SC4. 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.
- A chiral center is a tetrahedral atom connected to four different groups.
- R and S configurations refer to the three-dimensional relationship of these groups around the chiral center.
- Assignment of R and S configuration follows a well-defined set of arbitrary rules. Anyone can follow these rules and arrive at the same configuration for the same structure.
- About half of S enantiomers rotate light in the (+) direction and about half rotate light in the (-) direction. R and S configurations do not correlate directly with optical rotation values; these are two unrelated systems for describing enantiomers.
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Ball & Stick
Model SC4.1. One enantiomer from a pair of simple tetrahedral carbon compounds, CHBrClF.
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Ball & Stick
Model SC4.2. The other enantiomer from a pair of simple tetrahedral carbon compounds. It cannot be rotated so that it is the same as the first one, but it can be rotated to make a mirror image.
Rules for assigning R & S configuration:
Find the chiral center(s).
Compare the four atoms attached to the chiral center. What are their atomic numbers from highest to lowest?
The highest atomic number has first priority; the second highest has second priority and so on.
Turn the molecule so that the group with the lowest priority is away from you.
Look at the three other groups. If they proceed in a clockwise direction from highest to second to third highest priority; the chiral center has the R configuration.
If they proceed in a counterclockwise direction from highest to second to third highest priority; the chiral center has the 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.
Assign the configurations R and S in the models above.