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Definitions: Optical Isomers

Optical isomers are molecules that differ three-dimensionally by the placement of substituents around one or more atoms in a molecule. Optical isomers were given their name because they were first able to be distinguished by how they rotated plane-polarized light. These molecules are not necessarily locked into their positions, but cannot be converted into one another, even by a rotation around a single bond.

For example, consider the following two molecules.

In the molecule on the left, the chlorine is oriented upward, and in the molecule on the right, the chlorine is oriented downward. (These molecules are presented in Wedge-Dash Notation, which will be covered in more detail in a later section in the tutorial. To learn about this notation now, click here. To navigate back to this page from the Wedge-Dash information page, use the Back button in the browser, not within the tutorial.)

These structures can be viewed from a different angle, using Newman Projections, as shown below. (Converting between two different types of structural representations is complicated and will be covered in detail in a later section. For now, simply note that in the above structures, the chlorines were opposites in the sense of being "up" and "down," and now they are opposites in the sense of being "left" and "right.")

By viewing the animation below and comparing it to the stationary structure, it can be seen that the structures are never identical at any point throughout the rotation around the central carbon bond. Therefore, these molecules are optical isomers.

There are several different types of optical isomers. In order to better understand these, some general terminology should be introduced first.

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