Basic Organic Nomenclature
7. Stereochemistry (iii)
|So there are only ever two enantiomers, therefore
requiring only two names to distinguish them. The two designations
are R and S, used in parenthesis in front of the compounds name in the
same fashion that E and Z are used for alkenes.
To determine whether a particular isomer is the R or the S isomer, the Cahn-Ingold-Prelog priority rules are used in the same way that they are used to determine E or Z for alkenes. For a review of these rules, see the page on alkene nomenclature. In the case of a chirality carbon atom, there are four groups attached and so priorities 1 (lowest) to 4 (highest) must be determined.
As a review make sure that you see the priority order in the following compounds:
Priority order is:
Priority order is:
Identifying the R or S isomer.Once the four groups/atoms around the chirality centre have been prioritized, the R or S prefix can be determined as follows:
Worked examplesFollow through the examples.
Prioritize the groups on the chirality centre:OH (4), C2H5 (3), CH3 (2), H (1).
The lowest priority group in this example is the H atom. Rotate the model so that the hydrogen is pointing away from you:
Now trace the motion as you go from the highest priority group (the hydroxyl) to the second highest (the ethyl) and finally to the third highest (the methyl). In this case the motion is clockwise (to the right) and so the enantiomer is designated R.
The full name for the isomer shown is (R)-butan-2-ol.
Prioritize the groups: yne (4), ene (3), CH3 (2), H (1).
The lowest priority group in this example is the H atom. Rotate this model so that the hydrogen is pointing away from you:
Now trace the motion as you go from highest priority to lowest in
the three remaining groups. That is from yne through ene to methyl.
The resulting motion is anticlockwise or to the left: the isomer is
the S enantiomer.
Examples for you to work on.The following structures are named. Make sure that you obtain the same name by following through the process as given above.
To repeat it is:
ExercisesTry the following exercises.
Review More the One Chirality Centre.
Next page: Naming: More than One Chirality Centre.
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