1H NMR:E/Z Diastereoisomerism - Online Tutor, Practice Problems & Exam Prep

E and Z diastereoisomerism occurs with terminal double bonds, creating a trigonal center similar to cis and trans isomerism. The q test helps identify these relationships, indicating that protons in different positions (cis vs. trans) are diastereotopic and will yield different NMR signals due to varying shielding effects. The proximity of substituents affects the chemical shifts, leading to non-equivalent signals. Understanding these concepts is crucial for analyzing molecular structures and predicting their behavior in reactions.

E/Z Diastereoisomerism

There's one more type of proton relationship that we should be aware of and that's called E and Z diastereoisomerism. Big word. So how does that work? Well, this exists when your q test is used on a terminal double bond and it yields what we call a new trigonal center. Now, this is a term we haven't been over in a long time, but if you guys recall, a trigonal center is basically like a cis and trans isomer. A trigonal center would be like the relationship between a cis double bond versus a trans double bond. That's what we'd call a trigonal center. Notice if I use the q test on a terminal double bond, let's say I scratch out this H, replace it with a q, notice that what I just did by putting the q at the bottom cis version. Cis or if you're using E and Z notation, that would be Z. Cis slash Z. But who says I have to put it at the bottom? What if I had put the Q at the top? Well, if I had put the Q at the top, then that would be trans. That means that the fact that I can make 2 different isomers depending on where I put the q means that this E-Z relationship is possible. These protons are always going to be diastereotopic. We don't have to look for prior chiral centers, anything like that. Anytime you can make an E or Z or cis and trans relationship with the q test, it's always diastereotopic. Based on what you learned about diastereotopic before, do you think that these hydrogens will share a peak, share a signal, or do you think they'll get different signals? It says it right there. They're going to be non-equivalent and they're going to get different signals.

Now, I know that I've gone a lot through the rules of how to figure this out, but I just want to explain this really quickly intuitively because you might be struggling to think, well, why would they be non-equivalent? Well, think about it like this. Let's say you've got this double bond. Right? And we draw one of my H's there and we draw one of my H's there. That's basically the same exact sample that we have at the top. I just told you that because there's an E-Z relationship between these H's, they get their own peak. But how does that make sense in terms of shielding? Remember, in terms of putting on the wool coat, how does that make sense? Well, notice that this H is always going to be closer to that ethyl group because it's cis to it. This H is always going to be further from that ethyl group because it's trans to it. That means that the red H and the green H will be shielded slightly differently. One of them is going to be a little more bundled up and one of them is going to be a little less bundled up because of how far they are away or how close they are to this group over here. That means that they're going to have to each get their own peak. They're each going to have to get their own signal. In this example specifically, this would be H_a and this would be H_b. They would each get their own signal because of their unique position on the molecule.

That said, we have a few more practice problems. Go ahead and try to figure out if you think that these two hydrogens on the end of that double bond deserve to have the same signal or different, and then tell me the total number of signals you would get. Alright. So check it out.

Determining Diastereoisomerism

So let's figure this one out. When you use the q test on these H's, what was your conclusion? Guys, they're actually exactly the same and they're going to share a signal. The reason is, let's say I replace the H, replace it with a Q. Do we get an E Z or a cis-trans relationship? No, we don't because notice that if I make the Q on the right side, then it's going to be next to an ethyl group. However, if I put the Q on the left side, it's also next to an ethyl group. Since this double bond is symmetrical, there is no cis and trans possible. Since there's no cis and trans possible, that means that they're going to share a peak. So these are actually homotopic. They're homotopic. So let's go ahead and then just count out the peaks like normal. It would be that these are carbon. We already counted that carbon. Those are those H's. We've got this carbon, which we're going to scratch out because it doesn't have any H's. We've got this plane of symmetry, so that means that this is b and that means that this is c and that's it. We've got 3 different ones. The other sides are also b and c, so this would get 3 signals. Alright. Try that with the next one and let me know what you get.

