1H NMR:Number of Signals - Online Tutor, Practice Problems & Exam Prep

In proton NMR, the number of signals corresponds to unique non-equivalent protons. Equivalent protons share the same environment, while non-equivalent protons differ due to factors like electronegativity and molecular symmetry. For example, in a molecule with an oxygen atom and multiple carbons, distinct hydrogen types arise from their proximity to the oxygen, affecting their deshielding and chemical shifts. Recognizing symmetry can reduce signal counts, emphasizing the importance of understanding molecular structure in NMR analysis.

General Assumption for 1H NMR Signals

Let's discuss the first piece of information that we can derive from a proton NMR, and that's the total number of signals. On a typical proton NMR, there will be as many signals on the spectrum as there are unique non-equivalent types of protons. For that, we need to understand what is an equivalent or non-equivalent proton. An equivalent proton is going to be a proton that has the same perspective on the molecule as another proton. So, if 2 protons are pretty much in the same place on a molecule, like, for example, let's say the 3 protons that are attached to a methyl group. A methyl group usually has 3 H's on it. So all 3 of those would be said to have the same position on the molecule, so all 3 of them would be what we call equivalent. For right now, how are we going to be able to determine if something's equivalent or non-equivalent? We're just going to go with a really easy rule which is that let's go ahead and assume that hydrogen is bound to the same atom or equivalent. So like I just said, three hydrogens on a methyl group or on a carbon would be equivalent.

So that would apply to other atoms as well, not just carbon. 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 going to have some protons that are the same as other protons on the other side of the molecule. So symmetry is something we have to watch for when we're using this type of information. What we're going to do is I'm going to go ahead and do practice problem as a worked example and then I'll save the other 3 for you guys to do on your own.

Let's just go ahead and read this question. How many different types of protons or signals are there on each molecule? Let's look at What we notice is that has obviously a bunch of hydrogens on it that aren't drawn, but it has 4 different atoms. Now what I'm wondering is how many different signals do you think this is going to have? Now notice 3 of them are carbon. One of them is oxygen. Is there any plane of symmetry, etcetera. Those are the things we need to be thinking about. Well, I'm just going to tell you right now. The answer is that there are going to be 4 different types of hydrogens here.

Let's see why. The reason is because we could just start counting from the oxygen. Let's say that the oxygen is going to be letter A. The oxygen has a hydrogen that's attached to it and there's no other hydrogen like that. For sure, that's one type of hydrogen. No other hydrogens on that molecule look like that one. Now we have all the rest of these hydrogens. You might have thought that we could group them all together since they're all on carbons. Maybe you're thinking you have one type of hydrogen with the O and the second type is on the carbons.

But it turns out that no, they're actually more separated than that because for example, the hydrogens that are attached to this carbon are closer to the oxygen than the hydrogens attached to this carbon. That means that theoretically, the red hydrogens, the 2 hydrogens that are on this red carbon are going to be a little bit more deshielded than the blue ones because they're going to be closer to something electronegative. So I would actually expect that the red ones would be a little bit more downfield. You guys remember those words downfield and upfield?

Anyway, because of the fact that those 2 red hydrogens are attached to the same atom, we're going to say that's the second type. The 2 hydrogens attached to this atom are the third type. And then finally, the 3 hydrogens on this last carbon are the fourth. So in total, we get 1, 2, 3, 4 different types of protons. So not so bad.

Now another thing to note 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 oxygen is on one side and then you've got this asymmetry that goes through the whole molecule. That's why every single atom needed its own peak or its own signal. Now go ahead with that knowledge, try to do question and then I'll go ahead and solve it.

Identifying Proton Signals

Right, guys. So what was the answer for question B? 3. 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 hydrogens on one side of that dotted line, the same exact thing applies to the other side, so you don't even count the ones on the other side. For example, I notice that this one is on a double bond, so I'm going to make this as hydrogen type. Then I notice that these hydrogens are on an alkane, a regular sp³ hybridized carbon. That's going to be another type of hydrogen. Now I also notice 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 hydrogens. Remember, this is called proton NMR because it only responds to protons. Even though that is a unique position on the molecule, it doesn't have hydrogens so we don't count it. Then finally we have over here, we have the hydrogens that are on that one. So that's its own unique place. We've got those 3 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 B and this one is also C. That's what you do with a plane of symmetry. It means that any conclusions you have about one side are going to be the same exact ones on the next side. Okay. Awesome. So 3 different ones. Let's go ahead and move on to the next question.

