Prioritize the substituents at each chiral center and then, by...

Question

Prioritize the substituents at each chiral center and then, by each of the two methods discussed in Section 6.3.2.4, determine the absolute configuration. [Do not use your models, except to check your answers.]
(c)

(c)

Accepted Answer

OK. Hello everyone. So for this question, let's go ahead and assign priorities for each substituent in the compounds or excuse me in the given compounds chiral center and predict its absolute configuration R or S. So let's go ahead and get started a chiral center in a given compound, otherwise known as an asymmetric carbon is a carbon atom that has four different groups attached to it so that it has a non superimposable mirror image. Now analyzing the given structure, the only carbon here that is bound to four different groups is the carbon in the ring here that lies on the bottom right corner. So now that we know where the Cairo center is, let's go ahead and talk about some rules for the RS nomenclature system recall that priority is primarily determined by the atomic number of the atom directly connecting to the Cairo center. So let's go ahead and assess what our substituents actually are relative to our asymmetric carbon. So first we have an atom of chlorine. Next, we have an oxygen atom inside of the ring structure. We have a methyl group outside of the ring and we have another carbon from the ring itself attaching directly to the Cairo center. Now of all of these substituents, the highest priority is going to fall on chlorine because chlorine is going to have the highest atomic number. Out of all the substituents we have available to us. Chlorine specifically has an atomic number of 17. So I'm going to go ahead and label chlorine as number one being our highest priority. So at this point, the question is to find the second highest priority now that chlorine has been decided as the first. Now out of the remaining three substituents, removing chlorine from the list because we've already decided its priority. We have one substituent containing an oxygen and two others containing carbon. Now, oxygen has an atomic number of eight, whereas carbon has an atomic number of six. So because of this, the oxygen inside of the ring has a higher atomic number than the other two substituents and therefore, is our second priority here. So now the question is to decide between the 3rd and 4th highest priorities out of the two substituent here that contain carbon. Now, the problem here is that both of these substituents have the same atom connecting directly to the Cairo center. So the question is which one is going to break the tie, so to speak? No starting off at the carp in here, that's directly attaching to the Cairo center. The idea is to branch further down the substituent to see which atoms have a higher atomic number as you go farther away if you need to. So at this point, directly, excuse me, directly connecting to the Cairo center are two carbon atoms. So at this point, I would consider what atoms those carbons are attaching to to see which has the highest atomic number. So if I consider the methyl group for a second, this carbon right here is attached only to hydrogen which has an atomic number of one. On the other hand, the carbon inside of the ring that's connecting to the Cairo center is connected to yet another carbon, right? So because this other carbon has a higher atomic number than hydrogen, then the carbon atom inside of the ring is going to take priority over the methyl group in this case. So I'm going to go ahead and label the carbon inside of the ring as number three and the metal group as number four. And what I'm going to do also is I am going to go ahead and circle the Cairo center in dashes. Let me go ahead and do that. Here we go. OK. So, so far, so good, right. At this point, we've established our priorities from first to least. So now the question becomes, how do we determine or distinguish between R and S configuration? Well, to do this, you have to imagine tracing a circle from the highest priority group to the lowest priority group. So long as the least priority is in the back or behind you. So depending on the direction of rotation, in that case, you can distinguish between R and S configuration. No, I like to imagine this as imagining if I were to write the letters R or S respectively. Because when I consider the letter R, I can imagine going slightly clockwise as I go ahead and write this letter out. So because of this, if I'm going in a circle from the first to the least priority in a clockwise direction, then that is R configuration. Now, on the other hand, when I begin to write the letter, si go slightly counterclockwise at the very beginning. So similarly, if I'm going in a clockwise rotation, sorry counterclockwise, then that is S configuration. So in this case, considering the fact that the methyl group which is the least priority is pointing downwards, I can trace my path from the first to the least priority from the top of the molecule structure to ensure that, that the least priority stays behind me. So when I do this, I can see that as I go from the first to the fourth priority, I am going in a clockwise direction. So because of this, the configuration is going to be R and there you have it. So here is our final answer. Our chiral center. In this case was the carbon atom inside of the ring that lies on the bottom right corner, we have labeled our priorities. And after doing. So we determined that this compounds Cairo center is in the R configuration. So with all that being said, thank you so very much for watching. And I hope you found this helpful.

Prioritize the substituents at each chiral center and then, by each of the two methods discussed in Section 6.3.2.4, determine the absolute configuration. [Do not use your models, except to check your answers.]
(c)

Key Concepts

Chirality and Chiral Centers

Cahn-Ingold-Prelog Priority Rules

Absolute Configuration

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