Meso Compound - Video Tutorials & Practice Problems
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Another exception to the rule, these are compounds that remain achiral even though they have chiral centers!
1
concept
Defining meso compounds.
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Video transcript
So now I want to talk about another exception to the chi rally rules that I taught you. And this exception has to do with the rule about two or more Cairo centers. Remember that I told you guys that if a molecule has two or more Cairo centers that it's pretty much always gonna be Kyrill. Just assume that it's Carol. Well, it turns out that there is one exception to that, and that's me. So compounds, let's go ahead and get into this. So missile compounds or specific types of molecules where you basically have to Cairo centers that perfectly canceled out that yield two identical dia stereo MERS. Okay, so what that means is that after forming the stereo customers, two of them are gonna wind up being the same, meaning that it's actually gonna be a Cairo. Okay, so there's a few facts you should know about me. So compounds first of all, missile compounds have an internal line of symmetry. Remember, Test one had to do with internal line of symmetry. That means many times we actually will be able to use test one to predict that they're actually a Cairo. Okay, but sometimes we're not gonna be able to use that test. Which is why I'm going to give you guys some other rules specifically from eso compounds. Okay, It turns out that because two of the stereo customers are identical instead of following the two to the end rule for total number of stereo customers, we're actually gonna follow the two to the end, minus one rule for stereo I summers. What that means is that that minus one accounts for the fact that two of these stereo customers are gonna turn out to be identical. So instead of, for example, having four total stereo customers about two stereo centers, it would turn out to be three instead of four, because two of them would be turned out to be identical.
Meso compounds have an internal line of symmetry (TEST 1), meaning they are actually achiral.
Follow the 2n – 1 rule.
2
concept
The 3 rules of meso compounds.
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But I know what you guys are interested in is thes rules, right? So talk about these rules. There's three rules that will help us predict, if a molecules me. So, believe me, this is gonna help you so much because in your textbook and according to Professor, it's very difficult to tell when something's miss or not. A lot of times they just expect you toe recognize it. But I prefer to have rules. So here's the criteria. First of all, it has to have two or more Kyrill centers. I can't have less than two. All right, So if it has one, it's not me. So it has to Maybe it's me. So three, maybe it's me. So okay, two or more. Um, the second criteria is that it has to be atomic Lee. Symmetrical. Okay, now, this is actually really tricky to communicate because what? That doesn't mean it doesn't mean perfectly symmetrical. It just means that everything has to be connected in the same way on both sides. OK, even if wedges and dashes don't match up, that's okay. I see the atoms to be in place, okay, symmetrically. And then finally, I need an even number of the Cairo centers to be opposite to each other. Okay, so that means that if I have to Carl centers, I need two of them to be opposite to each other. That means that I need my first one. If it's our I need the other one to B s. If this first one is s, I need the other one to be our Why is this Because this has to do with the part about canceling out that I was talking about. Okay, I need for these Cairo centers to cancel out so that overall, the molecule, even though, has to carl centers, it will overall be a Cairo because the perfectly cancel each other out. Okay, so let me go ahead and give you a really simple version of a missile compound, and it would be something like this. Let's say that I had a cyclo vaccine and I had a wedge here, and they had a wedge here. Okay, this is an example. There's lots of different missile compounds out there, but this is an example of a missile compound. Why is that? First of all, do I have any Cairo centers? Yeah, I happen to have to. This is a Cairo center, and this is a Cairo center. Okay, so both of these air Cairo centers, because if you count the groups the direct, if you go around the directions air different, depending on which side you take. So the our group's heir difference, these air two different carl centers Is this molecule Cairo? No, it's not. Because if you look, it actually has an internal line of symmetry. Okay, If it has an internal line of symmetry, that means it's a Kyra. All right. But another way we could tell is by looking at the actual configurations of the Kyrill centers. And what I would see is if I went ahead and if I went ahead and figure out the Kyrill centers like the priority 12 and three. What I would find out is that this one goes around this way, okay? And then I would find that this one goes around this way. Okay, So what I would find is that one of these is in our and one of these is an s, and they're symmetrical to each other. They're both the same distance from the middle. So what? That means is that this is another way of proving that it's me. So by looking at the configurations and saying Hey there opposite and they're symmetrical and there's two or more of them. So this is me. So all right, now you guys might be wondering, Johnny, that looks so much more complicated than just doing the internal line of symmetry. Why don't I just do the test? One Internal line of symmetry test you can if it's a ring. But remember that Onley really works for rings for other kinds of compounds. It doesn't work. So, for example, a and B these compounds that I'm gonna give you down here, you really shouldn't use that test because that test is not gonna work very well for these. Okay, So what I want you guys to do is go ahead and start on a and try to do try to figure out the three things how many Carol centers there are what their configurations are. And if there if it's symmetrical, then once you figure that out, tell me if you think it's me so or if you think it would just be a normal Cairo compound, alright, So go ahead and try to solve a and then I'll show you guys how to do it
A compound will be meso if it meets the following 3 criteria:
It has 2 or more chiral centers.
It is atomically symmetrical, meaning that is symmetrical in connectivity, not shape.
An even number of chiral centers have opposite configuration to each other (i.e. if one is R, the other is S).
Is the following molecule meso and therefore achiral?
