wave number. So therefore, right, our molecule number three is going to be our first one. It's going to appear at the highest wave number. So from here, I would like to go ahead and redraw molecule two. So I'm going to go ahead and redraw molecule two so that we can go ahead and discuss how resonance affects its double bond character. All right. So for the molecule two, what I'd like to bring to your attention is the fact that this single bound oxygen, right? It can only participate in resonance with our carbon because in molecule three, our pi bond in our alkene was in conjugation with that particular oxygen, right? So that means that the carbon and the pi bond could participate in resonance. But here that is not the case, right? Because our pie bond is in conjugation with our car bottle. So here we still have two possibilities for residents, right? But the first possibility is in which our pipe bo is going to move and it's going to attach itself to the carbonyl carbon. So that forces the pip bos in our carbon to shift upwards towards oxygen. And so here we're going to end up with a negatively charged oxygen from our car bottle. And we're going to have a pie bond in between an S P two carbon and the carbon from our carbon. And so that creates a carol C, right, where all keen used to be. Now, on the other hand, right, our second resonance structure is with the oxygens in our ester, right? Because in our S P three oxygen, a pair of electrons can move in between itself and the carbonyl carbon. And that once again forces the pi electrons on oxygen, sorry, on our car bottle to shift upwards towards oxygen. So therefore, we're going to end up with both oxygens having a charge, right? The S P three oxygen ends up positively charged and our carbon oxygen ends up negatively charged. Right. So notice here how notice here how de localization of this lone pair of oxygen in our S P three oxygen, it's reduced, right? Because it is not directly involved in resonance the way that our molecule three was, right. So therefore, the double bond character is actually reduced, it's actually reduced. And therefore, we would expect our molecule to here to show up at the lowest wave number. So therefore, right, if I go ahead and scroll up to the original structures, our molecule three is going to show up at the greatest wave number and our molecule two is going to show up at the lowest wave number which leaves us with molecule one somewhere in between. So this is going to be your final answer. And I'm actually going to go ahead and draw the original molecule. So it's a, it's a little bit easier to see. So if I scroll down, once again, molecule three is going to appear at the highest wave number because it has the most double bond character. Molecule one was going to be somewhere in between, right, because resonance was not possible. So double bond character wasn't really affected. And then our molecule two here is going to appear at the lowest wave number because it had the least double bond character. So this is going to be your final answer. And if you stuck around to the end, thank you very much for watching and I hope you found this helpful.

Rank the following compounds from highest wavenumber to lowest wavenumber for their C-O absorption band:

Key Concepts

Infrared Spectroscopy (IR)

Wavenumber and Bond Strength

Effects of Substituents on C-O Bonds

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