Configuration and the R-S Convention

When referring to a particular enantiomer, we would like to be able to specify which configuration we mean without having to draw the structure. A convention for doing this is known as the R-S or Cahn-lngold-Prelog system. Here is how it works. 

The arrangement of four groups attached to a stereogenic center is caiied the configuration of that center. 

The atoms directly attached to the stereogenic center are ranked according to atomic number the higher the atomic number, the higher the priority. 

If one of the four groups is H, it always has the lowest priority, and one views the stereogenic center looking down the C—H bond from C to H. 

Assign a priority order to the following groups —H, —Br, —CH2CH3, and —CH2OCH3. 

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