1H NMR: Number of Signals

Concept: Concept: General Assumption for 1H NMR Signals

5m
Video Transcript

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's going to be as many signals on the spectrum as there are unique non-equivalent types of protons. For that, we need to understand what's an equivalent or non-equivalent proton. Well, an equivalent proton is going to be a proton that has the same perspective on the molecule as another proton. 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 methyl group. A methyl group usually has three H's 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.
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, the 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.
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. Symmetry is something you 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 (a) as a worked example and then I'll save the other three for you guys to do on your own. Let's just go ahead and read this question. It says how many different types of protons or signals are there on each molecule. Let's look at (a).
What we notice is that (a) has obviously a bunch of hydrogens on it that aren't drawn, but it has four different atoms. What I'm wondering is how many different signals do you think this is going to have. Now notice three 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 four 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 so the oxygen has a 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 hydrogens on that molecule look like that one.
Now we have all the rest of these hydrogens. You might have through that we could group them all together since they're all on carbon, so 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, there are 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 two 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 because remember those words downfield and up-field.
Anyway, because of the fact those two red hydrogens are attached to the same atom, we're going to say that's the second type. The two hydrogens attached to this atom are the third type and then finally the three hydrogens on this last carbon are the fourth. So in total, we get one, two, three, four different types of protons. So not so bad.
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 so that's why every single atom needed its own peak or its own signal.
Now go ahead with that knowledge, try to do question (b) and then I'll go ahead and solve it. 

Concept: Example 1: Identifying Proton Signals

2m
Video Transcript

All right guys, what was the answer for question (b). 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 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 to 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 noticed that this one is on a double bond, so I'm going to make this as hydrogen type A. 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. That's its own unique place.
We've got those three different signals. Now would we also have to 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.
Awesome. So three different ones. Let's go ahead and move on to the next question. 

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

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

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

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

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

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

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Concept: Concept: Q-Test (Proton Relationships)

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Concept: Example 4: Identifying Proton Signals using Q-Test

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Concept: Example 5: Identifying Proton Signals using Q-Test

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Concept: Example 6: Identifying Proton Signals using Q-Test

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Problem: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic. 

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Problem: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic.

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Problem: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic.

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Concept: Example 7: Determining Diastereoisomerism

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Concept: Example 8: Determining Diastereoisomerism

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1H NMR: Number of Signals Additional Practice Problems

Protons Ha and Hb in the molecule below are………….and therefore will give rise to ……………… signal(s)

Watch Solution

How many signals would you expect for the 1H-NMR and 13C-NRM spectra of the following compounds respectively? 

 

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Which compound below would give rise to 4 signals in the proton NMR spectrum and 6 signals in the carbon NMR spectrum? (Assume you can separate and see all peaks.)

A) I

B) II

C) III

D) IV

E) More than one of the above.

Watch Solution

How many 1HNMR signals would trans-1,2- dichlorocyclopropane give?

A) 1

B) 2

C) 3

D) 4

E) 5

Watch Solution

The 1HNMR spectrum of which of these compounds would consist of a triplet, singlet and quartet only?

A) 2-chloro- 4-methylpentane

B) 3-chloro- 2-methylpentane

C) 3-chloropentane

D) 1-chloro- 2,2-dimethylbutane

E) 3-chloro- 3-methylpentane

Watch Solution

For the following compound how many different signals would you see in the carbon NMR? (Assume that you can see them all.)

A) 3

B) 4

C) 5

D) 8

E) 9

Watch Solution

An unknown molecule X has 4 signals in the 1H NMR spectrum. Which of the following corresponds to molecule X?

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How many signals would you expect for vanillin?

A) 4

B) 5

C) 6

D) 7

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How many signals would you expect to find in each range of the 1H-NMR spectrum of the following compounds? Enter a number in each box. 

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How many signals would you expect to find in each range of the 1H-NMR spectrum of the following compounds? Enter a number in each box. 

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How many signals would you expect in the 1H NMR spectrum of the following compound?

a. 4 
b. 5 
c. 6 
d. 7 
e. 8

Watch Solution

In each molecule below, identify the number of unique 1H and 13C NMR signals you would expect to observe on a spectrum and write the number in the box provided. 

 

 

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How many signals do you expect to see in the 1H NMR spectrum of the following molecule?

a) 4

b) 5

c) 6

d) 7

e) 8

Watch Solution

For the structure below, label the types of protons using the a,b,c labeling protons. 

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Labeling: For each structure below, label the types of protons, using the a,b,c labeling protons. (This is only a labeling exercise.)

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Labeling: For each structure below, label the types of protons, using the a,b,c labeling protons. (This is only a labeling exercise.)

Watch Solution

Labeling: For each structure below, label the types of protons, using the a,b,c labeling protons. (This is only a labeling exercise.)

Watch Solution

Labeling: For each structure below, label the types of protons, using the a,b,c labeling protons. (This is only a labeling exercise.)

Watch Solution