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Current time:0:00Total duration:8:44

Video transcript

let's get some more practice with the RS system so we'll start with this compound right here we already know from earlier videos that this carbon is a chiral Center so let me go ahead and redraw everything because it's going to help us assign a configuration so that's that carbon we have a methyl group coming out at us I'm going to draw in the carbon with the hydrogen's here we have a hydrogen going away from us and going to the right around the ring we hit a ch2 so I'm drawing in a carbon with two hydrogen's here and then we hit CH so this carbon bonded to a hydrogen that's this carbon on the ring notice that this carbon that I just marked is double bonded to this carbon and for the purposes of our and s system we're going to pretend like this carbon is bonded to two different carbons even though it's really one so that's how to handle a double bond going this way around the ring we hit a ch2 so let me draw that in so here's our ch2 and then we hit another ch2 right here so a ch2 and then this carbon is bonded to this carbon so I'm just going to draw a line in there like that all right let's think about priority so this is our chiral Center let's look at the four groups attached to the chiral Center so this is step one prioritize the four groups using atomic number so what's directly attached to this carbon there's a hydrogen there's a carbon there's a carbon and there's a carbon so carbon beats hydrogen in terms of atomic number so hydrogen is the lowest priority group so we assign that a group number four all right now we have a tie we have three carbons we have three carbons so we need to see what those carbons are directly bonded to let's start with this top carbon here this carbon is bonded to hydrogen hydrogen hydrogen so let me write that down so hydrogen hydrogen hydrogen let's move this carbon on the right this one's bonded to carbon hydrogen hydrogen so carbon hydrogen hydrogen we're thinking about the atoms directly bonded to it and we're going in decreasing atomic number which is why I put the carbon first now let's look at this carbon so this carbon is bonded to a carbon a hydrogen and a hydrogen so CH H alright let's compare now well first point of difference we look at the first the first atom here this is a hydrogen and this is a carbon so carbon beats hydrogen and then over here we have a carbon so this one this one doesn't win this one must be third in terms of priority so I put a 3 here for this methyl group and now we continue on we have hydrogen vs hydrogen so that's a tie another hydrogen another hydrogen so we have another tie so we need to go to the next atom to break this tie so we go to our next carbon so this one right here what atoms are is this carbon directly bonded to well it's bonded to carbon carbon hydrogen so carbon carbon hydrogen this carbon is directly bonded to carbon hydrogen hydrogen so carbon hydrogen hydrogen we look for the first point of difference this is a carbon versus a carbon this is a carbon versus a hydrogen so the carbon wins and that means that this this way around the ring this is the higher priority path around the ring so this gets a number one and then this path around the ring going this way gets a number two so now we've assigned we've assigned a priority to our four groups so now we're ready for step two or en't the group so the lowest priority group is projecting away from us in space so let's go back to our original our original dot structure here we said that this way around the ring was the highest priority so this got a number one next going this way around the ring was second highest priority so number two our methyl group was a number three and our hydrogen was a number four our lowest priority group is going away from us our hydrogen or hydrogen is on a dash so that's going away from us in space so now we have now we have finally on to step three and let me change colors again because we're it's getting a little busy here so step three determine if the sequence 1 2 3 is clockwise or counterclockwise so if I look 1 2 3 and I go around a circle here's one here's two and here's three so going around one to three in a circle is this way right that is that is clockwise right you can see that's clockwise right here and so clockwise is R so the configuration of this chiral Center is R now let's look at this compound so we have only one chiral Center to worry about this one right here let's think about the atoms that are directly bonded to our chiral Center well there's a bromine directly bonded to it a chlorine and over here would be a carbon and then we have another carbon so we prioritize our groups in terms of atomic number bromine has the highest atomic number out of those atoms that we give bromine a number one next would be chlorine with atomic number of 17 so that's number two now we have a tie for our carbons so we need to see what is directly bonded to those carbons so for the carbon on the right here this carbon is directly bonded to a carbon here a carbon here and then of course a hydrogen so we write that in as carbon carbon hydrogen and then for the carbon on the left so this carbon that carbon is bonded to a carbon and two hydrogen's so we write in carbon hydrogen hydrogen we look for the first point of difference so we have carbon versus carbon so that's a tie so we keep going and we get carbon versus hydrogen so the carbon wins and this group gets the higher priority so this isopropyl group is a higher priority than this ethyl group so that means the isopropyl group is going to get a number three so this is three and the ethyl group is the lowest priority it gets a number four now that we've assigned priority to our groups we need to orient the molecule so the lowest priority group is pointing away from us and the lowest priority group is group number four so I'm go to the video in a second and in the video I'm going to show you two different ways to think about putting this group going away from you in space so one way would be to just think about an axis through this carbon here and then rotate and then rotate around this axis until your lowest priority group is pointing away from you another way to think about it is like a Newman projection if you stare down if you stare down this bond let me go ahead and change colors here if you look down this carbon-carbon bond here so if you put your eye along this axis so here's your eye that would mean your lowest priority group your ethyl group would be going away from you so that's another way to think about looking at the looking at the model or the molecule I should say in a way where the lowest priority group points away from you so here's our compound let's say that red represents bromine so there's bromine and yellow represents chlorine so our goal is to rotate this to put our lowest priority group going away from us so we think about an axis through our chiral Center we'll rotate it so the ethyl group is pointing away and now we can see that our red bromine is to the left our yellow chlorine is to the right and the isopropyl group is up in space we go back to where we were this time let's think about a Newman projection so we're staring down starting down this carbon-carbon bond so let's rotate the molecule and it's a little bit different perspective but it's still it's we're still able to see a red bromine to the left a yellow chlorine to the right and an isopropyl group up so here's what we saw we stared down our carbon-carbon bond so this carbon is our chiral Center so that's this one right here and then I used red for bromine and then I used yellow for chlorine so we can see that our bromine our bromine is to the left here and this was the highest priority group the chlorine is to the right that was the second highest priority and then we have our isopropyl group here up so that's 1 2 3 with our lowest priority group pointing away from us so always do now is go around in a circle and see what we get so we're going from 1 to 2 to 3 in a circles that means we're going this way around our circle and that of course is counterclockwise so counterclockwise is s so the configuration of this chiral Center is s