1H NMR:Number of Signals - Video Tutorials & Practice Problems
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1
concept
General Assumption for 1H NMR Signals
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let's discuss the first piece of information that we can derive from a proton NMR. And that's the total number of signals. So, on a typical proton NMR, there's gonna be as many signals on the spectrum as there are unique, non equivalent types of protons. Okay, And for that we need to understand what's an equivalent or non equivalent proton? Well, an equivalent Proton is gonna be a proton that has the same perspective on the molecule as another proton. So if two protons are in pretty much the same place on a molecule like for example, let's say the three protons that are attached to a metal group, right? Um, Ethel Group usually has three h is on it. So all three of those would be said to have the same position on the molecule, so all three of them would be what we call equivalent. Okay, so for right now, how are we gonna be able to determine if something's equivalent or non equivalent? We're just gonna go with a really easy rule, which is that Let's go ahead and assume that hydrogen is bound to the same Adam are equivalent. Okay, So, like I just said the three hydrogen on a metal group on a carbon would be equivalent. So that would apply to other atoms as well, Not just carbon. Okay. And in general, a rule that we can go by is that any type of symmetry is going to reduce the total number of signals. This is because if you have any planes of symmetry, then you're by definition gonna have some protons that are the same as other protons on the other side of the molecule. So symmetry something that tow watch for when we're using this type of information. Okay, so what we're gonna do is I'm gonna go ahead and do practice problem A as a worked example. And then I'll save the other three for you guys to do on your own. So let's just go ahead and breed this question says how many different types of protons or signals are there on each molecule? Let's look at a So we notice is that a has obviously a bunch of hydrogen on it that aren't drawn, but it has four different atoms Now. What I'm wondering is, how many different signals do you think this is gonna have now? Notice three of them are carbon. One of them is oxygen. Is there any plane of symmetry, etcetera? Those are things we need to be thinking about. Well, I'm just gonna tell you right now, the answer is that there are gonna be four different types of hydrogen is here. Let's see why. The reason is because we could just start counting from the oxygen. Let's see if the oxygen is gonna be, uh, later. Okay. Okay. So the oxygen has ah, hydrogen. That's attached to it. And there's no other hydrogen like that. So for sure, that's one type of hydrogen. No other hydrogen on that molecule look like that one. Okay, now we have all the rest of these hydrogen, and you might have thought that we could group them all together since they're all in carbon. So maybe you're thinking you have one type of hydrogen with the oh, and second type is on the carbons. But it turns out that no, they're actually mawr separated than that. Because, for example, the hydrogen is that are attached to this. Carbon are closer to the oxygen, then the hydrogen attached to this carbon. So that means that theoretically, the red hydrogen is the two hydrogen is that are on this red carbon are gonna be a little bit more d shielded than the blue ones because they're gonna be closer to something. A lecture. Negative. So I would actually expect that the red ones would be a little bit more down field. You guys remember those words downfield in upfield? So anyway, because the fact that those to red hydrogen dare touch the same Adam, we're going to say that's the second type. The two hydrogen is attached to this. Adam are the third type. And then finally, the three hydrogen on this last carbon are the fourth. So, in total, we get 1234 different types of protons. Okay, so not so bad. Now, another thing to know is you might have been thinking. Maybe there was symmetry here, but really, this molecule isn't symmetrical. The way that it's drawn, the the oxygen is on one side. And then you've got this asymmetry that goes through the whole molecules. That's why every single atom needed its own peak, okay? Or its own signal. So now go ahead with that knowledge. Tried to question be, and then I'll go ahead and solve it
2
example
Identifying Proton Signals
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Alright, guys. So what was the answer for question? Be three. Let's go ahead and check it out. First of all, did you guys find any symmetry in this molecule? Actually, yes. This is a molecule with a plane of symmetry down the middle. That means that whatever conclusion I make, let's say about this carbon over here also applies to the one across from it meaning that if you are able to identify the amount of unique hydrogen on one side that dotted line, the same exact thing applies to the other side. So you don't even count the ones on the other side. Okay, So for example, I noticed that this one is on a double bond. So I'm gonna make this as hydrogen type a. Then I notice that these hydrogen are on an al cane. A regular SP three hybrids. Carbon. That's gonna be another type of hydrogen. Now, I also noticed that there's this carbon here. I'm wondering, Did you guys give that a signal or not? Actually, this carbon doesn't even count because that carbon doesn't have any hydrogen. Remember, this is called proton NMR because it on Lee responds to proton. So even though that is a unique position on the molecule. It doesn't have hydrogen, so we don't count it. Then finally we have Over here, we have the hydrogen that are on that one. So that's its own unique place. Okay, so we've got those three different signals now. Would we have to also draw the signals on the other side? No. Because this one is also A This one is also be, And this one is also see. Okay, so that's what you do with the plane of symmetry. It means that any conclusions you have about one side are going to be the same exact ones on the on the next side. Okay, awesome. So three different ones. Let's go ahead and move on to the next question.
