Determining R and S for Organic MOLECULES
Let’s talk a bit more about stereochemistry. When we need to determine R/S, we know that the lowest priority group needs to be placed in the back (away from us). So, let’s look the following example:
After we assign our priorities, we can see that the lowest priority is in the plane of the paper. How do we get the hydrogen (lowest priority) in the back? In other words, how can we place it where the R3 group is in this example? What we are going to do is spin the molecule on a particular axis. Let’s look at the axis that contains the hydrogen atom:
If we now take this and push the hydrogen into the page, we can get R1, R2, and R3 in the same plane. If you have a model kit, make this structure and rotate it so you can better see what is going on (see post entitled Stereochemistry). From here, we can easily see how to rotate the groups.
However, this plane does not help up move the hydrogen. So, we need to look for another plane. What about the one containing R1?
Just as before, we can rotate (move) the group that is in our plane to the back (into the page). Now, let’s rotate our groups so we place the hydrogen where the R3 group is located:
Finally, let’s take this rotated structure and slowly reposition it to the same orientation as the starting compound:
Here is the overall change:
If we read groups 1-3 in order, we see that it moves in a counterclockwise direction; therefore, the assignment is S.
Additional thoughts
Did we have to look at the plane that contains the R1 group? No, we could have looked at the plane that contains the R2 group as well. All that is important is that we do not chose the planes that contain our hydrogen or the R3 group. This is because we are trying to ‘switch’ the positions of the hydrogen and R3. As we have seen, if we choose a particular plane, the group that is in that plane does not move.