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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

5. Chirality

Enantiomers vs. Diastereomers

We can use our knowledge of chiral centers to predict what types of stereoisomers molecules will make. 

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concept

Using chiral centers to predict types of stereoisomers.

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Now let's talk about ways to use Carl centers to predict the types of stereo customers that we're going to get. All right, So it turns out that the types of sterilized tumors can be roughly divided into how many different carol centers you have. Okay, so let's go ahead and just talk about the most simple situation. Which is that How about if you have a compound with no Kyrill Centers? Okay. Well, earlier today I told you guys okay, if there's no Carl centers, assume that it's a Cairo and that's still gonna hold. Okay, As you can see here, I said that compounds with no Carl centers are usually a Cairo. But now notice that I did put the world were usually in there. It's because there is a subset of these weird molecules called a tro py summers that we're gonna pay special attention to later. That happened to kind of break the rules, and they happen to be Cairo for other reasons. Okay, But other than that, most molecules can be really predict well predicted by the number of Carl centers they have. If they have no Cairo centers, then they're not Cairo. Okay? Except for the case of a trial by servers, and I'll teach you exactly which ones Those are there Exceptions. Okay, so let's go on to the next category. How about if a compound has exactly one Kyrill center? Okay, then that compound is always gonna be Kyra. All always, always. Okay, there's no way to avoid it. And since has one carol center that means that it's going to be able to a form in an teamers. Okay. And remember what an anti MERS were. They were the mere images that you would get that were different from the original. The reason is because if you have one Carl center, that means it's gonna be able to arrange itself in two different ways. A right handed way and a left handed way. So what that means is, if you put it upto a mirror, you are going to get the Indians here because there's one Cairo center. Okay, Now let's talk about the last situation. What if you have two or more Cairo centers? Okay. Well, those were gonna be usually Cairo. Okay? As I've been telling us today, as long as you have Cairo centers, assume that it's Cairo it turns out that there's also gonna be an exception to that. The exception to that comes in the form of molecules called. I'm gonna just point right here. The exception is me. So compounds okay, but we're going to pay special attention to those as well. So what I'm trying to say here is that except for a tro py summers which are weird and me so compounds which are also weird, these rules always hold, which is that compounds with two or more carl centers are gonna be Cairo unless they're me. So okay, now, if they have two or more carol centers, then they will not form Ananta MERS necessarily. They also have the ability to form what's called DIA stereo MERS. Okay, now die. Osteria MERS are It's a very weird word. What's the definition of that? Okay, well, the definition of a dia steri um er is simply a non mirror image stereo a summer. So basically it's a stereo. I summer that doesn't fall neatly into the nancy more category. Remember that in Nancy, MERS are always 100% the mirror image of the other one die. Astronomers are anything that doesn't look exactly like the mere image, but it's still a stereo. I summer cool, and we're gonna talk about the differences between Nancy MERS and Die astronomers in a second. Okay, by the way, sometimes it's really fun to hear your professors say Die astronomers because there's so many different pronunciations and ah, lot of them have to do with foreign accents, and it's kind of funny.
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How to predict the total number of stereoisomers.

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Let's talk about one more thing, and that's following the two to the end rule. Okay? It turns out that you can predict the total number of stereo system is possible by using the two to the end rule where N is equal to the number of stereo centers. Okay, so what that means is that you count up your stereo centers, and whatever your stereo centers are, you take two to the end that will tell you how many stereotypes summers you're gonna get. So, for example, I just give you guys a really easy example. If I had four stereo centers, okay, remember that a stereo center could be a Cairo center and or a tribunal center, all of both of those counts. A stereo centers. Okay, so if I had four of those thin, the total number of stereo items that I could get would be to to the four, which is equal to 16 different ice summers. Okay, which is crazy. Okay. You're never going to be asked to draw 16 different customers, but you may be asked to draw for Okay,

Follow the 2n (n = stereocenters) rule to predict total number of possible stereoisomers.

Note:Remember that stereocenters include both chiral centers and trigonal centers!

