Organic Chemistry

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15. Analytical Techniques:IR, NMR, Mass Spect

1H NMR:Spin-Splitting Patterns


Common Splitting Patterns

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So, guys, one more note on splitting. I'm sure that at this point you're getting pretty sick of this subject. But it turns out that certain combinations of splits when they're seeing on the same Proton NMR spectrum are highly indicative of certain types of molecular structures. If we can learn these combinations of splits and learn to associate them with those structures, we can get way ahead with our knowledge level of analytical techniques. In fact, this is the kind of knowledge that can really catapult you to the top of your class because this is stuff that your classmates probably aren't gonna be able to do right away. So let's talk about four really important splitting patterns and what they mean when you see them. Okay, so here the four that we're going to discuss, we're gonna discuss what an ethyl group looks like, what an ethylene group looks like. Isopropyl and quaternary. Let's start off with Ethel, which is probably the most common. It's probably the one that your professor mentioned in class. So what an ethyl group typically looks like is that notice that Ethel Group is always ch two ch three. So you've got a two Next two or three, right? That means that if we're using n plus one, which we would because, you know, this isn't very complicated example with different J values, etcetera. What you're gonna wind up getting is a triplet and a quartet. This is what it would look like. You'd have a quartet somewhere and a triplet somewhere. Now the order of them doesn't matter. It doesn't matter. That one's in front of the other. It just matters that you have both a triplet and a quartet on the same spectrum. If you see a triple and quartet in the same and, um, are spectra, then you have to start thinking to yourself, there might be an ethyl group there. Why? Because we know that Ethel groups produce a pair of a triple with the quartet. Okay, now in the same manner. An ethylene group would just be too next to a two. So that means that one would split into a triplet and the other one would also split into a triplet. Okay, they would split each other into the same thing. So if you see a triplet triplet that tells you that you might have a dual dual triplets might tell you that you haven't ethylene group pressing. Okay, so just it's not always the case, but it's very likely. Okay, now, what about isopropyl? This was actually the most distinctive. If you see this, you're pretty much for sure Haven't isopropyl. That would be a combination of a doublet and a septet because noticed that this hydrogen in the middle is being split by how many other hydrogen? Well, six. It's got three in the top, plus three in the bottom. So three plus three plus one member, its end plus one equals seven. So we would expect to see a septet here. Now, let's look at the hydrogen at the top. The hydrogen is at the top are only being split by one h. So then you get n plus one equals two and you get a doublet. Okay, so when you have isopropyl groups present, you're going to see a combination of doublet and septet. It's okay. And you've really If you see a double and a septet in the same spectrum, you know for sure they haven't is appropriate group okay, which by the way, can help a lot when it comes to structure termination, which is a topic for another conversation. But it's coming up. Lastly, we have Quaternary groups, and this would be an example of when you just have single. It's kind of for no reason. If you just have single, it's a bunch of single. It's then that tells you that you must have hydrogen is that aren't being split by anything. Okay, now remember that we already talked about one type off molecule that can create single. It's so it would be also or hetero atoms, right? So we know that hetero atoms can cause single. It's a swell, but in the absence of hetero items, if you don't have oxygen, you don't have nitrogen, and you still have a bunch of single. It's popping up everywhere. Then that tells you that you might have carbons that have no h is attached. Okay, so in this case I'm calling it a Quaternary group because it's got four things around it that aren't hydrogen. But, I mean, there's other examples. It doesn't have to just be our groups. It could be a carbon Neil, and in our it could just be meaning that you have four bonds to carbon that air. Not to h that air, something other than H, which means that when you go ahead and try to split this thing, is it gonna split? No, because it's next to a carbon without hydrogen so it can't split. Got it? So obviously, hetero atoms are our first thought. When you see single, it's. But if you can't figure out that it's a hetero autumn, then you might want to look into the fact that this carbon might not have any hydrogen attached to it. So, guys, that's really it. That's just the common splitting patterns that we're gonna use for structure. Determination now just does a really quick example. Here's a sample NMR Um, is there a common splitting pattern seen here that could help us to deduce the structure of the molecule before even looking at it? Now? Notice. I included the structure of the molecule, so that's a huge hint. But just by looking at the signals and the types of signals we have, could we already to do some stuff about this molecule? Okay, so what do this as a worked example? Don't worry about starting pausing the video or doing yourself. Let's just talk about what we have. Well, we have a quartet. We have a single it and we have a triplet. Do any of thes splits give us a hint as to what the molecule could look like? Well, I see one big hit right away, which is I see that we have a single right would do single. It's indicate well, single. It's indicate either hetero atoms or carbons that don't have any. Hydrogen is okay. And in this case, do we have Let's say that we were just given the molecular formula, which would be C two h six, No, age 60 Okay, so C two h, 60 Well, would we have an idea of where that single it could be coming from? Yeah, we could think that that might be ahead or Oh, Adam. Okay, so we would we would think to ourselves. Maybe this is a hetero atom. Maybe it's an alcohol, since I haven't Oh, present. Okay, so let's just, you know, one way to think. Then there's another hit. There's a quartet and a triplet on the same exact spectrum. What does that tell us? That means that I need to suspect Ethel group. Okay, So already I have a suspicion that I might have maybe an alcohol, right? And then I have a suspicion that I might have an ethyl group. Right, Because I have a triple in the quartet. What does an ethyl group, plus an alcohol equal my final structure? Okay, so obviously, I'm being a little bit crazy with the way I'm applying this. Meaning that, like, I'm probably making some extra connections that you haven't learned how to make yet. But I'm just letting you know that just by using splitting patterns, we have a huge heads up on what the structure already is. Okay? Obviously you're never going to determine the structure just due to splitting patterns. But it's amazing how much extra help this provides when you understand it. Okay, So awesome. Guys hope that made sense. Let's go ahead and move on to the next topic.