3
example
Which of the following molecules are meso?
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2m
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Video transcript
So let's just go down the list. First of all, how Maney Cairo Centers did have it had to. Okay, so that's good. Was it Atomic Lee? Symmetrical. Now, this is difficult to define a lot of times, but I would say yes. Okay. What I mean by Atomic Lee symmetrical just means that we have Adams in the middle that are connected to the same things on both sides. And in this case, yeah, that Quaternary carbon in the middle is connected to exactly the same things in terms of Adam's on both sides, even if the wedges and dashes don't match up, okay, But that's fine. Senators to figure out configurations R and s. So let's go ahead and work with this chlorine. First, the H would be in the back, So this is gonna be an easy situation. Where I have this is my one priority. This is my two. This is my three, and the H is my four. So then I would say Okay. Is my four in the back? Yeah, so I can go ahead and ignore it and just go ahead and draw one through three. And this side is gonna be in our okay. Now I do the same thing. I'm gonna have to erase a little bit. I'm gonna do the same thing with the other one. Okay, Let's go ahead and draw our H. This one's gonna be in the front this time. So that means that this is gonna be my four is gonna be my one. This is my two. And this is my three. Okay, so in this case, is my age in the back. No, you have to cross thes out and replace them. This is the swap. Now that my four is in the back, I'm gonna go ahead and go from 1 to 22 to 3321 I always ignore four. That looks like an s. But actually, it's gonna be in our because I had to. Swap system is gonna be our our. So now the question is, are these Cairo centers canceling out, or are they just building on each other? And the answer is that these air not canceling out, okay? They're both making the molecule. Cairo. So this would be not me. So and since it's not me, so I'm going to go with my general rule of if I have two or more to, um, or cow centers. This is Cairo. Okay, so remember that the general said if you have two or more Kyle centers, it's Cairo except me. So compounds in this case, since this is not me, so it has to be Cairo. Does that make sense? Cool. So I want you guys to do the same thing with this one. Now, I do want to clarify the notation a little bit. This is the beginnings of what's gonna be a Fisher projection. Okay? And fisher projections are specific thing that I may or may not teach you later, depending on whether you're professor in your book wants you to know it. Okay, But in the meantime, just know that these center balls are supposed to be carbons. Okay, so those air carbons you see, see that there are things in the front, things in the back. The same rules apply in terms of front and back. So go ahead and try to figure out our and the ass all that stuff and then get back to me.
Is the following molecule meso and therefore achiral?
4
example
Which of the following molecules are meso?
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3m
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Video transcript
All right. So are there to Cairo centers? Yes, there are. This is a Cairo center. This is a Carl center. Okay, so that's good. Are they Atomic Lee? Symmetrical. Now, this is rough, but let's look, if I were to split this in half, would it have the same groups on both sides? And actually, yeah, would it would have a carbon here on a carbon here. And even if the even, if the wedges and dashes don't match up perfectly, the atoms do notice that that one of the carbons on the top is attached to an O. H. And a metal notice that the carbon on the bottom is attached to an O. H. And the method. Okay, same groups, different rotation. But that's fine. At least its atomic lee symmetrical. So this is getting to check so far. Now, I just need to figure out the r and the s. So let's go ahead and do this. So let's go ahead. Number are priorities for the top one. It would be one. Okay, then this would actually my two down here. Okay? The reason is because the carbon in the middle doesn't count. That's just like my Kyrill center. Okay. And two is a carbon with an oxygen on it, whereas three is just a carbon with hydrogen. So obviously to is gonna win over three, and then this is for cool. All right, so now let's go ahead. Is four on the dash? No, it's not. So we have to replace. This is gonna be four. This is gonna be one. Okay, now, we're gonna go ahead and go around 2122223321 This looks like an s, but actually, it's gonna be in our because I swapped. Okay, Now it is the same thing at the bottom with a new Carl center. I'll make this one red. Okay, so my priorities are one to three and four is my four on the dash? No. So I'm gonna go ahead and cross out and swap snow. This is my four. This is my three. I'm gonna ignore one, and I'm gonna go around like this. So 12 to 3 31 This looks like an R. But actually, it's gonna be an s because I swapped. Okay, so this is actually getting three check marks. This was two or more. It was symmetrical. And it was also opposite candle those words or small. Okay, this getting three check marks. What that means is that this actually is me. So Okay, this is a mess. Oh, compound if this is a mess. Oh, compound. What does that mean? In terms of higher ality, that means that it's actually a Cairo. Okay, so check this out. These rules that I gave you are really clutch. Honestly, because the fact that there's no way that you would have been able to use the internal line of symmetry test to figure this out without doing some major rotating and rotating is something that I try to avoid in this class. Because, like I said earlier, people struggle with rotation. Okay, so if we can narrow it down to rules like we just did, it makes it better. So somehow, even though this molecule doesn't look like it has a plane of symmetry, it actually does. And the way that it does is you would have to rotate these guys around, so you have to rotate this. Ohh. Here, Stage three here and h there. And then you would get a plane of symmetry. But like I said, I try to stay away from rotation because it could get really tricky with some molecules. So let's just use these rules. And if you can use these three rules, you got it right every time. All right, so help that made sense to you guys. Let's go ahead and move on.