![]() The carbon on the left is bonded to 3 methyl groups ( C C C), and the carbon on the right is triple bonded to a carbon atom (also C C C). The chiral center is bonded to an oxygen atom (#1) and a hydrogen atom (#4) and two carbon atoms. While all of these approaches work, it has been my experience that undergraduate students tend to make fewer errors using the 3rd method which is why it's the method I present in the video lecture for this lesson. ![]() Once you've assigned R or S to this molecule, know that the original was in the opposite configuration. But with the #4 priority now in a dashed position it will be straightforward to assign R and S. By switching two groups you get the opposite configuration of the original molecule. There are 3 ways to approach correctly assigning R and S in such a scenario:ġ) Try to visualize the molecule in your mind from the correct perspective.Ģ) Rotate the other bond in the plane until the #4 priority group is in a position (dashed or wedged) from which you can more easily assign R and S.ģ) Switch the position of the #4 priority group with the group that has a dashed bond. If you simply make it the opposite you'll once again be correct 50% of the time on average. If you move from priority #1 to #2 to #3 and assign it as is you'll be correct 50% of the time on average. With the bond in the plane, you are not looking at the molecule from the correct perspective but neither is it the exact opposite of the correct perspective either. When the #4 priority group is attached by a bond in the plane you should have yourself a good cry before attempting to assign its configuration as this is the most challenging of the 3 scenarios. Below we'll show examples of how to assign R and Sġ) When the #4 priority group is attached by a dashed bondĢ) When the #4 priority group is attached by a wedged bondģ) When the #4 priority group is attached by a bond in the plane If the #4 priority group is attached by a dashed bond then you're set. To specifically determine the spacial arrangement the rules state that when the #4 priority group is attached by a dashed bond (facing away from you) then a right-handed turn (clockwise) indicates the R configuration, and a left-handed turn (counter-clockwise) indicates the S configuration. The complete rules are summarized in the table above. Once the priorities are assigned the spacial arrangement of these 4 atoms will be in one of two configurations: R or S. ![]() The idea is to assign each of the 4 atoms attached to a chiral center a priority, 1 through 4, based primarily on atomic number. The Cahn-Ingold-Prelog system is a set of rules for assigning R and S (absolute configurations) to chiral centers. ![]()
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