One of the ways to see if two molecules are going to be related to each other as stereoisomers is to look for something that we call chirality. So what is chirality? Well, what chirality says is this. Chirality says if you look into a mirror and you get a different image than the image of yourself, if you get a slightly different image and opposite image, then that means that you're going to be a chiral compound. That means that that image is going to be chiral and it's going to be different. And what we call that image is called non-superimposable. Non-superimposable is a really confusing way of just saying a different image. Okay? So I can give you a few different examples. For example, I look at my face in the morning. When I see my face in the mirror and I'm brushing my teeth, it looks a lot like the face that I actually have. That's because there's a line of symmetry in my face, so when I get the image back, I see the same exact image. Okay. So that would be something that is called achiral because the image is the same coming off of the mirror or at least it should be mostly the same as my face. Another example of something that is different would be like my hand. So my hand, I put it up against the mirror, the image I get is the opposite image. Okay? So that means that if I were to cut out the image that I see in the mirror and put it over my hand, I would get two opposite images. Okay? That's the idea behind chirality. What chirality says is that if you get an opposite image when you look in the mirror, that is a chiral compound. Now the name for that different image, it has a really funky name that you guys are just going to have to know and it's called an enantiomer. And that can get really fun depending on what your professor's accent is like, but enantiomer just means it's the mirror image of a chiral compound. Alright? So literally, it could be anything. It doesn't have to be a molecule. It could be a person, it could be a car, it could be a cat, whatever. An enantiomer is just the mirror image that's different. Okay? So here, let me show you guys an example with an actual molecule. Here's a molecule and what I want to show you is this fake mirror that I drew. It's just this dotted line here. So imagine that this molecule, like I said, maybe it's like brushing his teeth in the morning and it's looking at itself in the mirror. So it has an eyeball. Obviously, this is just a really cool molecule. So it has an eyeball and what does it see? Well, what it sees is okay, well, a lot of these a lot of the image looks the same. For example, what it sees is that the amine is towards the top. This is let's say that's its hair. So its hair is at the top. He thinks everything's good so far. Alright. And he also sees, for example, notice that I have some wedge and dash notation where the wedge has to do with something being in the front and dash has to do with something being in the back. That means if this is a 3D image, the dash would be behind and the wedge would be upfront. So what he sees is that okay, the hydrogen is still in the back. Cool. So this one is still in the back. Well, you know, everything looks fine, but then he notices something's weird. What he notices is that well, the OH used to be on the right side, but in the image that he's looking back, the OH is actually on the left-hand side of himself. Okay? So it's like his hand or something. His hand's on the other side of his body. Then the same way with the methyl group. The methyl group used to be on the left, but now it's on the right. Now I know this seems normal to you because you're thinking, Oh yes, this is a mirror, so that means everything is going to be flipped, Johnny. This isn't this shouldn't be new to you. This is just what happens when you look into a mirror. That's all your professor wants you to know. What your professor wants you to know is that some molecules when they look in a mirror, they're going to get that opposite image. What non-superimposable means is that if I were to take a cutout of this thing, okay, I'm going to erase some stuff here so it becomes more clear. Erase the eyeball. When I take a cutout of this thing and lay it over this one, is it going to look exactly the same? The answer is no because even though the NH2 would be in the same place and even though the H would be in roughly the same place because they're both facing toward the back, what I would get is that I would get an OH over here and I would get a CH3 over here. And what that means is these are non-superimposable because my groups are in different places if I overlay them over each other. Okay? So it turns out that we have a pretty easy rule that we can follow to test to see if something is going to be chiral or not. This is not the only way to figure it out. But it's just like a rule that I already kind of hinted at, which is this, If a molecule has an internal line of symmetry or anything, if anything has an internal line of symmetry, then it will have the same mirror image. And if you have the same mirror image, that's what we call achiral. That means it's not chiral. It's not a chiral compound.
Chirality - Online Tutor, Practice Problems & Exam Prep
Chiral is the chemistry term we use to describe objects whose mirror image is different from the original.
- The mirror image of any chiral molecule is called an enantiomer.
- If a molecule has an internal line of symmetry, it will have the same mirror image (achiral).
What is chirality?
Video transcript
Draw the mirror image of the following molecule.
