R/S nomenclature

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R configuration S configuration

Carbon (gray) with four substituents (labeled by priority 1-4) shows an R-configuration because when substituent 4 is in the back, substituents 1 - 3 run clockwise.

Carbon (gray) with four substituents (labeled by priority 1-4) shows an S-configuration because when substituent 4 is in the back, substituents 1 - 3 run counterclockwise.

R and S are used to describe the stereochemistry at an sp3 carbon center with four different substituents. First, each substituent is assigned a priority. Then, the priority 4 substituent is placed facing away from the viewer (as in the initial scenes), and the rotation going from 1 to 2 to 3 is considered. If it is clockwise, the configuration is (R), if counterclockwise, (S). You can rotate the model on the left and the right. It is impossible to orient them in a way that they superimpose perfectly. Thus, they are different molecules. If you inspect them closer, you will see that they are like image and mirror image, i.e. they are enantiomers (if we assume that the substituents are all different and are themselves achiral as they appear here).

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Karsten Theis

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