One of the many tasks that you will be required to master is determining the absolution configuration of a molecule, (R) and (S). Why do we have the R and S system? Have a look below. How would you characterize these molecules?
There is only one difference in the 2 structures, stereochemistry! It turns out that this is the structure of the drug called thalidomide and it was the result of a huge tragedy. You can watch a video on the thalidomide tragedy here. It turns out that one enantiomer has sedative effects and was hailed as the best thing for morning sickness while the other enantiomer has teratogenic effects (severe birth defects). So, we can definitely see that stereochemistry is important. With this importance, we need a way to differentiate one enantiomer from the other and we do that with assigning (R) and (S) or the absolute configuration.
What does R and S mean in chemistry? We get (R) from rectus (right in latin) and (S) from sinister (meaning left in latin). Ok, so now that we know a little bit about (R) and (S), let's talk about how we make our assignments. The first thing we need to do is to locate an element's atomic number. We get that by looking at the periodic table and looking at the number above the element symbol, as you can see from the picture below.
Once we locate the atomic number of each group connected to a chiral carbon, we rank them from high to low with the highest getting ranked 1 and the lowest getting ranked 4. Once the ranking is complete we can trace around the molecule as you can see below. If we go in a clockwise direction, then we get the (R) configuration (you can think of this as turning your steering wheel to the right). If we go in the counterclockwise direction, we get the (S) configuration. Note that for this to work, we need to make our assignment with the lowest priority group (group 4) in the back. View the picture below to check it out.
