Star (*) each asymmetric carbon atom in the following examples...

Question

Star (*) each asymmetric carbon atom in the following examples, and determine whether it has the (R) or (S) configuration.

(a) Molecular structure showing an asymmetric carbon atom with groups labeled for R and S configuration determination.

(b) Structural formula of a chiral carbon atom with attached groups, indicating R and S configurations.

Accepted Answer

All right. Hello everyone. So this question is asking us to consider the following compounds of which we have two encircle each asymmetric carbon atom in the compounds and determine their absolute configuration. That's R or S configuration. OK. So first and foremost, recall that an asymmetric carbon atom is a carbon atom that is connected to four different substituents. And when a carbon atom has four different substituents based on the priority order of those substituents, we can determine R or S configuration. First, the priority order of a given substituent is determined by the atomic number of the atom directly connecting to the chiral center or the asymmetric carbon. The higher the atomic number, the higher priority that group has. Now in cases where there's a tie between the atomic number, we would compare the atomic mass. However, what you most commonly encountered in organic chemistry is cases where the directly attached atoms are the same in more than one substituent. In this case, you would search for or find a point of difference between those substituents that you're comparing. So if there's a tie between the same atom, you would compare the next set of adjacent atoms until you find a point of difference between those substituents, double bonds count twice and triple bonds will count three times. So let's start with our first compound over here and I'm going to circle the carbon in the center because that is the carbon atom connecting to four different substituents. So, branching off of this asymmetric center, we have an amino group, a methyl group, an isopropyl group and a hydrogen. Now immediately hydrogen is going to be the fourth or lowest priority because it's going to have the smallest atomic number. On the other hand, nitrogen with an atomic number of seven would have the highest atomic number of all of the substituents present, which means that it's going to have highest priority. So that means there's currently a tie between the methyl group and the isopropyl group. Since both of these have carbon atoms directly connecting to the asymmetric center, this is where we find a point of difference. Now, when we consider the isopropyl group, this carbon is attached to two other carbons and one he ch on the other hand, our methyl group, specifically, our methyl carbon is only connecting to three hydrogens. So because the isopropyl carbon is connecting to other carbon atoms that have higher atomic numbers than hydrogen, that is going to break the tie, meaning the isopropyl group has second highest priority and the methyl has the methyl group as third highest priority. Now here R or S configuration be can be determined according to the position of the lowest priority group. In this particular case, in the case of part one here, our lowest priority group is hydrogen and hydrogen is in front. So with this in mind, let's trace a path from the first highest priority to the third highest priority in doing. So I notice that this path is going in a counterclockwise direction. So when the, when the lowest priority group is in the front and our path is going in a counterclockwise direction. This indicates our configuration. By contrast, if we're going clockwise, that would be s configuration when the lowest priority group is in the front. So here for part one, our asymmetric center is in the R configuration. So now let's go ahead and move on to part two. Now, once again, our carbon atom in the center is our asymmetric carbon. So thats what I am going to circle here. However, hydrogen, which would be our lowest priority group is neither in the front nor the back. So to make determining configuration a little bit easier, let's go ahead and redraw the structure by swapping our substituents because here it just so happens that chlorine is on a dash. A methyl group is on a wedge and both the isopropyl group and hydrogen are on the plane. So I'm going to swap the chlorine and the hydrogen and I'm also going to swab the methyl and isopropyl groups. This means that I would redraw the structure so that the isopropyl group is on a wedge, the methyl group and chlorine are on the plane and hydrogen would be on a dash pointing away from us. So from here, let's discuss priority, chlorine would have the highest atomic number. So it's first priority how Eugene would have lowest priority. Our isopropyl group would be second highest priority and our methyl group would be third highest. From here, we can now trace a path from the first to the third highest priorities. So after redrawing our second structure hydrogen, which is our lowest priority group is in the back and our path is being traced in a clockwise direction, excuse me, counterclockwise. So what hydrogen, the lowest priority group is in the back and we are going in a counterclockwise direction that is actually s configuration this time. So here our first compound was in the R configuration and our second is in the S configuration and there you have it. So here are the answers for parts one and two. And so with that being said, thank you so very much for watching. And I hope you found this helpful.

Star (*) each asymmetric carbon atom in the following examples, and determine whether it has the (R) or (S) configuration.
(a)
(b)

Key Concepts

Chirality

R and S Configuration

Cahn-Ingold-Prelog Priority Rules

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