Drawing Mirror Images and Determining Chirality
Video transcript
So let's go ahead and do this example where let's just do this as a free response. We just want to work through this together and see if we can figure this out. It says here draw the mirror images of the following molecules and determine if the mirror image is the same or if it's different. Then after we do that, we want to figure out if there is an internal line of symmetry and if there is, let's go ahead and annotate it. So for this first one, what I want to do, you guys can just follow along, I want to draw my fake mirror again. Okay? And that's just going to look like this. It's kind of rough. Okay? So imagine once again that this molecule is looking into the mirror. Alright. It's not even like a perfectly straight mirror or whatever. What is he going to get back? What kind of image is he going to get back? Well, what he's going to find is that okay, there's a 6 membered ring in the mirror. So let's write that down. And then in this first image, the bromines, imagine that the bromines are like something on his face. They are on the right-hand side. But for the mirror image, they would be on the left-hand side. Does that make sense? Okay. So man, I'm not that's a really ugly bond. Okay. So are you guys getting that so far? So that would be our mirror image. Notice that they're both still facing towards the back. Now what I want to know here is this mirror image the same as the original molecule or is it different from the original molecule? What do you guys think? The answer is that it's exactly the same And it's not because it's the reverse. The reverse has nothing to do with it. What it has to do with is that if I flip this around, if I rotate it a little bit, then I'm going to get the same exact thing. Now I told you guys I'm not going to do a lot of rotating. Okay. But for this part, just to illustrate this, I want to show you. So if I rotated this molecule like that, if I rotated it actually a 180 degrees to the right. Okay. I like to call that rotation like a DJ spin. Okay? Where I imagine that's a vinyl record and I'm just spinning it around. Alright? So if I DJ spin that molecule, what I'm going to wind up getting is a molecule that looks like this. All right, so is that the same molecule or is that a different molecule? That is the same molecule. So does that mean that this molecule is chiral or achiral? What do you guys think? What that means is if it says it's the same, that means it's achiral. Now I want you guys to look at if there is an internal line of symmetry in this molecule? Is there a line that you could draw that would split it perfectly in half? And the answer is yes. There's actually an internal line of symmetry right here. If I were to take scissors and I were to cut it in half like that, what I would find is I would get 2 perfectly symmetrical halves. Alright? So remember that I said if it has an internal line of symmetry, then it's going to be the same. Okay. I'm sorry. If it has an internal line of symmetry, then it's going to be the same, so it's going to be achiral. So all of that works out. Cool so far? You may be asked to draw mirror images on your exam.
Draw the mirror image of the following molecule.
Drawing Mirror Images and Determining Chirality
Video transcript
So let's do this. My mirror would look like this. What I would wind up getting is a 5-membered ring that looks like that. Okay? And I would get that the CH3 up here is now going this way, and then the one on the dash is now going this way. Cool? Awesome. So now those are why I mirror image. So now I'm wondering the same exact question. I'm wondering if this is the same molecule. So let's go ahead and do the same exact thing again. Let's do a DJ spin, 180 degrees, and figure out if this is the same molecule. So once I do that, what it's going to look like is like this. I'm going to get the ring facing the same way originally, but now notice that this bottom group, that was facing towards the dash, now moved up. So that means that now I have a dash facing up, and notice that the one on the wedge now rotated, so now it must be going down. Alright. So, is this the same molecule as the original one? What do you guys think? It turns out that this is a different molecule. This one is a different molecule. Why? Because there's actually no way that I can rotate this molecule for it to become the other one. I know you're in disbelief. I know some of you guys are thinking, hey, but what if I flip it like this or if I flip it like this or if I rotate it in a bunch of different ways, couldn't it become that? Never. You can actually never turn this one into that one, no matter how hard you try. Okay? So these are different compounds. So that means that after the mirror image is going to be different, so that means that this is a chiral molecule. Does that make sense? And then chiral means that I'm getting a different mirror image. Okay? What is the name of this mirror image? Do you guys remember I told you guys that it has a special name, kind of funky, enantiomer? Okay, you guys are already learning a lot. Okay? And we're like, we haven't even really gotten into it yet. I'm just introducing this. Alright. One more thing. Is there a line of symmetry? Is there a line that if I cut it down the middle, I'm going to have 2 perfectly symmetrical halves on the original molecule? And the answer is no. Because if I went ahead and drew this line of symmetry again, would I get 2 perfect halves? No, I wouldn't, because one side would have a methyl going up, one side would have a methyl going down. They would not match perfectly. So I'm just going to write here, and you guys should put this in your notes. I'm going to say here, no internal line of symmetry. Wow. Okay? And what does that mean if I have no internal line of symmetry? What that means is that it's chiral. Okay? So we can use that simple rule, and we can figure out if something's chiral or not. Now the whole point of this is to show you guys, to prove why the internal line of symmetry is important. Do you guys see that now? How if it does have one, you're going to get achiral. If it doesn't have one, you're going to get chiral. But now that we understand this, do we have to go through this mirror image crap every time? No. Instead, we can just look for a line of symmetry, and that's where I'm going with this. Alright? So let's go ahead and move on to the next topic, and we're going to talk about the line of symmetry.
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