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Current time:0:00Total duration:11:02

Cahn-Ingold-Prelog system for naming enantiomers

Video transcript

right now based on what we know so far if we wanted to name this molecule we would say well what's the longest carbon chain here well we have a two carbon chain so we're dealing and it's there's all single bonds so we're dealing with an ethane we're dealing with an ethane actually I'll write it all all at once and then we have on the one carbon we could call this the one carbon and call this the two carbon we have a bromine and a fluorine so we could call this we could call this one one bromo and we're putting the bromo instead of the fluoro because it becomes before F alphabetically one bromo one floor o one floor o and then we're dealing with an ethane right we have a two carbon chain all single bonds floor o ethane that's the name of that molecule there just a review of some of the earlier organic denominator videos we had done now we know immediately based on the last few videos that this is also a chiral carbon and if we were to take its mirror image if we were take its mirror image we would get another enantiomer of this same molecule or that they are enantiomers of each other so what is the mirror image of this of this ma of this one bromo one fluoro ethylene look like well you'd have the carbon right here you would still have I want to get all the colors right you would still have the bromine up above you would have this methyl group that's attached to the carbon now pointing in the left direction ch3 the fluorine would now still be behind the carbon so the way the fluorine would still be behind the carbon and now the hydrogen would still pop out of the page but it would now pop out and to the right so it would not pop out and to the right just like that that is the hydrogen now based on our naming so far we would name this one bromo one floor one bromo one floor Oh ethane and we would also name this one bromo one floor Oh ethane but these are fundamentally two different molecules even though they have the same molecules in them they have the same molecular formula they have the same I guess you could call it Constitution and that this the car this carbon is connected to a hydrogen to fluorine and bromine this carbon is connected to the same things this carbon is connected to a carbon three hydrogen so is this one these are these are stereoisomers these are stereoisomers and they're mirror images of each other so there are nine Chalmers enantiomers and actually they will one polarize light differently and they actually will have varied they can often have very different chemical properties in a chemical or biological system so it seems I guess not good that we have the same names for both of these so we're going to focus on this video is how do you differentiate between the two enantiomers enantiomers so how do we differentiate between the two so the naming system we're going to use right here is called the it's called the cahn-ingold-prelog system but it's a different cond it's not me it's CA hn instead of KH aan kaan in gold in gold pre log system pre log system and it's a way of differentiating between this enantiomer which right now we would call one bromo one fluoro ethane and this enantiomer one bromo one fluoro ethane it's a pretty straightforward thing really the hardest part is to just visualize rotating the molecules in the right way and for getting out in which direction it's kind of whether it's kind of a left-handed or right-handed molecule we're going to take it step by step so the first thing you do in the cahn-ingold-prelog system is to one identify identify your your chiral molecule here it's pretty obvious it's this carbon right here it is bonded to well we'll just focus on this left one the one we started with first it's bonded to three different groups and then what you want to do is you want to rank you want to rank the groups by atomic number so if you if you go up here out of bromine hydrogen fluorine in and a carbon this is what is bonded directly to this carbon which has the highest atomic number well you see bromine is over here let me do that in a darker color we have bromine at 35 we have fluorine at 9 we have carbon at 6 and then we have hydrogen hydrogen at 1 so all of these bromine is the largest we'll just call this number 1 then after that we have fluorine that is number 2 number 3 is the carbon number 3 and then hydrogen is the smallest so that is number 4 number 4 so now that we've numbered them the next step is to orient this molecule so that the smallest atomic number group is sitting into the page it's sitting behind the molecule so if we were to write now this hydrogen is the smallest of all of them Broll means the largest hydrogen the smallest so we want to orient it behind the molecule the way it's drawn right now it's oriented in front of the molecule so to orient it behind the molecule and this really is the hardest part is just to visualize it properly remember this fluorine is behind this is right in the plane of the paper this is popping out of the paper we would want to orient we would want to we would want to rotate you could imagine if we rotate we would wrote we'd be rotating the molecule in this in this direction so that let me redraw it so that now we have we redraw it we have the carbon here and now since we've rotated it like this we've rotated roughly about you know one 1/3 around the circle or so it's about 120 degrees now this hydrogen is where the fluorine was so now this hydrogen is where the fluorine was so that's where the hydrogen is the fluorine is now where this methyl group is so the fluorine is now where this methyl group is these dotted lines show that we're behind now this shows that we're in the plane and the methyl group is now where the hydrogen is now popping out of the page it's going to the left and out so this methyl group is now popping out of the page out and to the left that's where our methyl group is so all we've done is we've just rotated this around about 120 degrees that's the hardest we've just gotten this to go behind and that's that's kind of the first step after we've identified the chiral carbon and rank them by atomic number and of course the bromine is still going to be on the bromine is still going to be on top now once you put the smallest atomic number molecule in the back then you want to look at the rankings of one through three I mean we have four four molecules here so we look at the largest which is bromine number one then number two is fluorine number two and then number three is this methyl group that's the carbon that's bonded to this carbon so it's number three right there and in the con Ingold prelog system we literally just think about what would it take to go from number one to number two to number three and in this case we would go in this direction to go from number one to number two to number three we would go in the clockwise direction we're just kind of ignoring the hydrogen right now that's kind of sitting behind it that was the first step to orient it so it's sitting in the back the smallest molecule and then the three largest ones you just say what direction do we have to go to go from number one to number two to number three in this this case we had to go clockwise and if we go clockwise now then we call this we call this a right-handed molecule or we use the Latin word for right which is rectus rectus and so we would call this this molecule right here is not just one bromo one fluoro ethane this is our R for rectus or you can even think right although we'll see leftist use this S which is sinister so the Latin is really where the are comes from but this is our one bromo one bromo one floor O one floor o ethane that's this one right here so now you might guess well this must be the opposite so this must be the counterclockwise version we can do it really fast so same idea we know the largest one bro means number one that's the largest of the in terms of atomic number fluorine is number two carbon is number three hydrogen is number four what we want to do is put hydrogen in the back so what we're going to have to do is rotate it to the back to where fluorine is right now so if we to redraw it draw this molecule right here you have your carbon still you still have your bromine sitting on top but we're going to put the hydrogen now to the back so the hydrogen is now where the fluorine used to be the hydrogen's there this chlorine is going to be moved to where the hydrogen now is not the chlorine sorry this methyl group this carbon with the three hydrogen's is going to be rotated to where the hide where the hydrogen used to be it's now going to pop out of the page because we're rotating it in that direction so this is our methyl group right there and then this fluorine is going to be moved where the methyl group was so this fluorine will go right here and now using the cahn-ingold-prelog system this is our number one this is our number two just by atomic number this is number three you go from number one through number two to number three you go in this direction you're going counter clockwise counter clockwise or we are going to the left or we use the Latin word for it which is sinister and the word sinister comes from the Latin word for left so it's you know I guess right is good and people thoughts either left-handed people were bad or if you're not going to the right it's bad I don't know why you know sinister took on its sinister meaning now in common language but it's now the sinister version of the molecule so we would call this version this enantiomer of one bromo one fluoro ethane we would call this s we call this s s for sinister or for left or for counter clockwise s1 bromo one bromo one fluoro ethylene so now we can differentiate the names we know that these are two different these are two different configurations and that's what the s and the R tell us that if you have to go from this to this you would literally have to detach and reattach different groups you would actually have to break bonds you would actually have to Swit the swap two of these groups in some way in order to get from this enantiomer to this and antemer their different configurations really fundamentally different molecules stereo isomers enantiomers however you want to call them