Organic Chemistry

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15. Analytical Techniques:IR, NMR, Mass Spect

1H NMR:Spin-Splitting Simple Tree Diagrams


Splitting with J-Values:Simple Tree Diagram

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in this video, we're going to discuss spin splitting with Jay values and with tree diagrams. So essentially, this is the complicated version of the spin splitting explanation. Now that you understand the simple version, you might be wondering, Do I really need toe learn this more complicated version or not? And what I'll tell you is probably not okay. What is typical is that professors will briefly mentioned Jay values and will briefly show a picture of ah tree diagram as they're explaining spin splitting. What's less common is that a professor will tell you that you need to know specific J values or that you need to know how to draw tree diagram. If any of those two things come up, then you should watch this video. Okay, You can also just directly ask your professor, Will I be asked to draw a tree diagram? And if the answer is no, then you probably don't need to watch this video. Okay, But in the event that you do, here we go. Okay. So coupling constants, also known as J values Okay, describe the amount of interaction that a proton will have another. So it's kind of a quantification of the interaction. Okay. And here's some examples of common coupling constants that are frequently reviewed. So visceral protons, that would be to non equivalent protons that air just next to each other. This would be like a typical splitting example that we would have seen in the last video that would have a split of anywhere from 6 to 8 hertz. Okay. Thes interferences. Um, you know where these interactions are always described in a Hertz frequency. Okay, Now, sis protons that would mean protons that are split, um, that are separated by a cyst. Double bond usually interact anywhere from 7 to 12. And trans protons have a J value of anywhere from 13. 18 so kind of ordered it here in order. Now, this is not a comprehensive list of all the J values. If your professor says that they want you to know specific J values, then you should definitely refer to his resource is you can make sure that you know all the ones that they want you to know. But these are the three most common, that's for sure. Okay, so remember that we discussed in the more simplistic version of spin splitting that you could use Pascal's triangle to predict the shapes of splits that you get. But it turns out that Pascal's triangle Onley works if you assume that all of your J values are exactly the same. Okay, so basically, the whole reason that we could get those very predictable splits is because we're assuming that everything is splitting exactly the same that all of your hurts, all your values are the same. But once you introduce the idea of multiple J values multiple coupling constants, being involved with splitting the same proton Pascal's triangle no longer applies. In fact, Pascal's triangle is actually gonna give you the wrong answer because instead of everything splitting evenly, you've got different coupling constants, layering on top of each other, making weird shapes. So in that case, in order, predict what the split is actually gonna look like you have to use a tool that we call a tree diagram. The tree diagram is our way of visualizing how the splits are gonna work and how they're gonna happen in order so that we can get the final shape of the split meaning that if we predicted that something was a quartet before the shape might be different now that we're using a tree diagram. Okay, So what I want to show you first is drawing a simple tree diagram, and I'm gonna use a simple explanation of one that the end plus one rule would have worked on. So, for example, in this molecule noticed that it says that I'm basically I'm trying to figure out how h a my bold it proton is gonna be split. Okay? And what I notice is that Well, first of all, h A over here is that we're gonna split it. No, we're not going to get split by the other h A because that's not adjacent. That's actually on the same carbon. It's also not its's equivalent, so I don't even have to look at that. Okay, now, we also talked about how o H is a wall, so we're not going to get split over here either. So that means that this h A is only going to be split on one side, correct? It's gonna be split on the left side. How maney protons will be split by three. Notice that all three protons are the same exact type of proton. They're all home a topic, Okay? And they all have the same exact J coupling J value or coupling Constant of six hertz. Okay, so that means that in this case, are all of my j values the same? Yes, they are. Because I have three protons that are all splitting with a J value of six. Okay, So that means that Pascal's triangle should actually work in this example. I should not have to draw a tea tree diagram to figure out what it's gonna look like. Okay, if I used the end plus one rule here and is equal to what number three plus one. So that means that what type of split should I get? If I have three plus one, it means it should be a quartet. Okay, It should be a quartet. According to M plus one. And what shape does a quartet predict? According to Pascal's triangle, that means it should be a ratio of one, 23 23 toe one that should be familiar to you so far. Okay, but now what I'm gonna do is I'm actually gonna draw the whole tree diagram for this so you can see how it's actually correct. So let's start off with H A. H A is a single it by itself. Okay, this is a by itself. This is before it gets split. Okay, Now, in this graph, this is basically graph paper. I could give it any unit I want. Um, but let's just go with I think I have enough space to make, um, to make every single unit equal toe one. Okay? So that means that in my first split, I'm gonna split every I'm gonna go down as many layers as I have splits, um, or are coupling constants to split with. So in my first one, I'm going to split three on one side, three on the other, making my first split that is represented by H B one. Okay, notice that I'm gonna have HB one, HB two and HB three, and all three of these were gonna have a generation to split. Okay, So basically, what I have so far is that I had a single it, and now it just turned into a doublet with a distance of six hertz. Okay, this is now a double it. If I were to draw exactly if I were to represent this as a peak. It would look something like this would be a peek here and a peak here, a doublet. But I'm not done. If I were toe end there, that would be what it looks like. But I only split with the first proton. I still need to split with the others. So now let's go ahead and split with, um, HB two. HB two is gonna split both of these into six. So I'm gonna get six over here, and I'm gonna get six over here, okay? What that's going to give me is that now this is the split that I get from H B two and what I now get is I mean, these were still a distance of six hertz each. I'm not gonna write this every time. I'm just gonna write this again so you can see things now a distance of six hertz each. But the important part is that now, what would this look like? If, like, if I were to start drawing it right now? Well, what I would have is a ratio of one to to toe one. Now, why do I put a two in the middle? Well, because notice that two of the splits actually merged in tow. One line. That means that the basically the amplitude of the interference in that area is actually gonna be double that of the ones on the periphery. Does that 1 to 1 ratio look familiar? Yes, that's the ratio of a triplet. But we're not done yet because you still have that last Proton to split with. So I'm gonna race that for now. And we're gonna split with our last Proton H B three. All these lines have to get split, so I'm gonna split. Oops. I'm trying to use green here. I'm gonna split this one. I'm going to split this one, and I'm going to split this one. By the way, guys, just so you know, I know you've never done this before. Um, the height that I'm using, you know, I was using two units each. That's completely irrelevant. I just decided to do that because that was how much space I was given. The whole point is that you just try to keep it even however much downward you're drawing. Just try to draw that with every generation of split. Okay, so this is a church B three, our final split and what we notices what air the ratio is gonna be Now we'll think of this almost like Pascal's triangle. Whatever the top was, it's gonna add up to the bottom split. So that means that up here, I had a 1 to 1 ratio. That means that the splits here are gonna be now a one, 231 And to add together three, 23 toe, one ratio, Right? Because one and two again, make three. And then I got one on the side. Does this ratio look familiar? 1331 Yeah, guys, that is the ratio for our final answer, which is a quartet which, if I were to draw, would look something like this. Looks familiar, right? This is a quartet. So now a very legitimate question you should be asking me is Johnny. If we already have Pascal's triangle, if we already have the end plus one rule, why you just go through this whole exercise of doing something that just gave me the same exact answer? Well, because what I'm trying to show you is how you don't need to do this. If all your J values are the same. If they're all the same, please skip the hassle. We can already do this with Pascal's triangle. Okay, so then why would I ever need to use a tree diagram? You use a tree diagram if you have different J values in the same split. Okay, so let's go ahead and turn the page and see how that might be the case.