Enantiomers: Definition, Properties & Examples

Enantiomers: Definition, Properties & Examples
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  • 0:00 What Are Enantiomers?
  • 1:15 Properties of Enantiomers
  • 2:58 Chemical Properties
  • 3:29 Opposite Optical Rotation
  • 4:16 Some Examples of Enantiomers
  • 4:45 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Just like how our hands are mirror images of each other, molecules can also have mirror images that cannot be superimposed on each other. These molecules are called enantiomers. In this lesson, we'll discuss their properties and show some examples.

What Are Enantiomers?

Twins
twinenantiomers

Take a look at this pair of identical twins, Steven (who is right-handed) and Kevin (who is left-handed). One day you run into one of them in class, and you think you are looking at Steven. No, you're not. This is Kevin, because he is writing with his left hand. Just like the twins Steven and Kevin, there are pairs of substances or molecules that we think have the same identity because they are mirror images of each other. However, they are in fact different.

These pairs of molecules are called enantiomers. Enantiomers are pairs of molecules that are non-superimposable mirror images of each other. When we imagine a pair of enantiomers, we can compare them to a pair of hands. When we look down at our palms, like in the picture shown below, we can see that they are mirror images of one another. However, if we try to overlap our hands, we cannot superimpose them on each other - our thumbs and pinkies are on opposite sides.

We can also imagine superimposing the molecule on the left and right - they are non-superimposable, no matter how many ways you rotate them. The two molecules are therefore enantiomers.

Properties of Enantiomers

Not every pair of molecules can be enantiomers. They have to have specific properties. What are these properties? We will discuss this concept in the next several sections.

A pair of enantiomers must be a chiral compound, which means it has a chiral carbon. A chiral carbon, or a chiral center, is a carbon that has four different groups attached to it. This picture shows the different types of lines used to illustrate this. If we imagine the chiral carbon lying flat on a piece of paper, the thick, wedged line goes forward and juts out of the paper, while the broken lines go to the back of the paper. The chiral carbon and the first group (1) lie flat on the paper.

Chiral Compound and Example

Even though the compound has a chiral carbon, to be classified as an enantiomer, it must not have an internal plane of symmetry. For instance, in this example, we can see that it actually has two chiral carbons (found on the left and right), as well as an internal mirror plane.

This is not an enantiomer because while it does have a chiral carbon, it is not chiral because it has an internal mirror plane

If you put a plane within the molecule, you can see that left and right side are symmetrical, so this makes the molecule achiral (not chiral). If the molecule is achiral, then it cannot be an enantiomer.

Now that we know the compound is chiral - it has at least one chiral carbon and no internal mirror plane, we can draw the two mirror images and see if they are non-superimposable.

Enantiomers: Non-Superimposable Mirror Images

Let's imagine moving the images towards each other and overlapping them. It will be clear that they are non-superimposable mirror images of each other. Even if we imagine rotating the molecules, there is no way they can be superimposed.

Chemical Properties

A pair of enantiomers shares the same chemical properties but these become different when they are interacting with different chiral substances. Imagine two people shaking hands; they both use their right hands. However, if the other person shook hands using his left hand and the other person uses her right, the gesture would feel very different. Likewise, when a pair of enantiomers reacts with other chiral compounds, they will have different chemical reactions.

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