3
example
Identifying Proton Signals
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and the answer for problems. See Iwas. Just one. Okay, so I know some of you guys got that because it turns out, first of all, is there any symmetry? Actually, yes. There's actually two planes of symmetry. There's a plane of symmetry here. I'll make that the red plane. And there's also plane of symmetry here. I'll make that the blue plane. Okay. Now, what does it mean to have a plane of symmetry again? It means that basically whatever you figure out for one side of the mirror applies to the other. Okay, in this sides, in this case, since I have two overlapping planes of symmetry, that means all I need to do is figure out the number of unique positions in one quadrant and that will apply to all the other three. Okay, So all I need to figure out is how many different positions do I have in one quadrant? Okay. And what we notices in that one quadrant, we have only two different positions. We have the red position and we have the blue position. Okay, so my question is, which of these get signals. Okay, well, red notice. Red actually already has four bonds so red isn't gonna count. Red isn't going to get a signal. Now, Blue is going to get a signal. But notice that this blue is the same as this one. It's the same groups trying to use blue here. It's the same as this one. This one, This one It's also the same. Is these okay? Those are all the same. So the answer is that you're just going to get one. So I'm just gonna put the letter A Because there's nothing else. Okay, awesome. Next question.
4
example
Identifying Proton Signals
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2m
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So what was the answer that you got for D? Well, this one's actually kind of a high number. I want to start off by just talking about symmetry. Were you able to find any symmetry on this molecule if you did, you were kind of like deluding yourself because there actually isn't. This is an asymmetrical molecule. Now it would actually be easy to turn it into a symmetrical molecule if you didn't have this part. Let's say this Ethel Group didn't exist. That would be symmetrical, because now we would have a dotted line down the middle. And now anything that I determined for one half applies to the other. But that's not the molecule I gave you. I gave you a molecule that the two alcanzar symmetrical. But then one side has an ethyl on the other side. Doesn't that means there's no symmetry that literally means that every single atom needs its own signal. So I'm going to get Adam a signal. Be signals. See signal D signal E signal F signal G on signal age. Okay, so all of those were going to get their own unique signal. It's eight signals now. You might be wondering, guys, I know a few of you guys were wondering Johnny, Why did you give a and F different signals? They looked just the same to me. Okay. Why would you give them different signals? Well, notice F is closer to the Ethel Group, and A is further from the Ethel group. So that means there isn't perfect symmetry here, Right? If the Ethel group didn't exist, then A and F would have been the same thing. But since that Ethel Group is there, there's no symmetry. That means that F and A are their own unique peaks. Okay, so hopefully that makes sense. So far, let's move on to the next part.
5
Problem
Problem
How many types of electrically unique protons (peaks) are there in the following molecule?
A
4
B
5
C
6
D
7
6
Problem
Problem
How many types of electrically unique protons (peaks) are there in the following molecule?
A
7
B
8
C
9
D
5
7
Problem
Problem
How many types of electrically unique protons (peaks) are there in the following molecule?
A
3
B
6
C
5
D
4
8
Problem
Problem
How many types of electrically unique protons (peaks) are there in the following molecule?
A
5
B
2
C
3
D
4
9
Problem
Problem
How many types of electrically unique protons (peaks) are there in the following molecule?
A
1
B
4
C
3
D
2
10
Problem
Problem
How many types of electrically unique protons (peaks) are there in the following molecule?