Explain why compound A has two stereoisomers but compounds B a...

Question

Explain why compound A has two stereoisomers but compounds B and C exist as single compounds.

Chemical structures of three compounds labeled A, B, and C, illustrating stereoisomerism and stereocenters.

Accepted Answer

Hey everyone. And welcome back for the three compounds shown below. Explain why the first compound has two stereo isomers. While the second and the third compounds correspond to no stereo isomers. The three structures that we are given all have central nitrogen atom. The first structure has four different substitutions. The second one has three different substitutions out of four because we have two equivalent metal groups. And the third one has three different substitutions as well as a loan pair. So now let's consider them, as we said, the first structure has four different substitutions, right? Because they are all different, we can state that nitrogen in that structure is chiral because chirality requires us to have four different substitutions at a central atom so that it will have two stereo is or basically a pair of in an tumors which are non superimposable mirror images of each other. So that means it has a pair of inland tumors. Now, for the 2nd 1, the central nitrogen atom is not chiro because it has two equivalent methyl substitutions. And as we previously said, we want to make sure that the structure has four different substitutions comparing it to the previous one, the four different substitutions are hydrogen, our metal group ethyl group and our aldehyde. So we can say that the first structure which has chiro gen also corresponds to an asymmetric center. And now let's think about the 3rd structure. Now, the third structure is a bit more complex to think about. We have three different substitutions which are our methyl group, the aldehyde. And then what about the fourth group, which essentially is a lone pair of electrons, it's not an actual bond, it's just a lone pair of electrons. So even though this structure is chiral, that's really important to understand because it's not super imposable with its mirror image, but it will have no stereo isomers, which we also concluded about the previous structure. So let's understand why that happens. Now, if he some troll nitrogen with its long hair within its orbital and then our three substitutions. So we're going to draw one of them, the other one And the 3rd 1. Now we clearly see that the metal group is pointing up. So we will change the way we have added our bond. So let's fix that. We're going to draw an aldehyde on a solid line. So that's how we C O group, right, our Carneal with an implicit hydrogen atom. And then we also have our muscle group pointing up and then we have pointing down. Now we're going to consider a process called pyramidal or nitrogen inversion. And we will understand why no stereo isomers can exist. Or basically, they cannot be resolved. We will illustrate that inter conversion as follows. So there's a fast inter conversion in which the long pair of electrons from the hybrid orbital jumps into a P orbital of nitrogen. So there's a moment at which nitrogen becomes trigonal planar instead of being tetrahedral. In other words, we can draw nitrogen wet our three substitutions in a trigonal planar arrangement. So we are going to add our metal group as well as hydrogen. And then we are going to draw our orbitals. Now, this transition state exists for a very short time. We are going to label it as a transition state. And then we have to consider that the long pair of electrons from the p orbital of nitrogen can then enter our hybrid orbital from the opposite side. So we're going to show that process by nitrogen with a long pair at the opposite side. So that now we will get a mirror image of the first structure. And we understand that indeed this structure is chiral because the two mirror images are non superimposable, but they are not resolvable in stereo isomers because it exists as a single structure due to that process called the inter conversion or nitrogen inversion. So that would be the whole explanation. Now we are ready to conclude our answer. So let's think about this problem in general. Now the word first structure has a chiral center, it has four different substitutions so that we have, we can have a pair of an anti for the first structure. So we can label that the first structure has four different substitutions. That's our answer. That's why it can have a pair of enamors. For the second one, it does not have four different substitutions and therefore, it is not chiral. And for the 3rd 1, it undergoes nitrogen inversion, which we have discussed. So even though the structure is chiral, due to the fact that there is nitrogen inversion, we cannot resolve the structure into a pair of in an tumors. And those will be our final answers. Thank you for watching and I hope to see you in the next video.

Explain why compound A has two stereoisomers but compounds B and C exist as single compounds.

Key Concepts

Stereoisomerism

Chirality

Geometric Isomerism

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