Stereoisomers, Enantiomers, Diastereomers, Constitutional Isomers and Meso Compounds. Created by Sal Khan.
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- When we are talking about 'mirror images BEHIND the molecule', how do the groups change place? That is one strange mirror. I understand they will get closer and further, that is obvious. However, when I raise my left hand in the mirror, the opposite hand is not waving at me.(33 votes)
- That would be a strange mirror... :-)
Try pointing at your mirro with let's say your index-finger... While pointing at the mirror your index-finger is in the back (from your point of view) and your wrist in front. In the mirror it's actually other way around.. Your index-finger is pointing back at you (and is in front)and your wrist seems to be behind that, so in the back.. That's the way the groups change place...(79 votes)
- In the last example, if you flip the molecule as he says, wouldn't the bromines be coming in and the hydrogens coming out? How is that superimposable on the image where the bromines are going out and the hydrogens in?(34 votes)
- For the last example, to get a superimposable image, you wouldn't flip the molecule; instead you would rotate the molecule 180 degrees. If you spun the left image as if it were on a wheel, the bromines would still be coming out of the screen, but they would end up on the left side of the molecule rather than the right, exactly like you see on the right image.(45 votes)
- The first example that Sal makes in the video of stereo isomers, at05:39, if we flip the first around, don't we get the second ?? Are they different molecules ?? Thanks !! :)(15 votes)
- Thats a good question!
Yes, if you flip it, you do get the other one. This diagram is drawn in the normal way....
.....Thats why there is this whole thing about fisher projections. You should definately watch that video.
Fisher projections have a way of basically not mixing up the two.
It wouldn't be absolutely correct to say that they are different molecules, but you can say that the have the same molecular formula but different structural formulae.(10 votes)
- I don't think the last compound has any chiral centers. Yes it has Br and H but if you go around the ring in both directions you will get the same molecule attached to both ends of both carbons. A chiral center has to be connected to four different groups.12:00(3 votes)
- Take the upper carbon for example, if you go counterclockwise then you will meet -CH2- => -CH2- => -CH2- => -CH2- => -CHBr. On the other hand, if you go clockwise it should looks like this -CHBr => -CH2- => -CH2- => -CH2- => -CH2-. They are DIFFERENT. Sal is right.(11 votes)
- I don't understand when to use the 2 different types of mirrors, like when do you place it in the back of the molecule and when do you place it right next to it? At4:53if we had not known it was already an enantiomer, how would we know where to place the mirror? Thx.(5 votes)
- If you draw an enantiomer using a mirror behind the molecule, you can simply spin the enantiomer around (180 degrees around the y axis) and it will be as if you drew the mirror to the side of the original molecule.
It's sort of like when you put your feet together to stretch your legs (you push down on your knees in a butterfly formation). This is analogous to putting a mirror on the side of a molecule. However, when you spin your legs so that they are now straight out in front of you, it's as if you put the mirror behind one of your legs and the other one was the mirror image from behind.
Same molecule just spun in a different direction.(4 votes)
- At12:22min Sal wrote meso-compounds are superimposible on mirror image.
But on Wikipedia it says:
" A meso compound is "superposable" on its mirror image (not to be confused with superimposable, as any two objects can be superimposed over one another regardless of whether they are the same. If two objects can be superposed, all aspects of the objects coincide.)"
Which one is the correct definition?
P.S. I think this is not so relevant, but some teachers are very strict on this little details(4 votes)
- The Wikipedia article is right - the mirror images need to be superposable (or identical when superimposed) to be meso-compound. I've heard the definition given many times as Sal has written it, but you're right that it is not technically correct. To be safe, use the superposable definition of Wikipedia, or if you use Sal's definition modify it slightly to say "meso-compounds are identical to their mirror images when superimposed".(2 votes)
- I am totally confused..
when do you have to flip and see,
when do you have to rotate and see,
when to use the back mirror and when to use the adjacent one,
and also in that last example what was with the symmetry? and how can changing F with Br can make a difference since F will superimpose F and the lower Br will superimpose Br ?Pls help!!(4 votes)
- You have to go back and review the R and S naming. I believe the left image has an S config, but the right image has an R config.(1 vote)
- About the 1,2-dibromocyclohexane: i see the 2 representations as enantiomers. Because when you flip the molecule 180º around its vertical axis, the Br elements go away from the plane and the H- alkyls come forward. That would be a diferent molecule from its mirror image. Am I wrong?(2 votes)
- And then if you flip the molecule as you’ve described it over again the Br will be coming towards us and the H will be going away, so they are the same molecule.
