Test 1: Plane of Symmetry

The simplest test for chirality is symmetry. If a molecule has an internal line of symmetry, it is achiral

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

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Video Transcript

In this chapter, I'm going to give you guys three different ways to test for chirality. The first one is one that you already know. Let's go ahead and get started with it.
The test is for – it's called test one and it's the internal line of symmetry test. What we want to do is we want to look at these compounds and see if 1they 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. Do 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 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. I'm going to show you that in a second.
I've laid out a few different molecules here. What I would 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.
Go ahead and look at (a). Just so you know, (a) is a 3D structure of a ring. It just means that I took my ring and I flipped it a little bit like this so you can see the front and the back. 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 it's a five-membered ring. Notice that there's two methyl groups. 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.

Example: Does the following molecule contain a stereogenic center? Is it chiral?

 

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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. 

Example: Does the following molecule contain a stereogenic center? Is it chiral?

 

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This is a 2D ringed structure. It’s super easy to predict a plane of symmetry on this one. 

Example: Does the following molecule contain a stereogenic center? Is it chiral?

 

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The plane of symmetry is allowed to split atoms! Just look for any line that can create equal images on both sides. 

Example: Does the following molecule contain a stereogenic center? Is it chiral?

 

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Do you think TEST 1 will work well on this molecule? Why or why not?