Identifying Proton Signals

And the answer for the problem was just 1. I know some of you guys got that because it turns out first of all, is there any symmetry? Actually, yes. There are actually 2 planes of symmetry. There's a plane of symmetry here. I'll make that the red plane. And there's also a plane of symmetry here. I'll make that the blue plane. Now what does it mean to have a plane of symmetry again? It means that whatever you figure out for one side of the mirror applies to the other. In this case, since I have 2 overlapping planes of symmetry, that means all I need to do is figure out the number of unique positions in 1 quadrant and that will apply to all the other 3. All I need to figure out is how many different positions I have in one quadrant. What we notice is in that one quadrant, we have only 2 different positions. We have the red position and we have the blue position. My question to you is which of these get signals? Well, red, notice red actually already has 4 bonds, so red isn't going to 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 as this one. This one. This one. It's also the same as these. Those are all the same. So, the answer is that you're just going to get one. So, I'm just going to put the letter 'a' because there's nothing else. Okay? Awesome. Next question.

Identifying Proton Signals

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 in this molecule? If you did, you're kind of 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 ethyl group didn't exist. That would be symmetrical because now we would have a dotted line down the middle and now anything that I determine for one half applies to the other. But that's not the molecule I gave you. I gave you a molecule where the 2 alkenes are symmetrical but then one side has an ethyl and 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 atom signal signal signal d, signal e, signal f, signal g, and signal h. So all of those are going to get their own unique signal. It's 8 signals. Now you might be wondering, guys, I know a few of you are wondering, Johnny, why did you give A and F different signals? They look just the same to me. Why would you give them different signals? Well, notice, F is closer to the ethyl group and A is further from the ethyl group. That means there isn't perfect symmetry here, right? If the ethyl group didn't exist, then A and F would have been the same thing. But since that ethyl group is there, there's no symmetry. That means that F and A are their own unique peaks. Hopefully, that makes sense so far. Let's move on to the next part.

How many types of electrically unique protons (peaks) are there in the following molecule?

• A. 4
• B. 5
• C. 6
• D. 7

How many types of electrically unique protons (peaks) are there in the following molecule?  

• A. 7
• B. 8
• C. 9
• D. 5

How many types of electrically unique protons (peaks) are there in the following molecule? 

• A. 3
• B. 6
• C. 5
• D. 4

How many types of electrically unique protons (peaks) are there in the following molecule? 

• A. 5
• B. 2
• C. 3
• D. 4

How many types of electrically unique protons (peaks) are there in the following molecule? 

• A. 1
• B. 4
• C. 3
• D. 2

How many types of electrically unique protons (peaks) are there in the following molecule?

• A. 6
• B. 5
• C. 7
• D. 4

Here’s what students ask on this topic:

How do you determine the number of signals in 1H NMR?

To determine the number of signals in ¹H NMR, you need to identify the unique non-equivalent protons in the molecule. Equivalent protons share the same environment and produce the same signal, while non-equivalent protons have different environments and produce distinct signals. Look for factors like molecular symmetry and the proximity of protons to electronegative atoms, which can affect their chemical shifts. For example, in a molecule with an oxygen atom and multiple carbons, protons closer to the oxygen will be more deshielded and appear downfield. Recognizing these differences helps in counting the number of unique signals.

What is the significance of symmetry in determining the number of signals in 1H NMR?

Symmetry plays a crucial role in determining the number of signals in ¹H NMR. If a molecule has planes of symmetry, protons on either side of the plane are equivalent and produce the same signal. This reduces the total number of unique signals. For instance, in a symmetrical molecule like ethane (CH₃-CH₃), all six protons are equivalent, resulting in a single NMR signal. In contrast, asymmetrical molecules have more unique environments for protons, leading to more signals. Understanding symmetry helps simplify the analysis of NMR spectra.

Why do protons near electronegative atoms appear downfield in 1H NMR?

Protons near electronegative atoms appear downfield in ¹H NMR because electronegative atoms withdraw electron density from nearby protons, causing deshielding. Deshielded protons experience a stronger external magnetic field, resulting in a higher chemical shift value, which appears downfield (to the left) on the NMR spectrum. For example, in ethanol (CH₃CH₂OH), the protons on the carbon adjacent to the oxygen (CH₂) are more deshielded and appear downfield compared to the protons on the methyl group (CH₃).

How does the presence of multiple types of protons affect the 1H NMR spectrum?

The presence of multiple types of protons in a molecule affects the ¹H NMR spectrum by producing multiple signals, each corresponding to a unique proton environment. Each type of proton will have a distinct chemical shift based on its electronic environment, proximity to electronegative atoms, and molecular symmetry. For example, in a molecule like propanol (CH₃CH₂CH₂OH), you would observe different signals for the protons on the methyl group (CH₃), methylene group (CH₂), and hydroxyl group (OH), reflecting their unique environments.

What are equivalent and non-equivalent protons in 1H NMR?

In ¹H NMR, equivalent protons are those that share the same chemical environment and produce the same signal. Non-equivalent protons have different chemical environments and produce distinct signals. For example, in methane (CH₄), all four protons are equivalent because they are in the same environment, resulting in a single NMR signal. In contrast, in ethanol (CH₃CH₂OH), the protons on the methyl group (CH₃), methylene group (CH₂), and hydroxyl group (OH) are non-equivalent, producing separate signals in the NMR spectrum.