Ch. 15 - Analytical Techniques: IR, NMR, Mass SpectWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Concept #1: Common Splitting Patterns

Transcript

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 seen 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 going to be able to do right away.
Let's talk about four really important splitting patterns and what they mean when you see them. Here are the four that we're going to discuss. We're going to discuss what an ethyl group looks like, what an ethylene group looks like, isopropyl and quaternary.
Let's start off with ethyl which is probably the most common. It's probably the one that your professor mentioned in class. What an ethyl group typically looks like is that notice that an ethyl group is always CH2CH2CH3, so you've got a two next to a three. That means that if we're using n plus one, which we would because this isn't very complicated example with different J values, etcetera, what you're going to 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. The order of them doesn't matter. It doesn't matter what one's in front of the other. It just matters that you have both a triple and quartet on the same spectra. If you see a triple and a quartet in the same NMR spectra, then you have to start thinking to yourself there might be an ethyl group there. Why? Because we know that ethyl groups produce a pair of – a triplet with a quartet.
Now, in the same manner, an ethylene group would just be two next to a two. So that means that one would split into a triplet and the other one would also split into a triplet. 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 triplets might tell you that you have an ethylene group present. So it's just – it's not always the case, but it's very likely.
Now, what about isopropyl. This one's actually the most distinctive. If you see this, you pretty much for sure have an isopropyl. That would be a combination of a doublet and a septet. Because notice that this hydrogen in the middle is being split by how many other hydrogens? Well, six. It's got three on the top plus three on the bottom. So three, plus three plus one, remember it's n plus one, equals seven. So we would expect to see a septet here.
Now let's look at the hydrogen at the top. The hydrogens at the top are only being split by one H, so then you'd get n plus one equals two. And you'd get a doublet. When you have isopropyl groups present, you're going to see a combination of doublets and septets. If you see a doublet and a septet in the same spectrum, you know for sure that you have an isopropyl group, which, by the way, can help a lot when it comes to structure determination, 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 singlets kind of for no reason. If you just have singlets, a bunch of singlets, then that tells you that you must have hydrogens that aren't being split by anything.
Remember that we already talked about one type of molecule that can create singlets so that would be also or heteroatoms. So we know that heteroatoms can cause singlets as well. But in the absence of heteroatoms, if you don't have oxygen, you don't have nitrogen and you still have a bunch of singlets popping up everywhere then that tells you that you might have carbons that have no H's attached.
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 R groups. It could be a carbonyl and an R. It could just be meaning that you have four bonds to carbon that are not to H. That are something to other than H, which means that when you g ahead and try to split this thing, is it going to split? No, because it's next to a carbon without hydrogens, so it can't split. Got it?
Obviously, heteroatoms are our first thought when we see singlets, but if you can't figure out that it's a heteroatom, then you might want to look into the fact that this carbon might not have any hydrogens attached to it.
Guys, that's really it. That's just the common splitting patterns that we're going to use for structure determination. Now just as a really quick example, here's a sample NMR. 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 deduce some stuff about this molecule?
We'll do this as a worked example. Don't worry about starting, pausing the video or doing it yourself. Let's just talk about it. What do you have?
Well, we have a quartet. We have a singlet. And we have a triplet. Do any of these splits give us hint as to what the molecule could look like? Well, I see one big hint right away, which is I see that we have a singlet. What do singlets indicate? Well, singlets indicate either heteroatoms or carbons that don't have any hydrogens. In this case, do we have – let's say we were just given the molecular formula below, which would be C2H6O. Well, would we have an idea of where that singlet could be coming from? Yeah. We could think that that might be a heteroatom. We would think to ourselves maybe this is a heteroatom. Maybe it's an alcohol since I have an O present. That's just one way to think.
Then there's another hint. There's a quartet and a triplet on the same exact spectra. What does that tell us? That means that I need to suspect ethyl group. So already I have a suspicion that I might have maybe an alcohol and then I have a suspicion that I might have an ethyl group because I have a triplet and a quartet. What does an ethyl group plus an alcohol equal? My final structure.
Obviously, I'm being a little bit crazy with the way I'm applying this, meaning that 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 this structure already is. Obviously, you're never going to determine a structure just due to splitting patterns, but it's amazing how much extra help this provides when you understand it.
Awesome guys. I hope that made sense. Let's go ahead and move on to the next topic.