Test 1:Plane of Symmetry - Online Tutor, Practice Problems & Exam Prep
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Chirality can be tested using the internal line of symmetry test, which identifies achiral molecules that possess a mirror image identical to themselves. This test is particularly useful for cyclic compounds, such as rings. If a molecule has an internal line of symmetry, it is not chiral. Understanding chirality is essential in stereochemistry, as it influences the behavior of molecules in reactions and their interactions with polarized light.
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 chirality, and the first one is one that you already know, so let's go ahead and get started with it. The test is called test 1, and it's the internal line of symmetry test. What we want to do is 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 an achiral molecule. You guys remember that? That basically means that it has the exact same mirror image as itself because it has that line of symmetry, so then it's not chiral. Is that cool?
Now, it turns out that this test is going to be kind of limited. So it's not going to be the test that we use all the time. It's really only useful for rings. Okay? And I'm going to show you that in a second. Okay? So I've laid out a few different molecules here. What I'd 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. Okay. So go ahead and look. Just so you know, is a three-dimensional structure of a ring. It just means that I took my ring and I flipped it a little bit like this. You can see the front and the back. Alright? Later on, I'll tell you guys what kind of structure that is. It actually has a specific name. But for right now, just know that's a five-numbered ring. Notice that there are 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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Alright. So let's go ahead and get started. It did have an internal line of symmetry. It was right down the middle like this. Okay? If I split it down the middle, I cut it in half, I would get 2 exactly symmetrical halves. So this does have an internal line of symmetry, and that means that this would be achiral. Okay? At the end of the day, we're trying to figure out are these chiral or are these achiral?
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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Alright. So it turns out there is an example where I do not have a line of symmetry. Okay. So I'm going to put here no line of symmetry. And the reason for that is that no matter where I draw the line, if I draw it here, I'm going to get two different halves on both sides. One has a chlorine going up. One has a chlorine going down. So that means, would this be chiral or achiral? This would be chiral. So see how easy that is? Now we can just tell with this test. We can see is this ring chiral or is this ring achiral. Go on and see if you guys can do the same thing.
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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Alright, so 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. And 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 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 carbon (C) with three hydrogens (H) right? H, H and H, where one of those H's should be facing straight out. Sorry, there and there. Okay? And what we're literally doing is we're saying I'm going to cut that carbon in half, and I'm going to cut that bond in half and cut that hydrogen in half, and I'm going to get two 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 going to be achiral.
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 it was a trick question. Why was I sorry? I just had to do this so that you guys would see what I was talking about earlier regarding rings. Remember that I said that this is really only good for rings. Why is that? Because when you get into chains, these structures get very difficult to tell if they have a plane of symmetry or not. Let me show you. So, for example, what you're 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 an 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 and then I have some feet down here, obviously, I have some bigger issues going on h