Absolute Configuration and Stereochemical Nomenclature

- On the previous page, we referred to the two stereoisomers of 2-bromobutane, simply as "A" and "B". Chemists have devised a systematic way of naming stereoisomers, based on the concept of absolute configuration.

- The absolute configuration around a chiral center is designated as either "R" or "S".
These designations come from the Latin for left and right handed...

(R) from rectus for right-handed and (S) from sinister for left-handed
- The (R) or (S) designation is added to the IUPAC name of the compound as follows:
The absolute configuration around the chiral center in Isomer A is the "S" configuration. Thus, this compound is (S)-2-bromobutane.

Isomer B has the opposite or "R" configuration. This compound is (R)-2-bromobutane.
-The absolute configuration around a stereocenter is determined using the Cahn-Ingold-Prelog rules (named after the chemists who defined them).

Rule #1 -- Identify all four substituants at the chiral center under examination

Rule #2 -- Assign priorities to substituants by looking at the atoms 
           attached directly to chiral center. 

           The higher the atomic number the higher priority (use atomic mass for 
           isotopes of same element).

           In assigning priorties, follow each substituant away from the chiral 
           center until a point of difference is found.

Rule #3 -- Orient molecule with the lowest priority group pointed away from you.

Rule #4 -- Trace the remaining three groups numerically (i.e. from highest to
           lowest priority).

           If the trace goes to the right (or clockwise) the absolute 
           configuration is (R).

           If the trace goes to the left (or counterclockwise) the
           absolute configuration is (S).
- It is probably easier to show an example than to explain. So consider the two isomers of 2-bromobutane.
Identify the substituants on the chiral center (Rule #1) and assign priorities (Rule #2):

Rotate molecule so that the lowest priority group (the -H in this case) is in the back (Rule #3) and trace the remaining three substituants in numerical order (Rule #4).

Here are the "live" images of the two compounds, rotate them until the -H on the stereocenter is in the back.

Isomer A: should look somethig like this:

Since the trace goes counter clockwise, this is the (S) isomer.

Isomer B: should look something like this:

Since the trace goes clockwise this is the (R) isomer.

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