Test 1:Plane of Symmetry - Video Tutorials & Practice Problems
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The simplest test for chirality is symmetry. If a molecule has an internal line of symmetry, it is achiral.
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concept
How and when to use the internal line of symmetry test.
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So in this chapter, I'm going to give you guys three different ways to test for Cairo ality. And the first one is one that you already know. So let's go ahead and get started with it. So the test is for it's called Test One, and it's the internal line of symmetry test. So we want to do is we want to look at these compounds and see if they have an internal line of symmetry. If they do have an internal line of symmetry, then we would say that that would be in a Cairo molecule. You guys remember that? That basically means that it has the same exact mirror image as itself because it has that line of symmetry. So then it's not Cairo. Is that cool? Now it turns out that this test is going to be kind of limited, So it's not gonna be the test that we use all the time. It's really Onley useful for rings. Okay? And I'm gonna show you that in a second. Okay, so I've laid out a few different molecules here. What I like you guys to do is just pause the video in between and see if you can draw out a line of symmetry on these molecules, so go ahead and look. Look at a just, you know, A is a three d structure off a ring. It just means that I took my ring and I flipped it a little bit like this. You could see the front in the back. All right, later on, I'll tell you guys what kind of structure that is actually has a specific name, but for right now, just know it's a five numbered ring. Notice that there's two methyl groups, so go ahead and see if you can find an internal line of symmetry. If you can go ahead and draw it with a dotted line.
Unfortunately, this test is only practical for rings.
This is what we call a Hawthorne Projection. It’s a way to visualize rings in 3D. It’s really easy to tell if these molecules have a plane of symmetry or not.
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example
Determining Chirality with Plane of Symmetry
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All right, so let's go ahead and get started. A did have an internal line of symmetry. It was right down the middle like this. Okay, if I split it down the middle, if I cut it in half, I would get to exactly symmetrical halves. So this does have an internal line of symmetry, and that means that this would be a Cairo. Okay, at the end of the day, we're trying to figure out our lease Cairo or at least a Cairo.
This is a 2D ringed structure. It’s super easy to predict a plane of symmetry on this one.
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example
Determining Chirality with Plane of Symmetry
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All right. So it turns out be is an example where I do not have a line of symmetry. Okay, So I'm gonna put here no lying of symmetry. And the reason for that is that no matter where I draw the line, if I draw it here, I'm gonna get to different halves on both sides. One has a chlorine going up. One has a chlorine going down. So that means that would this be Cairo or a Cairo? This would be Cairo. Okay, so you see, how is that is now we can just tell with this test we can see Is this ring Cairo, or is this ring a Cairo Go on to see and if see, if you guys could do the same thing with C, so go ahead.
The plane of symmetry is allowed to split atoms! Just look for any line that can create equal images on both sides.
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example
Determining Chirality with Plane of Symmetry
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All right. So see, it was a little bit tricky to figure out, but it turns out that it actually does have a line of symmetry. Okay, Now, you might be confused because you were trying to draw it like this. If you draw it like that, there's no line of symmetry there. But it turns out there's another way to draw this compound, the line of symmetry, because it has groups facing perfectly opposite to each other. I could actually draw the line of symmetry like this right down the middle. Okay. And what that means is that I know that sounds weird, but I would basically have a C with three inches, right? H h and H where one of those ages should be facing straight out. Sorry, they're in there. Okay. And what we're literally doing is we're saying I'm gonna cut that carbon in half. I'm gonna cut that bond in half and cut that hydrogen in half. I'm gonna get to perfectly symmetrical sides of that carbon now. Is that possible? Obviously not, but I'm just saying that it does have a line of symmetry if you cut it right down the middle of all of that. Okay, so this actually does have an internal line of symmetry. So then the answer is that it's gonna be a Cairo.
Do you think TEST 1 will work well on this molecule? Why or why not?
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example
Determining Chirality with Plane of Symmetry
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all right. So d was a trick question. Why was the I'm sorry. I just had to do this, That you guys would see what I was talking about earlier about rings. Remember that I said that this is really only good for rings. Why is that? Because when you get into chains, thes structures get very difficult to talk. They have a plane of symmetry or not. Yeah, let me show you. So for example, what you were thinking is okay. There's no way to split this this way or this way. There's no way to split it that you're going to get in internal line of symmetry. Okay? But it turns out this molecule actually does have an internal line of symmetry. How? Okay, Yes, it does have an internal line of symmetry. How that's the whole point. It's very, very difficult to visualize, so you would never see it. But what it actually looks like is this if I were to pretend that this is a person and this is my head and these are my arms, okay. And then I have some feet down here, all right? Obviously, I have some, like Cem, bigger issues going on here. All right. I've got, like, two kinks in my back and everything. Let's say I'm laying it down. I'm taking a nap or something. Okay. Well, then I would have a plane of symmetry, because what I could do, Okay, this pillow just doesn't wanna exist. Okay, What I could do is I could take a chainsaw, so it's gonna get really graphic here. I could take a chainsaw and I could light this thing up, Turn it on. So maybe someone really hates me, okay? And I could start cutting down exactly down from my head down and start cutting me into two separate, perfectly equal halves. Alright, So that's messed up. And obviously, that's like a really gory way to communicate the point. Alright. But that just shows you It just illustrates how this molecule is very difficult to tell if as a line of symmetry, but it actually does have a line of symmetry. Okay, so and especially if you start adding more substitue, it's on there and start adding ones that are going up and down. It just turns into a huge mess. So you never want to use the internal line of symmetry test with chains. Okay, I'm just gonna put here. Yes, this is a Cairo, but don't use test one on chains for the exact reason that I was just telling you that if we use it, it's going to get very confusing. And it's gonna be Some of them will even be harder to visualize than this nasty chain saw a graphic that I just gave you. All right, So I hope that makes sense so far that we can use test one on rings to determine if there Kyra Laurey. Tyrell. But now you can see we've got some holes in this test. That's why I told you there's three tests because this test isn't very good. We're gonna need a more robust test that we can use on all types of structures, not just rings. And that's what I'm gonna get into next. So let's go ahead and start
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