Limonene exists as two different stereoisomers. The R...

Question

Limonene exists as two different stereoisomers. The R enantiomer is found in oranges and lemons, and the S enantiomer is found in spruce trees. Which of the following is found in oranges and lemons?

Structural representations of (+)-limonene on the left and (−)-limonene on the right, labeled accordingly.

Accepted Answer

Hello everyone. Let's do this problem. It says Vale has two stereo isomers. The S in anti occurs naturally identified the S valle among the following structures. So Vale is the commonly accepted name for this compound. But if we write the traditional Iupac name for veiling, it is two amino, three methyl Beano acid and it has the molecular formula C five H 11 N 02. So the first two structures satisfy this molecular formula and this name but compound C and D do not compound C has the molecular formula C 4h nine N 02. So that is not going to be our correct answer in compound D has the molecular formula C two H five and 02. All right. So we've already eliminated half the answer choices just by knowing they are simply not veiling. So now we see that on compound A, the amino group is dashed and on compound B, the amino group is wedge. So we have to check which one has the S orientation. So in order to do this, we're going to have to refer to the K OL pre log nomenclature rules to help us determine if we have R or S orientation. So the first step is to assign priorities to the four different groups attached to that chiral center, right. Chiral centers have four different groups attached to them. So when we assign priorities, we do so according to atomic mass, so the higher the atomic mass of the atom directly attached to that chiral center, then the higher the priority of that whole group. So what if there is a tie between the two first atoms attached to two different groups, right? Or the first two atoms of two different groups attached to that coral center rather. Well, then we simply look at the adjacent atoms that are also connected to that first atom of the group. And we then compare those adjacent atoms using the atomic mass. What if we have a double bond or a triple bond? Well, then we simply count those atoms as being attached multiple times. So let's say if we have a carbon double bonded to an oxygen, we treat that as if there is a carbon single bonded to two oxygen atoms. OK. So once we've assigned all of our priorities, we're just about ready to assign R or S configuration, but it's going to depend on the orientation of our lowest priority group. So is group # four wedge or is group number four dashed. And once we acknowledge this, then we connect groups 12 and three in a circle and see which direction that circle is being drawn? Is it being drawn counterclockwise or is it being drawn clockwise? So we use a different rubric for whether group four is wedge or dash. If group four is wedge and group 12 and three go counterclockwise, then that is our orientation. If they go clockwise, that is S orientation. And the opposite is true for when group number four is dash if group one, groups 12 and three go counterclockwise, that will be S configuration. And if they go clockwise, that is our configuration for when number four is dashed. OK. So let's look at answer choices A and B and determine if that chiral center is R or S. And remember we're looking for the S and anti. All right. So starting with answer choice A I see that our chiral center is going to be on the carbon that has the amino group because we have the amino group A hydrogen, which is not shown here. But that will be wedged right? Since it is on the same carbon and the, the amino group is dashed. And then we have this propal group, right, the isopropyl group and we have a carboxylic acid group. So let's assign the priorities of these four groups. Hydrogen is always going to be number four, right? It has the lowest atomic mass possible. And then we have a nitrogen, a carbon and a carbon while the nitrogen is going to get highest priority. And then we have two carbon atoms. So those are tied. So we need to look at their adjacent atoms. Well, the carbon in the isopropyl group is bonded to a carbon, a carbon and a hydrogen, right where the carboxylic acid group is bonded to an oxygen oxygen oxygen, right, we count this double bond oxygen twice. So since the carbon and the carboxylic acid group is bonded to three oxygen atoms, the oxygen has a higher atomic mass than those carbons in the isopropyl group. So the carboxylic acid group will get second highest priority. The isopropyl group will get third priority. All right. So let's take note of our lowest priority group. The hydrogen is it wedge or dash, it is wedge. So when we connect groups 12 and three in a circle, that circle is going counterclockwise. So we have group number four wedge, the circle is going counterclockwise. So checking our list, that means this center has R configuration. So it looks like that is not going to be our answer choice. So the correct answer should be B let's double check and see if it has s configuration. I'm going to give you a hint. We know it will because the N H two group is wedged. Whereas in answer choice A, the N H two group was dashed. So since those two groups, the hydrogen and the immuno group are switching orientation, I know that the configuration is also going to switch, but I will show you how we can figure that out even if we don't have that hint from answer choice A so N H two is wedge that will be priority number one, we have the dashed hydrogen which will be priority number four. And we have the same other two groups. So those are the same priorities. Carboxylic acid group is second priority. Isopropyl group is third priority. So now since number four is dashed, all we're changing is our rubric that we're using, right. The circle is still going counterclockwise From groups 1 2 and three. So now we have s configuration. So answer choice B is going to be the correct answer to this problem. I'm going to redraw it nice and clear without any writing on it. So we have 1234 carbon chain with a carboxylic acid group on the end, an amino group on carbon number two and it is wedged And a methyl group on carbon three. So B is the correct answer choice for this problem. That's it for this one. I hope this video was helpful and thanks for watching.

Limonene exists as two different stereoisomers. The R enantiomer is found in oranges and lemons, and the S enantiomer is found in spruce trees. Which of the following is found in oranges and lemons?

Key Concepts

Stereoisomerism

Enantiomers

Chirality

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