imgur>com?4wBfp>jpg (change ? to / and > to . )
In the picture above, why does the second stereocenter have an S configuration and not an R? Based on priorities / atomic weights:
OH (left) = 1
CH2OH (bottom) = 2
CH3 (top) = 3
H (right) = 4
Since the lowest priority group, H, is horizontal, the configuration is the reverse of my answer: Original answer is S and then reversed it to R. But this answer is wrong... why?
Well, looking at your priorities, I would say it's because life can be full of surprises! Just when you think you've got it all figured out, chemistry comes along and throws you a curveball. It's like that saying, "Expect the unexpected." In this case, the unexpected is that the configuration is actually S and not R. Chemistry loves to keep us on our toes, just like a clown riding a unicycle! So, even though your initial answer got reversed, don't be too hard on yourself. Sometimes, chemistry just wants to have a little fun and keep us guessing.
Based on the priorities/atomic weights provided:
OH (left) = 1
CH2OH (bottom) = 2
CH3 (top) = 3
H (right) = 4
To determine the configuration of a stereocenter, you need to assign priorities to the substituents based on atomic number. In this case, the OH group has the highest priority (1), followed by CH2OH (2), CH3 (3), and H (4).
To determine the configuration, you need to imagine looking down the bond between the stereocenters. If the substituents are arranged in a clockwise direction, it is an R configuration. If they are arranged in a counterclockwise direction, it is an S configuration.
In the case of the picture provided, you correctly assigned the priorities to the substituents. However, you made a mistake in determining the configuration. Since the lowest priority group, H, is horizontal (pointing towards you), instead of reversing your answer from S to R, you should keep the original answer of S.
Therefore, the correct configuration for the second stereocenter in the picture is S, not R.
In order to determine the configuration of a stereocenter, we use the Cahn-Ingold-Prelog (CIP) priority rules. The CIP priority rules assign priorities to the substituents attached to the stereocenter based on their atomic numbers.
According to the CIP rules, we assign a priority of 1 to the substituent with the highest atomic number, a priority of 2 to the second highest, a priority of 3 to the third highest, and a priority of 4 to the lowest.
In the given example, let's assign the priorities as follows:
OH (left) = priority 1
CH2OH (bottom) = priority 2
CH3 (top) = priority 3
H (right) = priority 4
Now, we need to orient the molecule so that the lowest priority group (H in this case) is pointing away from us. However, if the lowest priority group is already in a horizontal position, we need to visualize the molecule from the opposite side to determine the configuration correctly.
In this case, the lowest priority group, H, is already in a horizontal position. So, we need to mentally flip the molecule and observe it from the opposite side. When we do that, the OH group, which had the highest priority of 1, will move to the right, and the CH3 group, which had the third highest priority of 3, will move to the left.
After flipping the molecule and observing it from the opposite side, the priorities would be:
H (left) = priority 1
CH2OH (bottom) = priority 2
CH3 (top) = priority 3
OH (right) = priority 4
Now, if we apply the CIP priority rules again, we can see that the molecule has an R configuration rather than an S configuration. This means that the original answer of S configuration was incorrect.
So, to summarize, the error in assigning the S configuration initially was due to not considering the need to mentally flip the molecule when the lowest priority group is in a horizontal position. By mentally flipping the molecule and observing it from the opposite side, we can correctly determine that the second stereocenter has an R configuration.