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Identifying chirality centers

Video transcript

once you understand the concept of chirality the next skill is to be able to identify a chirality Center and so I'm using the term chirality Center here but you also might hear chiral Center or a stereo Center or stereo genic Center or asymmetric Center and they're all pretty much referring to the same concept a tetrahedral carbon sp3 hybridized carbon has four different groups attached to it and so let's look for some chirality centers in these molecules and we'll start with this alcohol here and so I'm going to redraw this I'm just going to hun draw out all of the atoms here so we have four carbons the carbon on the left has three hydrogen's attached to it and 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 right we have an OHA and then we also have a hydrogen attached to it all right so that's this carbon over here right and this is a chirality Center we have four different groups attached to this carbon so we can mark this carbon right here this is a chirality Center there's a methyl group on this side all right so that's one different group there's an O H group alright there's an ethyl group and then there's also a hydrogen right so go ahead and draw in that hydrogen so 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 let me go ahead and draw in the other hydrogen's alright so I have two hydrogen's on this carbon so that's not a chirality Center right I have two of the same thing bonded to this carbon and then finally this carbon over here with three hydrogen's right three of the same thing so that carbon is not a chirality Center so one chirality Center in this alcohol alright for our next example let me go ahead and draw in the carbons right so we have three carbons and the carbon on the left has three hydrogen's bonded to it alright so that carbon is not a chirality Center same with the carbon on the right three hydrogen's 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 right that's an sp2 two hybridized carbon with trigonal planar geometry and so immediately you know that it's not a chiral 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 none of these carbons are chirality centers so there are zero chirality centers in this molecule so that's acetone all right let's do a ring example so let's do this ring example right here all right so let me go ahead and once again draw out the drawn on the molecule this time so we have a hydrogen here alright we have two hydrogen's on this carbon right two hydrogen's on this carbon all the way around our rings let me go ahead and draw in all of these hydrogen's alright and then we look for we look for chirality centers or chiral centers all of these carbons let me highlight them in magenta all these carbons have two hydrogen's bonded to them so that's two of the same thing so there's no way that's going to be those are going to be chiral centers what about this carbon right here so it looks like it might be a chirality Center right we have a chlorine bonded to it two hydrogen and then we have these things going in opposite directions but in extra in actuality this is not a chiral Center and that's because there's a there's the same path around this ring here so the hydrogen is like one different group the chlorine is another different groups 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 yet a ch2 and then you hit a ch2 so it's the same path around the ring so it's like it's like two of the exact same groups bonded to that carbon so another way of thinking about this is if I go ahead and focus on that carbon the top there again so I have a hydrogen here and a chlorine here and I draw in a molecule like that right 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 methyl group so two the same thing attached to that carbon so this is not a chiral Center and so the our zero chirality centers for this molecule however if we 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 right because we know we have a hydrogen bonded to that carbon here so the paths around the ring alright there's a different paths around the ring let me draw in the hydrogen's once again so there are two hydrogen's on this carbon right only one hydrogen on this carbon and so if you go if you go to the left around the ring alright you hit a ch2 right here if you go to the right around the ring you hit a carbon bonded to only one hydrogen and so it's a different path and so it's like there are four different things attached to this carbon right here so this carbon is a chirality Center alright if we look at some of these other carbons let me use me use red for this all right so if we look at if we look at this carbon right here this carbon is sp2 hybridized right there's a double bond there so it's not a chirality Center same with this carbon this carbon has two hydrogen's bonded to it this carbon has two hydrogen's bonded to it this carbon has two hydrogen's bonded to it and so there's only one chirality Center in this molecule alright and let's finally do let's do one more example which is just looks a little bit more challenging than the ones we've been doing alright so it's a little bit scarier looking this is the ibuprofen molecule all right so let's let's go let's go through this one by one but let's start with this benzene ring here in the center alright so let's just start with this carbon right here alright so this carbon is sp2 hybridized and as you go around the benzene ring right all these carbons are sp2 hybridized so that immediately means that these can't be chiral centers right need be to have tetrahedral geometry to be a chirality Center and so those carbons are all out let's look at this carbon right here right I know that there are two hydrogen's 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 and so that carbon is out let's look at this carbon I know there are three hydrogen's bonded to that carbon so that one's out all right same with this carbon so these these are three of the same things so none of these carbons over here on the left side of the molecule are chirality centers let's go over here to lets me use a different color for this carbon right here alright so I know that there's only one hydrogen bonded to this carbon I'll just put in right here so let's think about the different groups I have a methyl group right here I have a hydrogen alright I have I have this carboxylic acid over here and then I have I have my benzene ring over here and the rest of the molecule so that's four different groups attached this carbon and so this carbon that I've marked in blue here is a chirality Center so this is a chiral Center and go ahead and mark it alright let's focus in on this carbon on the methyl group there are three hydrogen's 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 and so we have only one chiral Center in this molecule and so this is a very important skill to practice too so look at the carbons think about what's bonded to them and if they're 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