This is a bit weird to me. We're doing R and S configurations and I think of it as a steering wheel - steering to the right is an R configuration and steering to the left is an S configuration.
These questions are drawn out as molecules in our book and I listed them going clockwise.
So..
If a chiral center has an H, OH, C-CH3 and CH3 shouldn't it be an R configuration? Why is it S..(according to the solutions manual)
If a chiral center has a H, OCH3, CO2H, HOCH2, then shouldn't that be R as well.. Why is it S?
In the compound Prostaglandin E1 there are 4 chiral centers. Apparently there are 3 Rs and 1 S. I don't understand the S. Shouldn't that be an R?
Is there anyway anyone can explain the priority thing to me because that's where I'm confused when it comes to bigger molecules. Thanks!
Study the references below:
1. For the R/S system in general:
http://en.wikipedia.org/wiki/Chirality_(chemistry)
2. For assigning priorities:
http://en.wikipedia.org/wiki/Cahn_Ingold_Prelog_priority_rules
I understand your confusion regarding the R and S configurations. The R and S designations are used to describe the absolute configuration of chiral centers in molecules. To determine the R and S configuration, we need to assign priorities to the substituents around the chiral center.
The key concept in determining priorities is the Cahn-Ingold-Prelog (CIP) priority rules. These rules are used to assign priorities based on the atomic numbers of the substituents attached to the chiral center.
Here's a step-by-step process to determine the R or S configuration:
1. Identify the chiral center: In each molecule, identify the carbon atom that is bonded to four different substituents. In your first example, it would be the carbon atom with H, OH, C-CH3, and CH3 attached.
2. Assign priorities: To assign priorities, compare the atomic numbers of the atoms directly bonded to the chiral center. The atom with the highest atomic number gets the highest priority (1), the atom with the second-highest atomic number gets the second-highest priority (2), and so on.
3. Break ties: If two substituents have the same atom directly bonded to the chiral center, compare the next atoms until a difference is found. Continue down the substituents until a difference is found. For example, if you have two CH3 groups, compare the next atoms in each: if one of the groups is attached to a carbon atom and the other to a hydrogen atom, the one attached to the carbon atom has higher priority.
4. Visualize the molecule: Orient the molecule so that the lowest priority substituent (4) is pointing away from you. In your case, this would be CH3.
5. Observe the remaining three substituents (1, 2, and 3): If the priorities go clockwise (R configuration), it is an R configuration. If the priorities go counterclockwise (S configuration), it is an S configuration.
Now let's apply these rules to your examples:
Example 1: H, OH, C-CH3, and CH3
If you assign priorities based on the atomic numbers, the order will be: OH (1), C-CH3 (2), CH3 (3), H (4). In this case, the priorities go counterclockwise (S configuration), so it should be labeled as S instead of R.
Example 2: H, OCH3, CO2H, HOCH2
If you assign priorities based on the atomic numbers, the order will be: CO2H (1), HOCH2 (2), OCH3 (3), H (4). Again, the priorities go counterclockwise (S configuration), so it should be labeled as S instead of R.
Prostaglandin E1 with 4 chiral centers:
To determine the configuration for each chiral center, you need to apply the same steps to assign priorities to the substituents around each chiral center. If the priorities go clockwise, it is an R configuration, and if they go counterclockwise, it is an S configuration.
I hope this explanation clarifies the prioritization process for determining the R and S configurations in bigger molecules. It may take some practice, but by following the CIP priority rules and breaking down the steps, you will be able to assign the correct configurations. If you have any specific examples or further questions, feel free to ask!