1H NMR:Q-Test - Online Tutor, Practice Problems & Exam Prep

In proton NMR, the q test helps determine the relationships between protons based on chirality. Protons can be homotopic (same signal) if no new chiral center is formed, or enantiotopic (same signal) if a new chiral center is created without an original one. If both a new and an original chiral center exist, protons are diastereotopic (different signals). Understanding these relationships is crucial for interpreting NMR spectra and predicting molecular behavior in organic chemistry.

Q-Test (Proton Relationships)

We just learned an assumption that stated that all of the hydrogens on a single atom are going to share one peak on proton NMR. And while that assumption is very helpful for most cases, there are going to be some exceptions where these protons will actually have different relationships depending on the original chirality of the molecule. This is where the exceptions begin. And if you learn to accept the exceptions and just roll with them, this is going to go a lot smoother for all of us. So let's dig right into proton relationships and the q test. As I just mentioned, sometimes these hydrogens are going to have different relationships based on their original chirality. And the test that we use for that is called the q test. So what the heck is the q test? Well, the q test is simply this. What we do is let's just look at this example here. We have 3 hydrogens. We take 1 hydrogen. We cross it out and we replace it with a random letter. In this case, q. The only reason we're using q is just to be random so that we don't confuse it with an atom. We're going to replace H with the symbol q and we're going to analyze that carbon now, that atom that the q is attached to. I'm going to say, did I just achieve a new chiral center by adding that Q there? Imagining that this H just randomly turned into some atom, would this now be a new chiral center? So that's what the q test is. And now we're going to see how the q test can yield some different results based on the original molecule. Let's just take this example into consideration. Do you think that that is a new chiral center because the Q is there? No guys. Remember from Organic Chemistry 1 that chiral centers only exist when you have 4 completely different atoms all around the same carbon or all around the same central atom. And in this case, why is this not a new chiral center? Well, the q is different. This bromoethyl group is different. But I've got 2 hydrogens, 2 of the same exact atom on that carbon, This obviously cannot be a chiral center. What do we do? What do we conclude when your q test does not generate a new chiral center? Well this is going to be the relationships with your protons are going to be based on results like these from the q test. If your q test does not yield a new chiral center, then these protons are always going to have the same relationship that's called homotopic. These protons are always comotopic and because homotopic means the same, that means they're considered equivalent. That means that they actually do share a signal on proton NMR. Meaning that the assumption that I taught you about how you can just assume that all the hydrogens on a carbon have the same peak, It holds true in this case because we just said that they're homotopic so they're the same. Perfect. And in general guys, you don't have to use the q test on stuff like CH₃, right? Because notice that CH₃ is always going to have 2 hydrogens left over after you use the q test. So at this point, we don't need to be wasting time drawing q tests for methyl groups because we just know that it's going to yield no chiral centers. Perfect. Now the next two relationships that we're going to discuss are going to be when we actually do find a new chiral center. When it does create a new chiral center, what's that relationship now? It's not homotopic anymore. Let's check it out. This next category, let's just go ahead and look at the picture first and see what's different about this one. Notice that here instead of starting off with CH₃, I'm starting off with a CH₂. When I use the q test on one of the H's and get a q instead, do I get a new chiral center? Yes guys. This is a chiral center. This is a prime perfect example of a chiral center. Why? Because notice that this carbon now has 4 completely different groups on it. If we were labeling chiral centers, we could label them as 1 q, some kind of high priority atom. 2, there's a double bond there. That double bond is different from a single bond. Then lastly, my hydrogen would be in the back. That's number 4. Now we're not going to be naming R and S here. That's not important. All I care about is that you decide is it a chiral center. Yes, it is. Now is that enough to tell the relationship between those protons? No. We still need one more piece of information. Now that we know that it makes a chiral center, we need to analyze the chirality of the original molecule. We have to look at the original molecule and say, did that original molecule have a chiral center? What do you guys think? Now I'm going to inspect this one and you guys are going to tell me, is there a chiral center on that molecule? Remember that in order to have a chiral center, I need to have 4 different groups on an atom and absolutely not. There are no chiral centers. This has 2 hydrogens. This has 2 hydrogens. The other one has 2 hydrogens and then the double bond can't be a chiral center.

