1H NMR: E/Z Diastereoisomerism

There's one more type of proton relationship that we should be aware of and that's called e and z diastereomerism, whew, 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, okay? So, a trigonal Center would be like the relationship between a cis double bond versus a trans double bond as we call a trigonal Center. So, notice if I use the q test on a terminal double bond, let's say I scratch out this age replace it with an q. Notice that what I just did by putting the q at the bottom would be I just drew the cis version, right? cis, or if you're using a and z notation I would be you z, sis slash z, right? 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, right? So, that means that the fact that I can make two different isomers depending on where I put the q means that this e-z relationship is possible, okay? So, these protons are always going to be diastereotopic, okay? We don't have to look for prior chiral centers anything like that, anytime you can make E or Z or cis and trans relationship with q, with the q test, always diastereotopic. Now, based on what you learned about diastereotopic before, do you think that these hydrogen's will have share a peak, share 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, okay?

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 nonequivalent? Well, think about it like this, let's say you put this double bond, right? And we draw one of my H's there and we draw one of my H's there, okay? So, it's basically the same example that we have on 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, so that means that the red H and the green H will be shielded slightly differently, one of them is going to be a little bit more bundled up and one of those 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. So, that means that they're going to have to each get their own peak they're, each, going to get their own signal, okay? So, in this example, specifically this would be HA and this would be HB, they would each get their own signal because of their unique position on the molecule, okay? So, that said, we have a few more practice problems, go ahead and try to figure out for A, if, you know, you think that these two hydrogens on the 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.

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 because, let's say I replace the H, replace it with a q, do we get an easy relationship 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, since it's symmetrical there is no cis and trans possible, okay? Since there's no cis and trans possible that means that they're going to share a peak, okay? So, these are actually homotopic, okay? They're are homotopic. So, let's go ahead and then just count out the peaks like normal, it would be that these are A, we've got this carbon? Well, I mean, we already counted that carbon and 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 three different ones, the other sides are also B and C. So, this would get three signals. Alright, try that with the next one and let me know what you get.

So, were the hydrogen's on this one homotopic or diastereotopic and after using the q test I've got H, I've got a H, I use q over here and what I notice after using the q test is it actually does make a difference work with 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 hydrogen's are diastereotopic. So, I have a diastereotopic relationship and since they're diastereotopic that means I would, I would give each of them their own signal. So, I have HA and HB. Now, I just have to count up the rest, this would be C this would definitely be D. Now, what do you guys think about the last two? did you guys give them the same signal? Good, okay, 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 um, 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, right? So, 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 two methyl groups are on a single bond this thing can rotate as much as it wants, okay? So, that means that the E, let's say, this is E1, I'm going to 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 downwards 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 over complicate it and don't try to think too hard about these because I'm giving you a pretty good set of rules already, okay? So, let's move on to the next topic.