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example

Draw stereoisomers and determine relationship

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So let's go on to this example on. I want to use this more as a teaching moment than anything else. Just sit back and watch me do what I'm doing. Okay? So what I want you guys to do is us to do together is we're gonna draw all the sterilize MERS that air possible for this compound to bromo cyclo, hexane, Hexen one. All right. And then we're going to determine their relationships to each other. Okay, so first of all, how could we predict how many sterilizers we're gonna have? It says here, draw all of them. Doesn't say how many know. So we're gonna have to use some kind of method to figure out, like how many steroids summers there are. What are you thinking of? To to the end? Let me use to the end. Because I could just count up the number of stereo centers I have and that will tell me So how Maney stereo centers do I have? That means Cairo centers and tribunal centers combined. Well, I don't have any tribunal centers, but I do have to Cairo centers. This is a Carl center, and this is a Carl center. Both of these carbons have four different groups on them. So I'm gonna go ahead and plug this in to to the to, and that gives me four different I summers. Okay, so this is a situation we're gonna be drawing for. Okay? So let's go ahead and start off with the easiest combination possible. Which would just be Let's have them both facing towards me. Because remember that these steroids from the way, they're going to change that. Some will face towards the front some will face towards the back. So let's go ahead and draw our first one. And this one will be the easiest example. Let's make both e o. H and the BR. Let's make them both face towards me. Okay, That would definitely be one of the stereo I summers. Now, can you guys think of another stereotypes member that could possibly exist? Remember, we need four total. Another one could be that both of them face the back. Would that be the same exact molecule? No. It would actually be a completely different molecule. Okay, In terms of stereo, I summer. Okay, because there's no way I know some of you guys are thinking, but Can I just rotate that? And it will become the blue one? No. If I rotate that over the O. H and the BR will switch positions. So that means that if I rotate it like this, I like to call that sometimes, like a pancake flip. You flip it over like a pancake. The BR would now be on the top that it should be on the bottom. There be different compounds. So that's those are different compounds. Okay, so then are there any other combinations we can think of? Well, how did we keep the O. H in front? But we put the BR in the back. Would that be the same thing? No, that would be its own unique combination to that one is unlike the other two. Okay. And then finally, our last combination could be that I have my always in the back and my br front. Okay, so that's my fourth is, um er alright. So noticed that what I just did was I just went ahead and drew out all the possible stereo customers that were possible According to my two to the end rule to the end. All that does is it tells you all the different combinations of front and back that air possible. Okay, now we've drawn all the ice MERS. Now I want to know what their relationships are. So what I'm looking for is molecules that have perfectly opposite configurations meaning that if the O h is facing towards the front on one, I want to face it. I wanted to face towards the back on the other in order for it to be in an anti mur. Remember that an anti more means that it's the mere image, so that means every single configuration should be different. So if I'm looking for a new nan tumor in this case, I'm trying to figure out which of these are related to each other as AnAnd teamers. What I'm gonna look at is which of them have perfectly opposite configurations or perfectly opposite Carl centers? And the obvious trace to me is this top one right here? Okay, this relationship here has to be in an seamers. Okay? Why is that? Well, because if you'll notice here for the ohh, it's facing towards the front the B R is facing towards the front. When I look at the green one they're both facing towards the back. So this is the mirror image of the other one. And in fact, if I were to rotate it and put them up against the mirror, they would be mirror images of each other if I were toe rotate them both facing up. Okay. Both faces at the age and the br up. Okay, so those Aaron Nancy from each other remember that? Basically, in Antium er's have to be completely opposite. Okay, So are there any other pairs of an anti MERS here, for example, Are red and blue related to each other as an anti MERS? What do you think? Actually, no, because they're not perfectly opposite the b R is opposite. One is facing towards the front when is facing towards the back. But the O. H is the same. So that relationship can't be an anti MERS. Okay, Is there anything else here that's in Ann Summers? I think you guys might have found it red and black. So these two guys are also going to be an anti MERS meaning mirror images. Why? Because red and black, you can see that the O. H is opposite and the br is opposite. Okay, so those air also related to each other as an anti MERS. Okay, how about something like red and green? How are those related to each other? Well, that's not an anti MERS. Okay, so then what's the name that we give to all these other relationships that aren't really an anti MERS? But they're different sterilizers. Okay. For example, red and blue, they're not mere images, but they're still different. They're not the same. Well, the answer is these are all gonna be dia stereo MERS of each other. So this is gonna be dia steri. Um er, Homer. Okay, those air die Astronomers, this is die Osteria MERS. Okay, And all of these air die Osteria MERS. Okay, Why is that? Okay, So that applies to these in relation to each other and the's in relation to each other. Okay, why are they all related? Is Dia stereo MERS? The reason is because di astronomers is the word that we use for molecules that air stereo I summers, but not perfect mirror images. Every single relationship I just showed you either had basically one of the Carl centers, the same and the other one different. Okay, so we're going to say here is that Dia stereo MERS Okay, are basically not completely opposite. And this is gonna help you later once you start trying to figure out the relationships between ice summers. Okay, so but they're also not completely the same either. So they're kind of in the middle there, not completely opposite. But they're also not completely the same. Because if they were the same, they just be identical. So they're just like in between. Does that make sense? Cool.
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Determine the total number of stereoisomers for the following molecule.

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example

Determine total number of stereoisomers

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and the answer. Waas eight. Okay. Why? Because I've got three stereo centers. I've got this one. I've got this one and then that actually council the stereo center. My double bond because it conforms cis and trans. Okay, so that means that I've got eight different combinations, or basically two to the three combinations of ways that I could orient thes front back cis Trans. I could have combined all of them and I would get eight total. Okay, do you have to draw all of these? Please don't. That would be a really, really long task, but you could just visualized that would have to do with O. H. Moving front, back f moving front and back. And then the double bond moving system. Trans. Okay. And those would all be steroids, scores of each other. Cool, guys. Let's go ahead and move on to the next taco
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