Identifying Proton Signals using Q-Test

Notice that this question is asking the same thing that we've already answered before. How many signals will each molecule possess in proton NMR? But now that we know about the Q test, the very first thing we actually need to answer for all these problems is going to be, do I use the Q test or not? Because you don't have to always use the Q test. So you might be wondering, well, how do I know when I have to use it? Okay. So here's the hint: We use the Q test when you already have 1 or more chiral centers. Now the logic behind that is this: If you don't already have a chiral center, then for sure it's going to be either homotopic or enantiotopic if you don't have any chiral centers present from the get-go. Homotopic or enantiotopic. And remember that homotopic and enantiotopic both result exactly the same in proton NMR. They share a signal. That means the only time I have to worry about using the Q test is if I already had 1 or more chiral centers, in which case it might change the answer.

My first question to you is, for question A, do we have to use the Q test? The answer is no. We don't because there are no chiral centers present. Notice that you might think this is a chiral center, but it has two of the same exact group on it. It's CH₃ CH₃. So that's not a chiral center. There are no chiral centers here. That means I don't have to use the Q test. I'm just going to use the old rule that said that every single atom gets its own signal and watch for symmetry. I would go ahead and I would say this is signal A. This is signal B. This is signal C. This is signal D. And notice that after atom D, we actually do have a plane of symmetry developing where both of these are going to be equivalent. E and E because after that, you get a plane of symmetry. So we have symmetry on one part of the molecule but not on the rest. That's still okay. It helps us to determine that these methyl groups are the same as each other. The answer here would be 5 signals. Notice that it's going to become really important when you try to answer these questions that first you ask yourself, do I need the Q test? On this next problem, problem B, that should be your first question. Do you need to use the Q test? If so, where do you use it? When do you use it? If not, then just go with the old rule we learned. Go ahead and solve question B.

Identifying Proton Signals using Q-Test

Did we have to use the q test on problem b? The answer is yes because I do have a chiral center present from the beginning. If you're wondering where that chiral center is, it's right in the middle guys because notice that I have a methyl. I have a hydrogen. I have an ethyl on one side and I have an isopropyl on the other. That is one chiral center. Now does that mean that I have to use the q test on every single atom? Actually, no. You only use the q test when you have one or more chiral centers and only on CH₂s. Because the fact that I told you guys, CH₃'s are always homotopic no matter what because they don't make chiral centers. CHs, if you only have one H present, that kind of answers your question already because you only have one H. So, you don't have to worry about is it equivalent to another hydrogen. Let me just show you. For example, we can already conclude that this is going to get its own peak, A. We can conclude that these two are going to get their own peak. That's B because there's symmetry there. I'm trying to color code this for you guys. We can conclude that this is going to get its own peak C because it's a methyl group. On top of that, we've got this hydrogen which is obviously unique because it's the only one on that chiral center, so that must be hydrogen. We've also got a hydrogen here which, since it's the only hydrogen there, it must get its own peak because it's the only hydrogen that's in between two methyls like that. So far, we've been able to do all of this without the q test. Where does the q test really come into play? Only on any CH₂s that we have. Do we have a CH₂ present? Yes. We have a CH₂ right here. And on that CH₂, we need to use the q test. So, I'm going to take an H. I'm going to take an H and I'm going to replace one of the H's with a q. And now I'm going to ask myself, once I've done that, did I just make a new chiral center? What do you think? Is that a new chiral center now that I added a q? Yes, it is because I've got one group, two groups, a methyl and then four which is the rest of that junk. I can't even name it. It's a pretty big substituent. So that's definitely a new chiral center. So what does it mean when the Q test gives you a new chiral center and you already had one chiral center? What's the conclusion? It means that these hydrogens, hydrogen 1 and hydrogen 2 are diastereotopic. But most importantly, this question didn't ask me what the relationship was. It said, how many signals are we going to get. So, most importantly, what that means guys, and this is the important part, what that means is that this H gets its own letter F. It's an ugly own letter F. And this H gets its own letter G because remember that whenever you have diastereotopic protons, they each get their own signal. If you said 6 signals, that was the trick answer. The answer should actually be 7 signals because of the fact that those two diastereotopic protons get their own signal each. That was complicated. If you have any questions, let me know. But that's the way you got to approach these problems. That being said, what's the first thing you're going answer for C? You're going to tell me, do you use the q test or not? Go ahead and try to figure it out.

