Optical Isomers: Definition & Examples

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  • 0:00 What Are Optical Isomers?
  • 1:26 Stereoisomers
  • 2:05 Structural Isomers
  • 2:44 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Isomers are molecules that are the same...yet different. The difference might be in the structure, or it might be in the three-dimensional arrangement. In this lesson, you'll learn about one specific type of isomer that falls into the latter category, the optical isomer.

What Are Optical Isomers?

Take a good look at your two hands. They are very similar. They each have 4 fingers and a thumb, they're each connected to an arm, and there's one on each side of your body. But they're also different, because one is your right hand and the other is your left hand. If you tried to put one on top of the other; or superimpose them, it wouldn't work. Your thumb on one hand would be where your pinky is on the other. And this is because even though they are both hands, they are mirror images of each other.

Your hands are very much like a special type of isomer. Isomers are organic molecules that have the same chemical formula but different arrangements of their atoms. And optical isomers are isomers that are mirror images of each other. Like your hands, they come in pairs and are not superimposable. The different forms of optical isomers are called enantiomers, and the central carbon atom that holds each molecule together is referred to as being asymmetric. Also, just like your hand has four different fingers, this type of molecule has to have four groups in addition to that central carbon atom, and they must be different from each other as well.

Ok, so where did the name 'optical' come from? Well, these isomers got their name from how they rotate plane polarized light, or light waves that vibrate in only one direction. One enantiomer rotates it clockwise or to the right, and the other rotates it counterclockwise, or to the left.


Optical isomers are one of two types of stereoisomers. These are molecules that have different spatial arrangements, like we saw in the enantiomers before. They have the same structure and formula, but differ three-dimensionally ('stereo' means 3D').

The other kind of stereoisomer is a geometric isomer. These are different versions of organic molecules with a restrictive carbon-carbon double bond. That double bond is called 'restrictive' because it restricts rotation. Basically, if you can rotate the molecule so that it looks different, it's not actually an isomer. But, if you have to break the double bond to get the molecule to rotate, then it is in fact an isomer.

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