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Definitions: Geometric Isomers

Geometric isomers are molecules that are locked into their spatial positions with respect to one another due to a double bond or a ring structure.

For example, consider the following two molecules.

In the ring on the left, the methyl groups are on the same side of the ring (cis), and in the molecule on the right, the methyl groups are on opposite sides of the ring (trans). These are geometric isomers because the ring structure will not allow these molecules to interconvert. This can be seen in the animation below. Focus on the blue atoms: They start on the same side of the ring, and no matter how the bonds rotate, the blue atoms are always on the same side of the ring.
(Note: In a ring structure, groups are cis if they are both above or both below the ring. Here, this is represented by having both groups wedged (up). Groups are trans if one group is above the ring and the other group is below the ring; therefore, one group is wedged and the other is dashed. This will be shown in a more realistic, three-dimensional, way in a later section. To learn more about this notation in rings now, click here. To navigate back to this page from the Cyclic Wedge-Dash information page, use the Back button in the browser, not within the tutorial.)

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