About this video
This video explains chirality using the test of superimposability. An object is achiral if its mirror image can be rotated onto the original, as with a coffee cup or difluoromethane, and chiral if it cannot, as with human hands and a bromochlorofluoromethane-type molecule. A carbon bonded to four different groups is a chirality centre, and it is usually the reason a molecule is chiral.
You'll be able to
- define achiral and chiral objects using superimposability of mirror images
- test whether an object or molecule is chiral by rotating its mirror image
- identify a chirality centre as a carbon with four different substituents
Key words
- Achiral
- An object whose mirror image can be superimposed on the original.
- Chiral
- An object whose mirror image cannot be superimposed on the original, like a left and right hand.
- Superimposable
- Able to be rotated or moved so that it matches another object exactly.
- Mirror image
- The reflection of an object in a mirror, which is the comparison used to test chirality.
- Chirality centre
- A carbon atom bonded to four different groups, which typically makes a molecule chiral.
Transcript
Voiceover: Achiral objects are objects that are superimposable on their mirror images. And in a minute, I'm gonna show you that a coffee cup is an example of an achiral object. Chiral objects are objects that are not superimposable on their mirror images. And the word "chiral" comes from the Greek word for "hand".
And so I'm gonna show you how your hands are not superimposable on each other but your left and right hand are mirror images of each other. Let's take a look at a coffee cup reflected in a mirror. So, you can see on the left, here is the actual coffee cup. In the mirror is the mirror image of the coffee cup.
And I'm gonna pull out the coffee cup to make some space. And I'm gonna put what I saw in the mirror, the mirror image, right next to it. Here I have another coffee cup, so that's it's mirror image. And I'm going to take the coffee cup on the right and I'm going to rotate it.
And so, as I rotate it, you can see that it is superimposable with the object on the left. So the mirror image is superimposable. And that's the definition of an achiral object. So, we say that a coffee cup is achiral. Now let's try the same thing with a molecule.
So, this is difluoromethane. So, the green are the fluorine atoms. And in the mirror, you can see the mirror image. So, once again, I'm going to pull the molecule away and put what I saw in the mirror, the mirror image right next to it. And I'm going to rotate the mirror image.
So the one on the right, I'm going to rotate it to see if it's superimposable with the one on the left. And so, I take it and as you can see as I rotate, right there you can see that it is superimposable with the molecule on the left. And since the mirror image is superimposable, we say this is an achiral molecule.
So these are actually two of the exact same molecules represented here. Now let's look at my hands. So, my left hand and in the mirror you can see my right hand or what looks like my right hand. So I'm gonna take my left and my right hand together. We just showed that they are mirror images of each other.
Then I'm going to try to rotate my right hand to see if it's superimposable with my left hand. So you can see here, I have my palms both up but my thumbs are not pointing in the same direction. So they're not superimposable here. So I'm going to try again, I'm going to try to rotate it.
So now my thumbs are in the same position but my palms are not in the same position. So no matter what I do, I can never superimpose my right hand on my left hand. So, the mirror image is not superimposable upon the original object. So no matter what you do, you cannot accomplish this.
And since the mirror image is not superimposable, we say your hands are chiral. Now, finally let's take a look at a molecule. So the white is hydrogen, green is fluorine, red is bromine and yellow is chlorine. So the molecule is on the left and in the mirror is the mirror image.
So I'm gonna pull out that molecule and once again leave some space. And put what I saw in the mirror right next to it. So there's the mirror image. So I have these two mirror images of each other. I take the one on the right and I try to rotate it to see if I can superimpose it with the one on the left.
And so here I just rotated it a little bit there. And you can see that the red, the bromine atoms are in the same position. However, the chlorine and the fluorine, the yellow and the green are not in the same position. So I cannot superimpose it here. I try again and I rotate it some more.
And you can see the yellow is in the same position, the chlorine and also the hydrogen is. But, the red and the green are not. And so, no matter how I rotate the mirror image, the one on the right, I can never get it to look like the one on the left. And since the mirror image is not superimposable, we say that this is a chiral molecule.
And this carbon here, this is a very important carbon, with four different substituents attached to it, four different groups. We call this a chiral center or a chirality center.
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