One enantiomer cannot be rotated around to get the other enantiomer.
Something you will see is that if there is an internal mirror plane in the molecule (tricky to describe here but imagine cutting the molecule in half and reflecting the other half) then it cannot be chiral.
If one Br was coming towards us and one going away however it would be chiral and would have an enantiomer.
This article has some examples and may be helpful to read over (and the website in general is very useful for organic chemistry): https://www.masterorganicchemistry.com/2011/01/12/the-meso-trap/(3 votes)
- Hello :) I don't really get why , in the last ex., they became enatiomers if we change Br to a F ...(2 votes)
- They would be enantiomers because they wouldn't be supermposable to each other. Try it with a model set, I know it looks tricky from a drawing, but with a model set, it will make sense (:(2 votes)
- For the 2nd example at1:32, I know you mentioned that they're the same molecule. However, since the two molecules are different based on the rotation of the sigma bond between the carbon and the methyl substituent, would they also be considered conformational isomers?(2 votes)
- In terms of definition, you are correct about conformational isomers and the rotation around the sigma bond. However, since the molecules drawn are the same, they are not "different based on the rotation of the sigma bond between the carbon and the methyl substituent," at least not based on what's drawn. In fact, we don't have enough information from the video to know whether they are even isomers or, again, the same molecule because we don't know the spatial arrangement of the methyl hydrogens (and therefore also don't know the conformation: anti/gauche/etc) for either compound. That's why most conformational isomers are depicted in a Newman projection, looking down the bond axis gives us a better visualization of the substituents and their relationships to each other!(2 votes)
In this video, we're going to look at pairs of molecules and see if they relate to each other in any obvious way or maybe less than obvious way. So these first two right here, they actually look like a completely different molecules. So your gut impulse might be to say that these are completely different molecules. And it wouldn't be completely off, but we look a little bit closer, you see that this guy on the left has one, two, three, four carbons, and so does this guy on the right. It has one, two, three, four carbons. This guy on the left has two, four, six, seven, eight hydrogens. This guy on the right has two, four, six, eight hydrogens. And they both have one oxygen. So both of the molecular formulas for both of these things are four carbons, eight hydrogens, and one oxygen. They're both C4H8O. So they have the same molecular formula. They're made up of the same thing, so these are going to be isomers. They're going to be isomers, and they're a special type of isomers. In this situation, we don't have the same bonds. We're made up of the same things, but the bonds, what is connected to what is different. So we call this a constitutional isomer. So we are essentially made up of the same things, but we are actually two different molecule, actually, two very different molecules here. Now let's look at this next guy over here. So if we look at this molecule, it does look like this carbon is chiral. It is an asymmetric carbon. It is bonded to four different groups: fluorine, bromine, hydrogen, and then a methyl group. And so's this one. And they're both made up of the same things. You have the carbon-- and not only are they made up of the same things, but the bonding is the same. So carbon to a fluorine, carbon to a fluorine, carbon to a bromine, carbon to a bromine, carbon to hydrogen in both of then carbon to the methyl group in both. But they don't look quite the same. Are they mirror images? Well, no. This guy's mirror image would have the fluorine popping out here, the hydrogen going back here, and then would have the bromine pointing out here. Let's see if I can somehow get from this guy to that guy. Let me flip this guy first. So let me-- a good thing to do would be to just flip to see the fastest way I could potentially get there. Let me just flip it like this. So I'm going to flip out of the page, you can imagine. I'm going to flip it like this. So I'm going to take this methyl group and then put it on the right-hand side. And you can imagine, I'm going to turn it so it would come out of the page and then go back down. So if I did that, what would it look like? I would have the carbon, this carbon here. I would have the methyl group on that side now. And then since I flipped it over, the bromine was in the plane of the page. It'll still be in the plane of the page, but since I flipped it over, the hydrogen, which was in the back, will now be in the front. The hydrogen will now be in the front and the fluorine will now be in back because I flipped