Test 1:Plane of Symmetry - Online Tutor, Practice Problems & Exam Prep

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.

How and when to use the internal line of symmetry test.

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.

Determining Chirality with Plane of Symmetry

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?

Determining Chirality with Plane of Symmetry

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.

Determining Chirality with Plane of Symmetry

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.

Determining Chirality with Plane of Symmetry

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

Here’s what students ask on this topic:

What is the internal line of symmetry test in chirality?

The internal line of symmetry test is a method used to determine if a molecule is achiral. If a molecule has an internal line of symmetry, it means that one half of the molecule is a mirror image of the other half. This symmetry indicates that the molecule is not chiral, as it does not have a non-superimposable mirror image. This test is particularly useful for cyclic compounds, such as rings. Identifying an internal line of symmetry helps in understanding the stereochemistry of the molecule and its behavior in chemical reactions.

How do you identify an internal line of symmetry in a molecule?

To identify an internal line of symmetry in a molecule, follow these steps: 1) Draw the molecule in a way that clearly shows all its atoms and bonds. 2) Visually inspect the molecule to see if you can divide it into two mirror-image halves. 3) If such a division is possible, draw a dotted line along the plane where the molecule can be split into two identical halves. This dotted line represents the internal line of symmetry. If the molecule can be divided this way, it is achiral.

Why is the internal line of symmetry test limited to certain types of molecules?

The internal line of symmetry test is limited because it is most effective for cyclic compounds, such as rings. This is because rings often have symmetrical structures that make it easier to identify a plane of symmetry. For acyclic (non-ring) molecules, the test is less useful because these molecules can have more complex and less symmetrical structures, making it difficult to find an internal line of symmetry. Therefore, other methods are often needed to determine chirality in such cases.

Can a molecule with an internal line of symmetry be chiral?

No, a molecule with an internal line of symmetry cannot be chiral. Chirality requires that a molecule have a non-superimposable mirror image, meaning it cannot be identical to its mirror image. An internal line of symmetry indicates that the molecule can be divided into two identical halves, making it superimposable on its mirror image. Therefore, such a molecule is achiral.

How does the internal line of symmetry test relate to stereochemistry?

The internal line of symmetry test is crucial in stereochemistry because it helps determine whether a molecule is chiral or achiral. Chirality is a key concept in stereochemistry, affecting how molecules interact with polarized light and participate in chemical reactions. By identifying an internal line of symmetry, chemists can classify molecules as achiral, simplifying the analysis of their stereochemical properties and predicting their behavior in various chemical contexts.