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Definitions: Chiral


A molecule is chiral if it is not superimposable on its mirror image. Most chiral molecules can be identified by their lack of a plane of symmetry or a center of symmetry. Your hand is a chiral object, as it does not have either of these types of symmetry.



The molecule on the left has a plane of symmetry through the center carbon. This is a mirror plane; in other words, one half of the molecule is a perfect reflection of the other half of the molecule. This molecule is not chiral because of its mirror plane.

The molecule on the right has a center of symmetry, or an inversion center. An inversion center is a point in the molecule - not necessarily on an atom - through which all other atoms can be reflected 180 degrees into another, identical, atom. (In more accurate symmetry terms, an inversion through a center is equivalent to rotating groups by 180 degrees and then reflecting the groups through a plane perpendicular to the rotation axis.) This type of symmetry is rare in organic molecules, and is more common in inorganic molecules. The inversion center is represented by the blue circle in the above example. The same molecule is shown three-dimensionally below. The inversion center is in the center of the middle carbon-carbon bond. This molecule is not chiral because of its inversion center.

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