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.]
(d)

(d)

Accepted Answer

All right. Hello everyone. So for this question, let's go ahead and prioritize each substituent in the given compound chiral center and predict its absolute configuration R or S. So lets go ahead and get started. Recall first and foremost that a chiral center or stereo center refers to an atom commonly carbon that is connecting to four different groups or substituents. So four different substituents or four different groups now featured in the starting compound is a four member ring. And the only chiral center present in this compound is the carbon atom that lies on the bottom right corner of this four member ring. So I'm going to go ahead and circle it in dashes. So now the question becomes, what are the priorities of my substituents? And what is the absolute configuration? Well, before we can decide on the absolute configuration, we have to discuss substituent priority first. So let's go ahead and recall some can in gold Prel nomenclature rules otherwise known as the C I system. Recall it the first and most important rule of the CIP system is that at the atomic number decides primarily the priority of that substituent. And I'm specifically referring to the atomic number of the atom directly connecting to the Cairo center, the higher the atomic number, the higher the priority. So in this case, let's go ahead and consider the atoms directly connecting to our Cairo center. In this case, two of our substituents involve carbon. One of them involves the oxygen inside of the ring and the other involves hydrogen. Now, out of all of these oxygen is going to have the highest atomic number. Therefore, it receives highest priority. By contrast, hydrogen has the lowest atomic number. So it receives the lowest priority. But now the question is how are we going to distinguish between two substituents where carbon is directly connecting to the Cairo center? Well, to do. So let's go ahead and recall a few more rules in the C I system. If in situation where the first atoms are the same, the idea is to go ahead and find the first point of difference. So what does this mean? Well, when the first atoms of substituents are the same, the idea is to compare the next set of adjacent atoms and repeat this process until you can find a difference in the substituents themselves, for example, a different atom. So once that point of difference is established, you can determine the priority of those substituents based on that point of difference. Now, sometimes a point of difference can involve multiple bonds, for example, a double bond or a triple bond. In that case, a double bond is counted twice and a triple bond is connecting three times or counts three times. So, with these principles in mind, let's go ahead and recall or rather examine the two substituents that are not yet ranked. On one hand, we have a methyl group and on the other hand, we have another carbon inside of the four member ring. Now, even though carbon is connecting to the Cairo center in both substituents here, we can find a first point of difference by moving on to the next set of adjacent atoms. The methyl group in this case, only has the three hydrogens of the methyl group itself. Whereas on the other hand, this other carbon inside of the ring is connecting to yet another carbon directly next to it. This is our point of difference because carbon has a higher atomic number than hydrogen. The carbon atom inside of the ring is going to have higher priority because of this point of difference. So the metal group in this case is going to be third highest priority whereas the carbon atom inside of the ring is second highest priority. So now we can go ahead and talk about absolute configuration. Now, the first thing I want to point out here is that the lowest priority group, in this case, the hydrogen is in the back. So the rules that I'm going to describe right now apply in this specific scenario where the lowest priority group is already in the back in this case, what we're going to do right now is trace a path from the highest priority to the lowest priority depending on the direction of that path or rather its rotation that is going to distinguish between R and S configuration. Now, I like to distinguish them by imagining myself writing these letters when I talk about or rather when I find myself writing the letter R, especially in capital for a brief moment, I'm rotating in a clockwise direction. So if I'm tracing a path that goes in a clockwise direction, that corresponds to the R configuration. By contrast, when I'm writing the letter S for a brief moment, I'm actually going in a counterclockwise direction, meaning that if my path goes in a counterclockwise rotation that corresponds to S configuration. So when I go ahead and I trace my path from my first to my lowest priority, I can see that I'm doing so in a counterclockwise rotation. So according to the system just described, this corresponds to the S configuration, meaning that that is our final answer. And there you have it here is the priority order for all of our substituents. And additionally, according to this order, we were able to determine that this compound is drawn in the S configuration or rather its absolute configuration is S So with that being said, if you stuck around until the end, 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.]
(d)

Key Concepts

Chirality and Chiral Centers

Cahn-Ingold-Prelog Priority Rules

Absolute Configuration

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