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Current time:0:00Total duration:9:47

Video transcript

the RS system is used to describe the configuration of a chirality Center down here we have a pair of enantiomers on the left we have one compound on the right we have its mirror image we're going to assign an RS to each of our enantiomers so let's start with step 1 in step 1 we prioritize the four groups attached to our chiral Center and we do that according to atomic number let's start with the enantiomer on the left we know that this carbon is our chiral Center and we have four different things attached to this carbon over here to the right - a very shortened version of our periodic table and we can see that bromine has the highest atomic number out of these four atoms so bromine gets highest priority we're going to give the bromine a number one chlorine has the next highest atomic number at 17 so chlorine gets second highest priority fluorine has the next highest @ with a 9 so fluorine gets a 3 and finally hydrogen is the lowest priority group the lowest atomic number of 1 so hydrogen gets a 4 so step one is done step 2 orient the groups so the lowest priority group is projecting or pointing away from you our lowest priority group is hydrogen and it's already going away from us the hydrogen is on a dash here so step two is pretty much done step 3 determine if the sequence 1 2 3 is clockwise or counterclockwise so we're going to ignore our hydrogen here so I'll just kind of rub it out here and look at what's happening with 1 2 & 3 so 1 2 & 3 are going around in this direction that is clockwise therefore this is the R enantiomer so I'm going to write our bromo chloro fluoro methane let's do the same thing for the four it's mirror image so this compound on the right we know that this carbon is our chiral Center we already know how to assign priority bromine gets a number one chlorine gets number two fluorine gets number three and hydrogen gets a number 4 so step one is done step two point the lowest priority group away from you well that is already happening here so step two is done step three determinate the sequence 1 2 3 is clockwise or counterclockwise so let's ignore this group going away from us the lowest priority group let's go around 1 2 & 3 so going around in a circle 1 2 & 3 in this direction we know that is counterclockwise so this must be the S enantiomer so this would be s bromo chloro fluoro methane so step 3 is done so that's how you assign a configuration to a chiral Center here we have another pair of enantiomers so this alcohol on the left and mirror image on the right let's start with the one on the left we know from earlier videos that this is the chiral Center and our goal is to assign a configuration to this chiral Center let's go ahead and redraw the molecule so that carbon is our chiral Center attached to that carbon is our Oh H so I'll draw the Oh H on a wedge we know that hydrogen is there going away from us so that's a dash so even though it's not drawn in we already know it's there to the right we have a methyl group so a ch3 so let's draw in a ch3 and then finally to the left we have an ethyl group so that would be a ch2 and then ch3 all right let's go back to our chiral Center so here is our carbon that's our chiral Center let's look at the atoms that are directly bonded to this carbon so there's an oxygen directly bonded to the carbon there's a hydrogen and then we have two carbons so let's assign priorities if we look over here at our very shortened version of the periodic table we know that oxygen has the highest atomic number out of those atoms so oxygen gets highest priority the O H group gets the highest priority so this gets a number one hydrogen has the lowest atomic number so the hydrogen is the lowest priority group so we say that's group number four finally we have two carbons and two carbons would of course be a tie Carbon has atomic number of six so we have to find a way to break the tie so the way to do a tiebreaker is to look at the atoms that are directly bonded to those carbons so we'll start with the carbon on the right the carbon on the right is directly bonded to three hydrogen's one two three so let's go and write that out so hydrogen hydrogen hydrogen the carbon the carbon over here is directly bonded to a carbon a hydrogen and a hydrogen and we're going to put those atoms in order of decreasing atomic number so this carbon has a higher atomic number than these hydrogen's so we write carbon hydrogen hydrogen next we compare these lists so the left we have carbon hydrogen hydrogen on the right we have hydrogen hydrogen hydrogen and we look for the first point of difference well that's carbon versus this hydrogen here carbon has a higher atomic number than this hydrogen so the carbon wins so this group gets a higher priority so the ethyl group has a higher priority than the methyl group so the ethyl group must be the second highest priority so this gets a number two and the methyl group must get a number three so now we've assigned priority to all of our groups let's go down here and let's write it in the O H got highest priority so that gets a number one the ethyl group got second highest priority so that gets a number two the methyl group got third highest priority so that's a number three and the hydrogen our lowest priority group is pointing away from us so that takes care of step two step three is to see is to see what the sequence is doing so if we go around a circle one two three we're going around this way right we're going around clockwise and we know for going around clockwise that must be the R enantiomer so this is our two butanol if that's our 2 butanol the mirror image must be s 2 butanol so let's go ahead and let's double check and make sure that's true so this is our chiral Center this is our this is our chiral Center we know the O H is highest priority so that gets a number one let me go ahead and change colors for this so this one gets a number one we know that our ethyl group it gets a number two we know our methyl group gets a number three and we also know there's a hydrogen going away from us in space so our lowest priority group is projecting away so all we do now is look at what's happening with one two and three and one two and three are going around this way and that of course is counter clockwise and counterclockwise is s so this is s 2 butanol we just saw that this is s 2 butanol but what if you were given the dot structure on the right and asked to assign a configuration to the chiral Center so here is the chiral Center at the O H this time is going away from us that means that the hydrogen is coming out at us in space so we've already seen how to assign priority the Oh H gets a number 1 the ethyl gets a number 2 and the methyl gets a number 3 and the hydrogen gets a number 4 when you go to step 2 step 2 says to orient the group so the lowest priority group is projecting away from you but that's not what we have here here we have our lowest priority group coming out at us in space because this is a wedge so one thing you could do would be to take this compound and in your head mentally rotate it so the hydrogen is pointing away from you in space and when you do that you'll see that it's the same as the one on the left so this is just two different ways to represent the same enantiomer so in the video on drawing enantiomers I actually showed you the video where I rotated this compound to prove that these two drawings represent the same compound so now your hydrogen is going away from you in space and that's how we got that's how we got s because this was number one this was number two and this was number three so we went around this way and we saw that is counterclockwise and so we got s but what if you didn't want to rotate the molecule in your head sometimes it's pretty difficult to do for this one it's not too bad but it can get a little bit tricky so there is a trick that you can use there is a trick that you can do and you can just start with the drawing on the right you can go ahead and number your your groups in terms of priority and then you can go ahead and just ignore the hydrogen for the time being even though the hydrogen is coming out at us in space here if you ignore it and look at 1 2 3 so 1 2 3 is going around this way and that would be clockwise and we know that clockwise is our so I'm going to write it looks our here it looks our but since the hydrogen is coming out at us in space we can take the opposite of how it looks so if it looks our and the hydrogen is coming out in space you know it's actually the S enantiomer so that's a little trick that you can do instead of instead of rotating the molecule in your head you can just assign you can just assign your priorities come up with RS and then take the opposite