 Okay, so let's try this problem here. It's just as identify the stereocenter as stereocenters in the following molecule And label them by using an asterisks. So kind of like what we did over here We found the stereocenter. We put an asterisks. Okay, so remember. What is the stereocenter? Carbon or any atom with four different groups on it. Okay, so it's going to be a tetrahedral arrangement Okay, so do we have any of those here for different groups? Okay, so there's two that are kind of good candidates, right? Hopefully everybody sees that those two. Let's Just draw the hydrogens implicitly in there. Okay, so We'll show that this molecule has hydrogen. Okay, everybody's okay with what we've done so far, right? Okay, so when we look at this group here, what is that? Can anybody tell me? CH2 or a methylene group, right? So that definitely does not have four different groups on there because there's two hydrogens. Everybody's good with that, right? Same with this one, same with this one. Everybody's okay there, right? Okay, so what I Want you to know is that even though this is a methylene, this is a methylene. These two groups are different, okay? And I'm going to prove that to you. Okay, so in other words if I'm going to label the stereocenters That's one there, and that's one there. Okay, so now let's prove to ourselves that the two are different Okay, so Clearly the chlorine and hydrogen are different and clearly they're different than a carbon or a carbon. Okay, so we start there Now we look, okay Is there a difference between these two groups? This group's CH2 and this group's CH2 So there's no difference right now, okay? So since there's no difference right now, we have to move to the next carbon down the chain, okay? So no difference, no difference, now we're at this carbon, okay? And this carbon, okay? This one's a CH2. What is this one here? CHCl, is that different than a CH2? Yes, so these two groups are different, okay? So that's a stereocenter there, okay? And the same logic applies going the opposite way. Is everybody okay with that? So you have to you have to just accept that that's the case, okay? They are different Now some of us wanted to do R&S on this one, okay? This molecule even though it has stereocenters is not chiral, okay? Why? Because it's a mesocompound, got that internal plane of symmetry So what happens is the R and the S stereocenter actually cancel each other out So remember what I said chiral doesn't mean has a stereocenter. It means that it rotates plane polarized light, okay? Something that rotates plane polarized light will have a stereocenter in it usually, or it will, okay? As far as working What happens here is that these are opposite and Equal and opposite in rotation so they cancel each other out, okay? So it's like this is the positive. This is the negative or vice versa, okay? So now we have to identify well, which one's the R and which one's the X, okay? Because one's one and one's the other. So how do we identify that? Well first we put the smallest Group to the back, okay? Which we already have the hydrogen is in the back. We'll do this one first, okay? So now we go, okay? We have to prioritize the remaining three groups, okay? As one, two, and three, okay? So the first group, the biggest group is chlorine, okay? Why do I say that? Because I look on the periodic table, chlorine has the highest atomic weight out of the three remaining atoms carbon, carbon, and chlorine, okay? Is everybody okay with what I've done there? So that's number one, okay? Now we go to the second and third groups and try to identify which one's bigger. Well carbon and carbon, they're the same So we got to go to the next one, okay? Carbon and carbon, they're the same So we got to go to the next one. Carbon and chlorine, well, chlorine's bigger than carbon So this is two and that's three like that, okay? Is everybody okay with what I did there? So now we're going to just make a little arrow, okay? So going from one to two to three like that. So is that clockwise or counterclockwise? Counterclockwise? So that's sinister, S, okay? So that's the S stereocenter, okay? So now let's do it over here. Prove to ourselves that this is one, it's going to be R, okay? So what do we say? One is there, two is up there, right? Just by the same analysis as we did over here and three is there. So one, two, three What is that? Clockwise or counterclockwise? Clockwise, so that's R, okay? Ain't nothing, you just got to flip it to where the smallest priority group is in the back, okay? Cool. Thank you very much.