Return to Contents Chapter 7 : Stereochemistry Ch 7 contents
Enantiomers

If a pair of stereoisomers are non-superimposable mirror images of each other, then they are enantiomers.
Look at the two JMOL images of molecules shown below and make sure you can see this. This example was chosen so that each "group" is a simple coloured sphere.

If you don't know what is meant by non-superimposable mirror images then note that your left and right hands are a pair of enantiomers.
If objects are superimposable, it means you can not tell them apart.  If they are non-superimposable, then you can always distinguish them.

For example, If you look at your hands, you will see that while they are very similar, they are essentially mirror images. They are not identical, because you can always tell which one is which (I hope !)

The two enantiomers of 2-chlorobutane are shown below.


(R)-2-chlorobutane

(S)-2-chlorobutane

 

Questions

You should be able to draw molecules, such as those shown in the JMOL images above, to show that they are mirror images!

A molecule that can exist as a pair of enantiomers has the property of chirality (derived from the Greek meaning handedness), or is described as chiral.  A molecule that lacks this property is achiral.

You should be able to draw the two mirror images of an achiral molecule and then see how if you rotate one of the images the two molecules are actually the same (i.e. that they are superimposable). For example, compare 2-chloropropane and 2-chlorobutane systems :

 
Because the 2 alkyl groups are indistinguishable, the original molecule and its' mirror image are superimposable.
Because the 2 alkyl groups are distinguishable, the original molecule and its' mirror image are non-superimposable.

 
Important properties of enantiomers that you should know
  • Same physical properties except that they rotate plane polarised light in opposite directions (see later).
  • Same chemical properties except when another chiral molecule is involved.
Other things to know:
    A mixture that contains equal quantities of enantiomers is called a racemate or a  racemic mixture.

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  Dr. Ian Hunt, Department of Chemistry, University of Calgary