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Chiral vs. Achiral: Definition & Examples

Chiral vs. Achiral: Definition & Examples
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  • 0:00 The Mirror Test
  • 0:41 What is Chirality?
  • 2:32 What is a Chiral Molecule?
  • 3:33 A Chirality Center
  • 4:48 Lesson Summary
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Lesson Transcript
Instructor: Nancy Devino
Identifying chiral and achiral objects, including molecules, requires looking at many pictures, which is what you'll do in this lesson. You'll also learn the definitions of these terms and use the definitions to distinguish between chiral and achiral molecules.

The Mirror Test

When you were a kid, did you ever try to put your shoes on the wrong feet? It probably didn't feel very good! That's because the left shoe was designed to fit the left foot and the right shoe for the right foot. The same is true of gloves; one fits the right hand and the other fits the left. For example, if you're left handed, it's tough to share a baseball glove with someone who's right-handed.

Now, imagine rotating these two gloves so that the ball pockets are facing one another. It's as if one glove is looking in the mirror at the other glove. Did you know that certain pairs of molecules have this same mirror image relationship?

What Is Chirality?

The idea that left hands or feet and right hands or feet are mirror images has a word, chirality, which means the property of handedness. As we can see in the photograph, when placed beside each other, the left hand is the mirror image of the right hand.

Hands

But in order to be chiral, or possess the property of handedness, an object must be non-superimposable on its mirror image. As shown on the right side of the photograph, when you lay one hand directly on top of the other, facing the same direction, they don't match! That's what we mean by chiral.

So how do you know if an object is chiral or achiral? Well, the prefix a- means without, so achiral objects lack some key feature of chirality. If an object can be superimposed on its mirror image, then it's achiral. Socks are a good example, or at least, plain socks are. A pair of socks can be placed so that one looks like the mirror image of the other. But flip one sock around and lay it on top of the other, and they're a perfect match. The socks are achiral.

It turns out there are chiral and achiral molecules, just like there are chiral and achiral objects. Take a look at bromochlorofluoromethane and its mirror image.

Is bromochlorofluoromethane chiral or achiral? To figure this out, we need to see if its mirror image can be superimposed on the original. Imagine holding onto the green chlorine atom in the molecule on the left in the picture above. Now, suppose you were to spin or rotate the molecule around the Cl-C bond clockwise. You'd end up with the molecule on the left.

Since the molecule is not superimposable on its mirror image, this molecule is chiral.

What Is a Chiral Molecule?

Short of looking at every molecule in a mirror and rotating it to see if it's chiral, how else can you tell if a molecule has this property? The answer is pretty simple; a carbon atom that contains four different groups will be chiral. We call that atom a chiral center, or chirality center. When two of the groups on a carbon atom are the same, that carbon is not a chiral center. In this molecule, called 2-butanol, carbon two has four different groups: an ethyl group, a hydrogen atom, a hydroxyl group, and a methyl group.

Let's compare 2-butanol to butane, where carbon 2 has two hydrogen atoms. At first, it looks like we have a mirror image situation. But look at the third image! If we rotate one of the structures to retrieve the original molecule, then the molecule is superimposable on its mirror image. The requirement for chirality is that a molecule not be superimposable on its mirror image, so 2-butane is achiral.

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