2-Aminopropane is an achiral molecule, but 2-aminobutane is ch...

Question

2-Aminopropane is an achiral molecule, but 2-aminobutane is chiral. Explain.

Accepted Answer

Welcome everyone. Let's take a look at our next question. It says Y is three amino pentane a chiral. Well, three ameno he is chiral choice A because three amino pentane does not have four different groups attached to the carbon carbon and has a plane of symmetry. While three amino hexane has four different groups attached to the carbon and does not have a plane of symmetry. Choice B because amino three amending has four different groups attached to the carbon and does not have a plane of symmetry. All three amino hexane does not have four different groups attached to the carbon and has a plane of symmetry. Choice C because three amino pentane has four different groups attached to the carbon and has a plane of symmetry. Whereas three amino hexane does not have four different groups attached to the carbon and does not have a plane of symmetry or d because three amino pentane does not have four different groups attached to the carbon and does not have a plane of symmetry. Whereas three amino hexane has four different groups attached to the carbon and has a plane of symmetry. So our first step, of course, will be to draw these two structures. Both have an amino group on the carbon on carbon three, but three amino pentane is a five carbon chain while three amino hexane is a six carbon chain. So it's J pentane 12 345 and then amino hexane will have one more carbon. So extend one more line there. I always double check and count my carbons because skeletal structures are very easily to lose track of carbons in a long chain. For our pentane, we have 12345 carbons for hexane, 123456 that both have an amino group on carbon three. So we'll add NH two, two carbon three of both molecules. And now we have to figure out we know that three AOP pentane is a chiral, well, three amino hexane is chiral. So let's look at these and figure out why. Well, the overall chirality of a molecule is determined by the overall geometry of this molecule. So whether there is a plane of symmetry or not. So when we look at three aopen, it is pretty clearly a symmetrical molecule, we always do have to be careful when looking at models because they're drawn in two dimensions and the molecules exist in three dimensions. We want to make sure we're not being fooled by a symmetry that we see in our two dimensional drawing that is not present in the three dimensional structure. But in this case, it's a fairly straightforward structure. And we can see we have this plane of symmetry that runs through like a vertical line right down the middle cutting that amino group in half. And then you have an ethyl group on either side. So three amino pentane does have a plane of symmetry. But when we look at three amino hexane, the added extra carbon on the right side of the molecule means that it's no longer symmetrical. You have an ethyl group, you have two carbons on the left side, in this case of my structure and three carbons on the right. So no longer does it have a plane of symmetry? So we look at our answer. Choices, we see in choice A, we'll start with plane of symmetry. Choice A says three amino pentane has a plane of symmetry. So that's correct. Three amino exane does not. Choice B says three aane does not have a plane of symmetry. So we know choice B right off the bat is incorrect. Choice. C says three amino pentane has a plane of symmetry. So that's correct while three amino hexane does not, so we can leave that one alone and keep it still in the running. And then choice D says three aane does not have a plane of symmetry. So cross that out as well. So we're down to just choices A and C. So now the other element of our answer is it asks, does it have four different groups attached to the carbon. Well, we know right away that any carbon where there's no substituents in a straight chain like this is not a center of chirality because it will have more than one hydrogen present. So for instance, on our four amino cyclops, I'm going to erase my plane of symmetry here to make this molecule a little more clear to look at. So for instance, if we look at carbon one, and of course, we'll have three hydrogens around it. But let's look at carbon two, it has two bonds, one to each carbon on either side. And so therefore, it still has space for two hydrogens. So any carbon in our chain with no substituents cannot be a chiral center. So the ones we need to focus on then RC three, the carbon that has the amino group on it since it does have this substituent. So each of these carbons has three bonds, it has the bond on either side to the other part of the chain and then it will have hydrogen that isn't indicated in a skeletal structure. So we'll put that on there in each case. So in three amino pentane, let's look at and I'm going to highlight in blue, our carbon three. So we keep focused on what we're looking at. What are the four groups on this carbon. We know it has tetrahedral geometry because it has single bonds only. So our carbon has one hydrogen, it has one amino group. And now let's look at the other parts of the chain. So on the left side, we have ch two, ch three, we have an ethyl group. And on the right side, we also have an ethyl group. So we have two ethyl groups. So therefore, this carbon carbon three is not a chiral center. On the other hand, we look at the three amino hexane, this carbon, I think I'll label them both carbon three just so we keep this straight and on my other molecule, I'm just going to erase that C and I'll relabel it c three. So in three amino hexane, our carbon three has a hydrogen and an amino group just like the other. On the left side. Since it's carbon three, we still have a ethyl group. But on the right side, that's no longer an ethyl group. It has three carmens now. So instead it is a prop group. So this carbon is a center of chirality since it has four different groups attached to it. So going back to our answer, choices, let's look at choice. A, we already looked at the lane of symmetry portion of that answer. And then it says because three amino pentane does not have four amino groups, four different groups, excuse me attached to the carbon while three amino hexane has four different groups. So this is correct. I'm going to put a little dot there and then let's double check on choice C which says three aane has four different groups, but this is incorrect. It does not have four different groups because of those two hydrogens on it. So choice C is incorrect and we confirm choice A is our correct answer for Y three AOPA is a chiral while three amino hexane is chiral. Choice A because the three AOPA does not have four different groups attached to the carbon and has a plane of symmetry. All three amino hexane has four different groups attached to the carbon and does not have a plane of symmetry. See you in the next video.

2-Aminopropane is an achiral molecule, but 2-aminobutane is chiral. Explain.

Key Concepts

Chirality

Stereocenters

Molecular Structure

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