SC4. Structure in Chemistry


Simple Organic Enantiomers: R and S configurations

Stereochemistry is most important to us because of the role it plays in biology.  Biological compounds usually belong to a class of compounds called organic compounds, originally meaning compounds that come from organisms.  More generally, organic compounds contain carbon.  because carbon is usually a tetrahedral atom, there is great potential for enantiomers in organic compounds, including biological ones.

The compound shown below contains a carbon connected to a hydrogen, a bromine, a chlorine and a fluorine.  A tetrahedral atom connected to four different things is called a chiral center; it is a place where two different arrangements are possible. 

Figure SC4.1.  A pair of simple organic enantiomers.

This is not likely to be a naturally-occurring compound, although some marine organisms do make organic compounds containing bromine and chlorine.  However, compounds similar to this one are sometimes used as general anaesthetics for surgical purposes.  It is used as an example here because the chiral center is easy to see.

Figure SC4.2.  Ball-and-stick model of (R)-bromochlorofluoromethane.

Go to Animation SC4.1.   A three-dimensional model of (R)-bromochlorofluoromethane.


Figure SC4.3.   Ball-and-stick model of (S)-bromochlorofluoromethane..

Go to Animation SC4.2.   A three-dimensional model of (S)-bromochlorofluoromethane.


Rules for assigning R & S configuration:

Note:  If two atoms connected to the chiral center have the same atomic number, you have to use a tie-breaker to decide which one gets higher priority.  The tie-breaking rules are descirbed on the next page.

Problem SC4.1.  Assign the configurations R and S in the models above.  Assume red is bromine, bright green is chlorine and pale green is fluorine.

Problem SC4.2.  Which of the following compounds have enantiomers?  If they exist, draw them.


Circle the stereocentres in the following compounds.

Problem SC4.4

Circle the stereocentres in the following compounds.


This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License

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