it over. So the fluorine is now in the back. Now, how does this compare to that? Let's see if I can somehow get there. Well, if I take this fluorine and I rotate it to where the hydrogen is, and I take the hydrogen and rotate it to where-- that's all going to happen at once-- to where the bromine is, and I take the bromine and rotate it to where the fluorine is, I get that. So I can flip it and then I can rotate it around this bond axis right there, and I would get to that molecule there. So even though they look pretty different, with the flip and a rotation, you actually see that these are the same a molecule. Next one. So let's see, what do we have here? Let me switch colors. So over here, this part of both of these molecules look the same. You have the carbons on both of them. This carbon looks like a chiral center. It's bonded to one, two, three different groups. You might say, oh, it's two carbons, but this is a methyl group, and then this side has all this business over it, so this is definitely a chiral carbon. And over, here same thing. It's a chiral carbon. And this has the same thing. It's bonded to four different things. So each of these molecules has two chiral carbons, and it looks like they're made up of the same things. And not only are they made up of the same things, but the bonds are made in the same way. So this carbon is bonded to a hydrogen and a fluorine, and the two other carbons, same thing, a hydrogen and a fluorine. Carbon, it looks like it's a hydrogen. It's bonded to a hydrogen and a chlorine, so it's made up of the same constituents and they're bonded in the same way. So these look like-- but the bonding is a little bit different. Over here on this one on the left, the hydrogen goes in the back, and over here, the hydrogen's in the front. And over here, the chlorine's in back, and over here, the chlorine's in front. So these look like sterioisomers. You saw earlier in this video, you saw structural isomers, made up of the same things but the connections are all different. Stereoisomers, they're made up of the same thing, the connections are the same, but the three-dimensional configuration is a little bit different. For example, here on this carbon, it's connected to the same things as this carbon, but over here, the fluorine's out front, and over here-- out here, the fluorine's out front. Over here, the fluorine's backwards. And same thing for the chlorine here. It's back here and it's front here. Now, let's see if they're related in a more nuanced way. You could imagine putting a mirror behind. I guess the best way to visualize it, imagine putting a mirror behind this molecule. If you put a mirror behind this molecule, what would its reflection look like? So if you put a mirror behind it, in the image of the mirror, this hydrogen would now, since the mirror's behind this whole molecule, this hydrogen is actually closer to the mirror. So then the mirror image, you would have a hydrogen that's pointed out, and then you would have the carbon, and then you would have the fluorine being further away. And same thing in the mirror image here. You would have the chlorine coming closer since this chlorine is further back, closer to the mirror, and then you would have the hydrogen pointing outwards like that. And then, obviously, the rest of the molecule would look exactly the same. And so this mirror image that I just thought about in white is exactly what this molecule is: hydrogen pointing out in front, hydrogen pointing out in front. You might say, wait, this hydrogen is on the right, this one's on the left. It doesn't matter. This is actually saying that the hydrogen's pointing out front, the fluorine is pointing out back, hydrogen up front, fluorine back, chlorine out front, hydrogen back, chlorine out front, hydrogen back. So these are actually mirror images, but they're not the easy mirror images that we've done in the past where the mirror was just like that in between the two. This one is a mirror image where you place the mirror either on top of or behind one of the molecules. So this is a class of stereoisomers, and we've brought up this word before. We call this enantiomers. So if each of these are an enantiomers, I'll say they are enantiomers of each other. They're steroisomers. They're made up of the same molecules, so that they have the same constituents. They also have the same connections, and not only do they have the same connections, that so far gets us a steroisomer, but they are a special kind of stereoisomer called an enantiomer, where they are actual mirror images of each other. Now, what is this one over here in blue? Just like the last one, it looks like it's made up of the same things. You have these carbons, these carbons, these carbons and hydrogens up there. Same thing over there. You have a hydrogen, bromine, hydrogen and a bromine, hydrogen, chlorine, hydrogen, chlorine, hydrogen, chlorine, hydrogen, chlorine. So it's made up of the same