R and S
R and S Nomenclature
A method is needed to help distinguish between mirror images of a stereocenter. For instance, both of these enantiomers would be named 2-bromobutane.
But, since these two enantiomers are different compounds, they need to have different names. Robert Sidney Cahn, Sir Christopher Ingold, and Vladimir Prelog developed a system of rules to differentiate the names of compounds like these. The Cahn-Ingold-Prelog system is used to assign a letter (R) or (S) to each stereocenter of a compound based on its 3D orientation.
How is it done? Each stereocenter is considered all by itself.
1. Assign a priority to each group on the stereocenter. 1 is the highest priority while 4 is the lowest priority.
a. We begin by looking at the first atom of the group. Higher atomic numbers are a higher priority. If the atoms are different isotopes of the same atom, the heavier isotope is a higher priority.
b. If the first atoms of two groups are identical and there is a tie, we look at the next atoms along the chain in the group to break the tie. We keep going until the first place where the tie is broken.
In this example, we begin by looking at the first atoms attached to the stereogenic carbon. H is obviously the lowest priority and is assigned 4. The first atoms of the other three groups are all carbon, so there is a tie. Therefore, we look at the next atoms. For the methyl, CH3, the next atoms are three hydrogen atoms that are attached. For the ethyl, the next three atoms attached to the carbon are one carbon atom and two hydrogen atoms. For the isopropyl group, the next atoms are two carbon atoms and one hydrogen atom. This breaks the tie. (C,C,H) > (C,H,H) > (H,H,H), so the priorities of 1, 2, and 3 are assigned respectively.
c. We count double bonds as being two atoms and triple bonds as being three atoms when assigning priority.
In the example, we have a tie between three carbon atoms when we look at the first atoms of the groups. When we go to the next atoms, the methyl group has three hydrogen atoms. The ethene (C=C) group has two carbon atoms (one for each bond of the double bond) and one hydrogen atom. The ethyne (CC) group has three carbon atoms (one for each bond of the triple bond).
2. Take the 3D representation of the molecule and put the lowest priority group (#4) in the back. This makes groups 1-3 face you. Draw an arrow from 1 to 2 to 3. If the arrow is a clockwise arrow (to the right), then we assign an R to the stereocenter. “R” comes from the Latin word Rectus-“upright”). If the arrow is a counterclockwise arrow (to the left), we assign an S to the stereocenter. “S” comes from the Latin word Sinister-“Left”). Another way to think about it is if you turn the steering wheel of a car in the direction of an (S) arrow, the car would turn left. If you turn the steering wheel of a car in the direction of an (R) arrow, the car would turn right.
Group 4 in the back
The situation is very easy if group 4 is already in the back. We can draw an arrow from 1 to 2 to 3 and see if it is an S arrow or an R arrow.
Group 4 in the front
If group 4 is in the front, it is still fairly easy. There are three ways you can do it. Pick your favorite.
1. One way to tackle this problem is to rotate the molecule so group 4 is in the back. Then the arrow is drawn to determine (R) or (S). This takes a bit of mental gymnastics, though.
2. Another way to tackle the problem is to take yourself, in your mind’s eye, for a walk behind the page. From behind the page, the molecule would appear to have group 4 in the back. You can look at the arrow from that position and it would appear as an (R) arrow. This also takes a bit of mental gymnastics.
3. Probably the easiest way to tackle this problem is to leave the molecule as it is with group 4 in the front. Draw the arrow from 1 to 2 to 3. In this case, it is an (S) arrow. Since group 4 is in the front instead of the back where it should be, the arrow needs to be flipped. We know it is really an (R) arrow.
Group 4 to the side
If group 4 is to the side, it is the most difficult situation. Here are two ways to figure it out.
1. One way is to rotate the molecule. For simple molecules, this is possible, but for bigger more complicated molecules, it is more difficult.
2. A second way is to take a walk in your mind’s eye. Position yourself so that group 4 is in the back and take a look at the arrow. In this case, the arrow goes from 1 at the top to 2 in the back to 3 in the front. This is an (R) arrow.
Carefully think through the following compounds and make sure you see why they are named the way they are.
Example:
4. Find the stereocenters in the following molecules and label them as R or S.
a)
b)
c)
d)
e)
f)
g)
h)
Answers
4.
