Chiral Molecules & Ions: Definition, Identification & Examples

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In chemistry we often refer to chiral molecules. In this lesson we will learn what a chiral molecule is and how to identify a chiral molecule and chiral centers.

What Is Chiral?

Take a pair of gloves, and examine them. The two gloves look identical in every aspect, except if you try to put the left-hand glove on your right hand it just isn't going to work. No matter how you rotate that glove, it isn't going to fit comfortably on your right hand. So, although the two gloves look identical, they are actually opposites - they are a perfect mirror image of each other. They are also non-superimposable, which simply means that even though they are mirror images of each other, they can't be stacked on top of each other or they can't be rotated to be the exact same shape.

Even though gloves appear identical they are non-superimposable
Gloves

In chemistry, when a molecule can have an exact mirror images of itself and be non-superimposable it is referred to as chiral. Keep in mind that the molecule (or the ion in the molecule) itself is chiral, but when you're referring to the mirror image of a chiral molecule you call it an enantiomer. Sometimes it gets confusing to distinguish between the two terms, enantiomer and chiral, but chiral simply refers to any molecule that can have a non-superimposable mirror image of itself, while enantiomer is referring to how two molecules compare to each other.

Non-Chiral

In order to understand and identify chiral molecules, sometimes it's helpful to identify some molecules that are NOT chiral. First, let's look at things around us. We've already identified that gloves are chiral, but what about socks? If you held up a pair of socks they could be a mirror image of each other. Yet, you can still rotate the two socks to stack on top of each other - it doesn't matter which foot you put each sock on. So socks are not chiral.

Even though socks are mirror images of each other they are non-chiral because they are superimposable
Socks

Let's look at this molecule:


Non chiral molecule


And now let's look at the mirror image of this molecule:


Mirror image


These two molecules can be superimposed on each other. If we simply rotate the mirror image, we end up with the original molecule. Thus, they are super-imposable:


By rotating the image, we see that we end up with the starting molecule
Rotate image


Looking at this molecule, we can see that the carbon is bonded to an oxygen (red dot), a nitrogen (blue dot), and two carbon atoms (grey dots). A chiral ion will never be bonded to two of the exact same things, so that is the first thing to look out for: Does the carbon have 4 unique connections?

Chiral Centers

In order to determine if a molecule is chiral, we need to look at each ion or atom and determine if it is a chiral center. A chiral center is a carbon atom that is chiral at that point. If a molecule has even one chiral center, then it is a chiral molecule. But a molecule can have more than one chiral center, so we need to examine every carbon atom.

Let's look at a larger molecule now:


Chiral center molecule


We need to examine each carbon to determine if it is a chiral center.

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