things. They're connected in the same way, so they're definitely stereoisomers. Well, we have to make sure they're not-- well, let's make sure they're not the same molecule first. Here, hydrogen's in the front. There, hydrogen's in the back. Here, hydrogen is in the back. Here, hydrogen is in the front. So they're not the same molecule. They have a different three-dimensional configuration, although their bond connections are the same, so these are stereoisomers. Let's see if they're enantiomers. So if we look at it like this, you put a mirror here, you wouldn't get this guy over here. Then you would have a chlorine out front and a hydrogen. So you won't get it if you get a mirror over there. But if we do the same exercise that we did in the last pair, if you put a mirror behind this guy, and I'm just going to focus on the stuff that's just forward and back, because that's what's relevant if the mirror is sitting behind the molecule. So if the mirror's sitting behind the molecule, this bromine is actually closer to the mirror than that hydrogen. So the bromine will now be out front and then the hydrogen will be in back. This hydrogen will be in the back. I'm trying to do kind of a mirror image if it's hard to conceptualize. And then that would all look the same. And then this chlorine will now be out front, and this hydrogen will now be in the back in our mirror image, if you can visualize it. And then we have another one. And this chlorine is closer to the mirror that it's kind of been sitting on top of. So in the mirror image, it would be pointing out, and then this hydrogen would be pointing back. Now let's see, is our mirror image the same as this? So the mirror image, our bromine is pointing in the front, hydrogen in the back there. Then we have hydrogen in-- then in our mirror image, we have the hydrogen in back, chlorine in front. Same there. So far, it's looking like a mirror image. And then in this last carbon over here, chlorine in front, hydrogen in back. But here, we have chlorine in the back, hydrogen in front. So this part, you could think of it this way. This is the mirror image of this, this is the mirror image of this part, but this is not the mirror image of that part. So when you have a stereoisomer that is not a mirror, when you have two stereoisomers that aren't mirror images of each other, we call them diastereomers. I always have trouble saying that. Let me write it. These are diastereomers, which is essentially saying it's a stereoisomer that is not an enantiomer. That's all it means: a stereoisomer, not an enantiomer. A stereoisomer's either going to be an enantiomer or a diastereomer. Now, let's do this last one. Let's see we have two-- we have this cyclohexane ring, and they have a bromo on the number one and the number two group, depending how you think about it. It looks like they are mirror images of each other. We could put a mirror right there, and they definitely look like mirror images. And this is a chiral carbon here. It's bonded to one carbon group that is different than this carbon group. This carbon group has a bromine. This carbon group doesn't. It just has a bunch of hydrogens on it, if you kind of go in that direction. And it's hydrogen and then a bromine, so that is chiral. And then, same argument, that is also chiral. And obviously, this one is chiral and that is chiral. But if you think about it, they are mirror images of each other, and they each have two chiral centers or two chiral carbons. But if you think about it, all you have to do is flip this guy over and you will get this molecule. These are the same molecules. So it is the same molecule. So this is interesting, and we saw this when we first learned about chirality. Even though we have two chiral centers, this is not a chiral molecule. It is the same thing as its mirror image. It is superimposable on its mirror image. It is superimposable on its mirror image. So even though it has chiral carbons in it, it is not a chiral molecule. And we call these meso compounds. And we can point to one of them because they really are the same compound. This is a meso compound. It has chiral centers. It has chiral carbons, I guess you could say it. But it is not a chiral compound. And the way to spot these fairly straightforward is that you have chiral centers, but there is a line of symmetry here. There's a line of symmetry right here. These two sides of the compound are mirror images of each other. Now, these would not be the same molecule if I change that to a fluorine and I change that to a fluorine. Then all of a sudden, you do not have this symmetry. These are mirror images, but they would not be superimposable. So if that was a fluorine, these would actually be enantiomers. And this would not be only one meso compound, it would be two different enantiomers, and one of them would have an R direction and one of them would have an S direction if we go with the naming conventions that we learned.