Identifying Proton Signals using Q-Test

So, did we have to use the q test on C? The answer is no. Guys, this molecule has no chiral centers. You might be thinking, but Johnny, isn't that a chiral center? No. Because I have two of the same exact group on both sides. So there's no chiral center here, meaning that there's no q-test. So that means that I'm literally just going to give every atom its own peak. Is there any symmetry on this molecule? That's the harder question. Unfortunately, yes. There's actually symmetry right down the middle. You might be wondering how in the world is that possible. One side obviously has the alcohol. One side obviously has the methyl. Johnny, you're nuts. What's wrong with you? No, because guys remember that tetrahedral molecules don't really look like that. What they look more like is that you've got two groups to the side and then you've got one group in the front and you've got one group in the back. Now we don't know which one is which. We don't know if the OH is in the front or the OH is in the back since wedge and dash wasn't given to us. But really when you split this molecule down the middle, you're splitting it down that front and back molecule. So when you're splitting it down the middle, you actually have you're splitting let's say the alcohol's in the front. You're splitting it right down the middle and you're also splitting the methyl right down the middle as well, meaning that this actually is symmetrical. So how many different bonds would I have? Well, obviously that H is unique. That's going to be type A. Obviously, this methyl is unique. That's going to be type B. Nothing else is like that. We have a carbon here that we're not going to count because it doesn't even have any H's. And then we've got molecules C. We've got proton C and we've got proton D, which are going to be mirrored on the other side because of symmetry. So this is also and this is also. So that means that this only had 4 signals. Got it? Cool. So let me know if that made sense. I'm sorry, it was a little bit tricky. Just got to get practice with this, alright? So let's move on to the next part.

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

Hey, guys. Let's take a look at the following practice question. So, in this question, it says, Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic. Now, before we can answer this question, we have to understand what each of these terms means. Now if we say unrelated, unrelated just means the hydrogens are on different carbons and they're not the same. So the hydrogens are on different carbons and are not the same. Here we can obviously see that the 2 hydrogens in this example are both on the same carbon. So they wouldn't be unrelated. They're related to each other in some way. That leaves us either with them being homotopic, enantiotopic, or diastereotopic. Now, when we say homotopic, all this means here is if you replace one of them with a different element, you don't create a chiral center. So replacing either one creates no chiral center. Here, enantiotopic, we're going to say replacing 1 creates a product with 1 chiral center. Here, diastereotopic can take on 2 different definitions. So when we say diastereotopic, it can lead to two possibilities. One, we create a product with more than one chiral center. Or it creates an E or Z alkene. Remember, E is closely related to trans and Z is closely related to cis. So diastereomers are two possibilities. If you do either option, then you're diastereotopic. So if we take a look at this compound here, here, I'm asking, are those 2 hydrogens that I'm pointing the arrows towards, are they one of these 4? We know they're not unrelated. They're related to each other in some way because they're on the same carbon. Here, we're just going to replace one of them. Here, you may sometimes hear of the q test, where you replace one of them with Q where Q represents anything other than that initial hydrogen here. I like to replace the hydrogens with D for deuterium which is an isotope of hydrogen. Deuterium is different from hydrogen. So what we realize here is what happens to this carbon now? This carbon becomes chiral. It's connected to deuterium, an H, a methyl group, and then this whole group right here. So we just create the compound that has one chiral center. There are no other chiral carbons anywhere else within this molecule. This carbon over here is not chiral because it's connected to 2 methyl groups. Those 2 groups are the same, so that carbon wouldn't be connected to 4 different groups. We'd say the identity of these 2 protons here is that they are enantiotopic. Replacing either one helped create a product that has only one chiral center. Again, we can replace the Hs with Q, which represents anything other than that hydrogen or here, I like to use deuterium as an example. But either way, we'd still get the same result. We create a product with 1 chiral center so they're enantiotopic.