Determining Diastereoisomerism

So were the hydrogens on this one homotopic or diastereotopic? And after using the Q test, I've got H. I've got H. I use Q over here. And what I notice after using the Q test is it actually does make a difference where I put the Q. If I put the Q at the top, I get trans. If I put the Q at the bottom, I get cis. So actually, these hydrogens are diastereotopic. So I have a diastereotopic relationship. And since they're diastereotopic, that means I would give each of them their own signal. So I'd have Ha and Hb. Now I just have to count up the rest. This would be D. Now what do you guys think about the last 2? Did you guys give them the same signal? Good. I'm glad. I could tell a lot of you guys gave these the same signal because there's a plane of symmetry there. Okay. So both of them have the same.

Now there is that one detractor out there. Okay. And it's okay. I'm glad you're thinking so hard. But I know you're out there saying, Johnny, but isn't this E? Isn't this one closer to the double bond and isn't this one further away? Shouldn't they actually get different signals, right? Shouldn't they be different? And the answer is actually no, guys, because the only way that you can have diastereotopic in this sense is on a double bond because there's no free rotation. But notice that since these 2 methyl groups are on a single bond, this thing can rotate as much as it wants. That means that the E, let's say this is E₁ and let's say this is E₂, right? E₁ right now happens to be closest to the double bond but after it rotates, then E₂ can be just as close. So actually, there is no difference between E₁ and E₂. They can rotate. The only difference happens with Ha and Hb because they can't rotate. Hb is always locked in the downward position. Ha is always locked in the outwards position. So that's why they become diastereotopic in that sense, okay? I know the rest of you guys are like Johnny, you're overcomplicating it. But just letting you know, you have to think about just the rules that I told you. Don't overcomplicate it and don't try to think too hard about these because I've given you a pretty good set of rules already, okay? So let's move on to the next topic.

Here’s what students ask on this topic:

What is E/Z diastereoisomerism in 1H NMR spectroscopy?

E/Z diastereoisomerism in ¹H NMR spectroscopy refers to the different spatial arrangements of substituents around a double bond, leading to distinct chemical environments for protons. E (entgegen) and Z (zusammen) denote the relative positions of the highest priority substituents on each carbon of the double bond. E isomers have substituents on opposite sides, while Z isomers have them on the same side. These differences cause protons to experience different shielding effects, resulting in non-equivalent NMR signals. Understanding E/Z diastereoisomerism helps in analyzing molecular structures and predicting their behavior in reactions.

How does the q test help identify E/Z diastereoisomerism in 1H NMR?

The q test helps identify E/Z diastereoisomerism by examining the relationship between protons on a terminal double bond. By replacing a hydrogen with a hypothetical group 'q', you can determine if the resulting isomers are E or Z. If placing 'q' at different positions (cis or trans) creates distinct isomers, the protons are diastereotopic. This means they will have different chemical environments and produce separate NMR signals. The q test is a useful tool for predicting non-equivalent signals in ¹H NMR spectroscopy.

Why do E and Z isomers produce different NMR signals?

E and Z isomers produce different NMR signals because the spatial arrangement of substituents around the double bond affects the shielding of protons. In E isomers, substituents are on opposite sides, while in Z isomers, they are on the same side. This difference in proximity to other groups causes variations in the magnetic environment experienced by the protons. As a result, E and Z isomers have distinct chemical shifts, leading to non-equivalent NMR signals. Understanding these differences is crucial for interpreting ¹H NMR spectra accurately.

How does the proximity of substituents affect chemical shifts in E/Z diastereoisomerism?

The proximity of substituents affects chemical shifts in E/Z diastereoisomerism by altering the magnetic environment around the protons. In Z isomers, substituents are closer together, causing increased shielding or deshielding effects. In E isomers, substituents are further apart, leading to different shielding effects. These variations in shielding cause protons to experience different chemical shifts in the NMR spectrum. Understanding how substituent proximity influences chemical shifts helps in distinguishing between E and Z isomers in ¹H NMR spectroscopy.

What is the significance of diastereotopic protons in 1H NMR spectroscopy?

Diastereotopic protons are significant in ¹H NMR spectroscopy because they occupy different chemical environments, leading to non-equivalent signals. In the context of E/Z diastereoisomerism, diastereotopic protons arise when substituents around a double bond create distinct spatial arrangements (E or Z). These protons experience different shielding effects, resulting in separate peaks in the NMR spectrum. Identifying diastereotopic protons is crucial for accurately interpreting NMR data and understanding the molecular structure and stereochemistry of compounds.