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Article ID: 135
Last updated: 25 Jan, 2013
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Check out this interactive stereochemistry tutorial on the ChemEdDL

Molecules that are non-superimposable mirror images are called enantiomers, just look at the two molecules shown below.  The term enantiomers always refers to a relationship between two compounds.  Note that your left and right hands are a pair of enantiomers.  No matter how much you try, these two structures (or your hands) will never superimpose.  If you believe you can superimpose them, you've likely made a structure drawing error.

Example of enantiomers: (R)-2-chlorobutane and (S)-2-chlorobutane


Enantiomers are known as chiral molecules (derived from the Greek meaning hand), therefore, a molecule that is superimposable on its mirror image is said to be achiral.  Notice how the molecule on the left below, which does not contain a chiral or stereogenic center, can merely be rotated where the far right and far left structures are identical (or superimposable).  Simply rotating a molecule does not change its structure in any way.

Also important to know: a mixture containing equal quantities of enantiomers is called a racemate or a  racemic mixture. Enantiomers are characterized by the presence of a stereogenic or chiral center, see the article link below.

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Department of Chemistry & Chemical Biology