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Identifying chirality centers

How to determine which atoms in a molecule are chirality centers. Created by Jay.
Video transcript
Voiceover: Once you understand the concept of chirality, the next skill is to be able to identify a chirality center. So, I'm using the term "chirality center" here, but you also might hear "chiral center" or "stereo center" or "stereogenic center" or "assymetric center," and they're all pretty much referring to the same concept: a tetrahedral carbon, sp3 hybridized carbon that has four different groups attached to it. So, let's look for some chirality centers in these molecules, and we'll start with this alcohol here. So I'm going to redraw this. I'm just going to draw out all of the atoms here. So we have four carbons. The carbon on the left has three hydrogens attached to it. So there's no way that's a chirality center. I need four different groups, and I have three of the same thing on that carbon, so that one is not one. This next carbon here, we have an OH, and then we also have a hydrogen attached to it. So that's this carbon over here. And this is a chirality center. We have four different groups attached to this carbon, so I'm going to mark this carbon right here. This is a chirality center. There's a methyl group on this side, so that's one different group. There's an OH group. There's an ethyl group, and then there's also a hydrogen. So draw in that hydrogen. There are four different groups attached to that carbon, so that carbon is a chirality center. So this carbon right here is a chirality center. I'm going to draw in the other hydrogens. So I have two hydrogens on this carbon, so that's not a chirality center. I have two of the same thing bonded to this carbon. And then finally this carbon over here with three hydrogens, three of the same thing, so that carbon is not a chirality center. So, one chirality center in this alcohol. For our next example, let me go ahead and draw in the carbons, so we have three carbons. And the carbon on the left has three hydrogens bonded to it. So that carbon is not a chirality center. Same with the carbon on the right, three hydrogens, so there's no way it could be a chirality center. Let's focus in on this carbon right here. So let's think about the hybridization of that carbon. We know from earlier videos, that's an sp2 hybridized carbon with trigonal planar geometry. So immediately you know that it's not a chirality center. That has to be sp3 hybridized, giving you a tetrahedral geometry, and we don't have four different things bonded to that carbon, so none of these carbons are chirality centers. So there are zero chirality centers in this molecule. So that's acetone. Let's do a ring example. Let's do this ring example right here. Let me draw it out on the molecule this time. So we have a hydrogen here. We have two hydrogens on this carbon. Two hydrogens on this carbon all the way around our rings. Let me draw in all these hydrogens. Then we look for chirality centers or chiral centers. All of these carbons, let me highlight them in magenta. All of these carbons have two hydrogens bonded to them, so that's two of the same thing. So, there's no way those are going to be chiral centers. What about this carbon right here? It looks like it might be a chirality center. We have a chlorine bonded to it, a hydrogen, and then we have these things going in opposite directions. But in actuality, this is not a chiral center, and that's because there's the same path around this ring here. So the hydrogen is like one different group, the chlorine is another different group, so that's two different groups. But if you go around the ring this way, and you go around the ring this way, it's the same path both ways. You hit a CH2, and you hit a CH2. You hit a CH2, and you hit a CH2, and then you hit a CH2. So it's the same path around the ring. It's like two of the exact same groups bonded to that carbon. Another way of thinking about this is if I focus in on that carbon at the top again, so I have a hydrogen here and a chlorine here, and I draw in a molecule like that. So, I took out this last carbon down here, so I'm leaving out the last carbon. One way to think about it is, that's two of the same things attached to this carbon. Here's an ethyl group, and here's an ethyl group. So, two of the same thing attached to that carbon. So this is not a chiral center. There are zero chirality centers for this molecule. However, if we change things up, so let's look at this molecule now, we have a different path around the ring. So once again, we're focused in on this carbon because we know we have a hydrogen bonded to that carbon here. So the paths around the ring, there's a different path around the ring. Let me draw in the hydrogens once again. There are two hydrogens on this carbon, only one hydrogen on this carbon. If you go to the left around the ring, you hit a CH2 right here. If you go to the right around the ring, you hit a carbon bonded to only one hydrogen. So, it's a different path. It's like there are four different things attached to this carbon right here. So this carbon is a chirality center. If we look at some of these other carbons, let me use red for this, if we look at this carbon right here, this carbon is sp2 hybridized. There's a double bond there, so it's not a chirality center. Same with this carbon. This carbon has two hydrogens bonded to it. This carbon has two hydrogens bonded to it. This carbon has two hydrogens bonded to it. So there's only one chirality center in this molecule. Now let's finally do one more example which just looks a little bit more challenging than the ones we've been doing. It's a little bit scarier looking. This is the ibuprofen molecule. Let's go through this one by one, but let's start with this benzene ring here in the center. Let's just start with this carbon right here. This carbon is sp2 hybridized, and as you go around the benzine ring, all of these carbons are sp2 hybridized. So that immediately means that these can't be chiral centers. Need to be a type of tetrahedral geometry to be a chirality center. So those carbons are all out. Let's look at this carbon right here. I know that there are two hydrogens bonded to that carbon, so that carbon is out. Let's look at this carbon. Well, I know that this carbon is bonded to two methyl groups, which is two of the same thing. So that carbon is out. Let's look at this carbon. I know there are three hydrogens bonded to that carbon, so that one's out. Same with this carbon. So, these three are the same thing. So none of these carbons over here, on the left side of the molecule, are chirality centers. Let's go over here. Let me use a different color for this carbon right here. I know that there's only one hydrogen bonded to this carbon. I'll just put it in right here. So let's think about the different groups. I have a methyl group right here. I have a hydrogen. I have this carboxylic acid over here. Then I have my benzene ring over here in the rest of the molecule. So that's four different groups attached to this carbon. This carbon that I've marked in blue here, is a chirality center. So this is a chiral center. Go ahead and mark it. Let's focus in on this carbon on the methyl group. There are three hydrogens, so that's not a chiral center. This carbon right here has a double bond to it, so it's sp2 hybridized. So that cannot be a chirality center either. So, we have only one chiral center in this molecule. This is a very important skill to practice. To look at the carbons, think about what's bonded to them, and if there are four different groups bonded to that carbon, and it's a tetrahedral carbon, sp3 hybridized, then we can call it a chirality center or a chiral center, or whatever term your professor wants to use.