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

Question

Prioritize the substituents at each chiral center and then, by comparing them to the models you created in Section 6.3.2, label the absolute configuration as R or S.

(a)

Accepted Answer

All right. Hello everyone. So for this question, let's go ahead and determine the priorities for each substituent in the given compounds Cairo center and predict its absolute configuration R or S. So lets go ahead and get started. Recall first and foremost that a chiral center is an atom commonly carbon that has four different groups or four different substituents attached to it. So if we go ahead and consider the structure provided the only carbon atom or the only atom in general that's connecting to four different groups is this carbon atom at the very center of our structure. So I'm going to go ahead and encircle it in a dash so that we can identify it. So now lets go ahead and talk about the order of priorities and therefore the absolute configuration recall that the absolute configuration cannot be determined without first talking about priorities. So to understand priorities, let's go ahead and recall some basic rules for the K in gold preo nomenclature system or C IP. For short, the first and the most important rule to recall in the C I system is that the atomic number of the atom directly connecting to the Cairo center is what's primarily going to determine its priority, the higher the atomic number, the higher priority it's going to have. So let's consider what substituents are connecting to our Cairo center. Namely we have chlorine hydrogen, an ethyl group and a ring structure. Now, two of these substituents contain carbon which has an atomic number of six hydrogen has an atomic number of one and chlorine has an atomic number of 17. So therefore, chlorine is going to have the highest priority and hydrogen is going to have the least priority or lowest priority. And this is because it has the lowest atomic number. But now we've approached a sort of challenge, right? Because now we have to distinguish which is 2nd and 3rd highest priority amongst two substituents that both contain carbon. So at this point, we have to recall a few other rules in the C IP system, namely that when the first Adams are the same, the next set of Adam specifically along the chain of the substituents is compared. And so if necessary, this process gets repeated. Until you find the first point of difference. As soon as you can determine priority at that point of difference, then you've arrived at your answer and you finished analyzing those particular substituents. So what does it mean when we talk about a point of difference? Well as you go along a substituent chain, you can come across different atoms that have a higher atomic number than others in the other substituent. However, there's another rule that we can consider which in this case involves quote unquote Phantom Adams. This describes scenarios where double or triple bonds are featured in substituents. Now, a double bond is counted twice and a triple bond is counted three times. So for example, draw your attention to a moment for a moment to the ring structure. We have one carbon atom directly connecting to the asymmetric center or the Cairo center. This particular carbon atom is double bonded to one carbon and single bonded to another carbon within the rink. Because of this double bond, the double bond counts as two separate bonds to carbon. So the double bond, as well as the other single bond means that this particular carbon inside of the ring is counted as if it is connecting to three other carbons. Whereas the carbon atom of the EO group is only connecting to one other carbon. And so this right here is our point of difference. Our ring structure is going to have the second highest priority simply because it is connecting to more carbons, which has a higher atomic number than hydrogen. So I'm going to go ahead and label that right. Our ring structure is second priority or second highest and our eo group is third highest priority. So now we can talk about R or S. So at this point forwards now that the order of priorities has been determined what we're going to do now is trace the path or the track from the highest priority group to the lowest priority group. Now, if the lowest priority group is in the back, then the path that's been traced is going to directly decide the configuration. In this case, we have two possibilities. We have R and we have S so if the lowest priority is in the back and the path that we've been tracing goes in a clockwise direction, then that corresponds to the R configuration. By contrast, if this path goes in a counterclockwise direction, it is going in the S configuration. Now, the way that I like to remember this and again, this is only if the lowest priority is in the back is by imagining if I were to actually write the letters R and S for example, when I write the letter S in the very beginning, at least I'm going in a counterclockwise direction, whereas in a capital R, I'm slightly going in a clockwise direction. So here it just so happens that hydrogen, which is the lowest priority is in fact in the back. So I'm going to go ahead and trace a path going from my first to my least priority. And when I do so I can see that this is going in a clockwise direction. So this is in fact an example of our configuration and there you have it. So here is our final answer. We've decided our priorities in order of one through four. And we've determined that the absolute configuration is, in fact, R so with all that being said, if you stuck around until the end of this video, thank you so very much for watching. And I hope you found this helpful.

Prioritize the substituents at each chiral center and then, by comparing them to the models you created in Section 6.3.2, label the absolute configuration as R or S.
(a)

Key Concepts

Chirality

Cahn-Ingold-Prelog Priority Rules

R and S Configuration

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