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

Hey guys, let's take a look at the following practice question given to us. Here it says, Identify the indicated set of protons as being unrelated, homotopic, enantiotopic, or diastereotopic. Now remember, unrelated means they're on different carbons. Therefore, they're chemically non-equivalent. Homotopic means replacing either one creates no chiral center. Enantiotopic means replacing one of them creates a product with only one chiral center. And then diastereotopic means replacing either one creates a product with more than one chiral center or it creates an E or Z alkene. Here, we're going to replace one of the hydrogens. It really doesn't matter which one, with anything that's different from what it was originally. Replace one of these Hs with a q. Q represents anything other than that hydrogen that we had originally. We're going to say here, replacing it makes that carbon chiral because that carbon is connected to an H, a q, a methyl group, and then this whole group here which is different from H and different from the other guys. That carbon is chiral. But that is not the only chiral center that we have. We also have another chiral center here. It's connected to a methyl, a Br, this whole group here. And remember, a carbon must make 4 bonds. It's connected to an H that we don't see. That's 4 different groups. At the end, we have a product that has more than one chiral center. By definition, that will mean that those 2 hydrogens there are diastereotopic. Replacing either one creates a product with more than one chiral center.

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

Hey guys, let's take a look at the following practice question that was given to us. So here, we're saying, identify the indicated set of protons as either being unrelated, homotopic, enantiotopic, or diastereotopic. Now, if we take a look, we're going to replace one of these hydrogens with anything different from the initial hydrogen that was there. I'll just use the letter Q to represent anything different from that H. Now, here we're going to check to see. Did we create a chiral center or not? This carbon here is connected to an H, connected CH2CH2. Got to keep going until we find a difference. But the problem is they both reach this carbonyl before we ever find a difference. As a result of this, we're going to say that those two guys, because we replaced one of them, didn't create a chiral center. We're going to say that they are homotopic to each other. Replacing either one did not create a chiral center. So remember, homotopic means if I replace either hydrogen, I create a product with no chiral centers at the end, which is this example here.

Here’s what students ask on this topic:

What is the Q-test in 1H NMR and how is it used?

The Q-test in ¹H NMR is a method used to determine the relationships between protons based on chirality. To perform the Q-test, you replace one hydrogen atom on a carbon with a hypothetical atom 'Q'. If this substitution creates a new chiral center, the protons are analyzed for their relationships. If no new chiral center is formed, the protons are homotopic and share the same signal. If a new chiral center is formed without an original one, the protons are enantiotopic and still share the same signal. If both a new and an original chiral center exist, the protons are diastereotopic and have different signals.

What are homotopic protons in 1H NMR?

Homotopic protons in ¹H NMR are protons that are chemically equivalent and share the same signal in the NMR spectrum. This occurs when the Q-test does not generate a new chiral center, meaning the protons are in identical environments. For example, in a CH₃ group, all three hydrogens are homotopic because replacing any one of them with a hypothetical 'Q' does not create a new chiral center. Therefore, they all produce the same NMR signal.

How do enantiotopic protons appear in 1H NMR spectra?

Enantiotopic protons in ¹H NMR appear as a single signal because the NMR is not specific enough to distinguish between them. Enantiotopic protons occur when the Q-test creates a new chiral center, but the original molecule has no chiral centers. Although these protons are technically different, the NMR spectrum treats them as equivalent, resulting in a single peak. Advanced NMR techniques using chiral solvents can differentiate them, but this is beyond the scope of basic organic chemistry courses.

What are diastereotopic protons and how do they affect 1H NMR spectra?

Diastereotopic protons are protons that are in different chemical environments and thus produce different signals in the ¹H NMR spectrum. This occurs when the Q-test creates a new chiral center and the original molecule already has one or more chiral centers. Because these protons are not equivalent, they result in separate peaks in the NMR spectrum. Although the signals may be very close together, they are distinct, reflecting the different environments of the diastereotopic protons.

When should the Q-test be used in 1H NMR analysis?

The Q-test should be used in ¹H NMR analysis when you need to determine the relationship between protons on a carbon atom, especially if there is a possibility of chirality. Specifically, you use the Q-test if the molecule has an original chiral center. If there is no original chiral center, the protons will either be homotopic or enantiotopic, and the Q-test is not necessary. The Q-test helps identify whether the protons are homotopic, enantiotopic, or diastereotopic, which is crucial for interpreting NMR spectra accurately.