R

R & S Configurations

To properly name an enantiomer, the R and S configurations must be included. These configurations are used to distinguish molecules from each other and to assign a more in depth nomenclature.

R and S configurations begin with a Stereogenic center, which is a carbon molecule that has 4 different molecules bonded to it. The molecules have to be different and the carbon cannot be attached to a double bond. There can be several stereogenic centers in one molecule as well.

For example, the drug Taxol, an anti-cancer medication, has 11 stereogenic centers.

Each of these centers is going to have either an R-configuration or an S-configuration. R stands for Rectus and S stands for Sinister.

To assign a configuration, one must give priority to each molecule surrounding the stereocenter. It goes from highest priority(usually R group) to the lowest priority(usually H group). The molecules with the higher atomic masses will assume highest priority and the lower masses will assume lower priority. If two molecules are the same like CH3 and CH2, CH2 will take higher priority because of the higher total atomic number. However, if they’re the same then you move to the mext molecule beyond both and whatever has the higher atomic number will take priority.

For example,

Do you see how the biggest R group will take 1st priority and then the rest follow accordingly? Also, with one switch of the Br and Cl, the entire molecule switches from R to S. Notice how on the S group above that it seemingly goes 1,2,4,3. That is only true because you follow the direction from 1 to 2 and keep going even if the next in line isn’t the 3rd priority, also notice how R is clockwise and S is counterclockwise.

*Exception: when there is a Hydrogen group in the molecule and it is in the behind position, the entire molecule will switch configurations. This is explained in depth below:

This molecule only has one stereocenter, the carbon on the left. The priority order is assigned as such: 1. NH2 2. C 3. CH3 4. H.

Therefor, this is a S configuration. *However, if the H were in front, rather than the back, then the molecule would switch to a R configuration.