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Current time:0:00Total duration:13:52

Video transcript

Fischer projections are another way of visualizing molecules in three dimensions and let's use the example of lactic acids it's called lactic acids it has a carboxylic acid functional group over here on the right and this is the only chirality Center in lactic acids an sp3 hybridized carbon four different substituents attached to it so with only one chirality Center we would expect to have two stereoisomers for this molecule and those stereo isomers would be enantiomers of each other over here I picked one of those enantiomers and I've just drawn it in this fashion let's see which in the interior we have over here well this is my chirality Center the one attached to my Oh H and if I were to assign absolute configuration to that chirality Center I look at the first atom connected to that chirality Center well that's oxygen versus carbon versus a carbon over here in my carbonyl so obviously oxygen is going to win so we can assign oxygen a number-one priority since it has the highest atomic number and when I compare these two carbons to each other I know the carbon on the right is double bonded to an oxygen so that's going to give it higher priority than the carbon over here on the left since that's bonded to hydrogen's and then my other hydrogen attacks attached to my chirality Center is going away from me in space so when I'm assigning absolute configuration I look at the fact that it's going one two three it's going around this way it's going around clockwise therefore this is the R enantiomer of lactic acid so that's all from a previous video now if I want to draw a Fischer projection of our lactic acid what I would do is I would put my eye right here and I would stare down at my chirality Center so I would stare down at my chirality Center and I would draw exactly what I see well if I'm staring down this way I could draw a line right here to represent you know my flat sheet of paper and I can see that both my hydrogen and my OAH are above my sheet of paper whereas my carboxylic acid and my ch3 are below my sheet of paper so this carbon is my chirality Center carbon and I have my coming out at me and this is actually going to be on the right side so if take out your molecular model set you will see as a chill be coming out at you and we'll be on the right side of you and this hydrogen will be coming out at you'll be on the left side of you so that hydrogen would go over here like that this carboxylic acid functional group all right this is the top of my head right here then that would make this go at the top of what I'm looking at and so that is going away from me in space so we would use a dash to represent that and so we could go ahead and draw our C double bond to an O and then Oh H going away from me and then if I look at this this ch3 group over here is also going away from it's going down in space so I can represent it going down in space like that and this is the viewpoint of a Fischer projection so if I'm going to convert this into a Fischer projection a Fischer projection is just drawing you know a cross like that and then at the top you have your C double bond O 2 and O and then Oh H is just a way of abbreviating this carboxylic acid functional group and then I have a hydrogen over here and then I have an H group over here and then I have a ch3 here so this is a Fischer projection this is the Fischer projection for our lactic acid so this is our lactic acid and Fischer projections were invented by Emil Fischer who won the Nobel Prize in Chemistry for one of the things was for his research on carbohydrates and he drew Fischer projections to help him draw carbohydrates and so that's where you'll see Fischer projections used most often even though some chemists don't really like them very much so this is the Fischer projection for our lactic acid and if I wanted to draw the Fischer projection for s lactic acid I would just reflect this molecule in a mirror so let's see if I can fit my mirror in over here and I would have my OAH reflected in my mirror and then I'd go ahead and draw my Fischer projection and then my methyl group would be over here my hydrogen would be over here and my carboxylic acid functional group would be right there so this would be s lactic acid on the right and our lactic acid on the left s lactic acid is the lacked is the type of lactic acid that you find in in the build up of muscles after extreme exercise and the type of lactic acid that some people heard of from milk is actually a racemic mixture so the bacteria and sour milk will will break down the lactose into into a 50% mixture of our and 50% mixture s lactic acid let's take a look at a carbohydrate since Fischer used Fischer projections for carbohydrates specifically so here I have a carbohydrate and if I were to number this carbohydrate this carbonyl would get at number 1 and then this will get a number 2 over here a number 3 and a number 4 this is a four carbon carbohydrate how many stereoisomers does this carbohydrate have well this carbon number two is a chirality Center and carbon number three is a chirality Center so two chirality centers so I use the formula of 2 to the N where n is number of chirality centers so I would expect 2 squared or four possible stereoisomers for this molecule so you could draw four different stereoisomers for this molecule will draw them in a few minutes for right now I've gone ahead and drawn one of them as a sawhorse projection so here I have a sawhorse projection of one of the possible stereoisomers and what we're going to do is we are going to put our eye right up here and we are going to stare straight down at this bond right here and we're going to see if we can draw the Fischer projection for this molecule so what do we see well let's start with let's start with this carbon right up here so we'll make that carbon this one and you can see that the O H the O H attached to that carbon is going to the right and it's going up at us so that oh is going to the right and it's going up at us and then if I look at this hydrogen over here it's on the left that it's going up at us so my hydrogen is on the left and it's going up at us and this aldehyde functional group this CH oh you can see is going is going down all right so this aldehyde functional group is going away from us so we can go ahead and represent that aldehyde as going away from us in space select that well this chirality Center carbon is connected to this chirality Center carbon so we'll go ahead and draw straight lines it's looking straight down at it and once again we will find that our Oh H group is on the right coming out at us our hydrogen is on the Left coming out at us so let's go ahead and put those in which group is on the right coming out at us hydrogen is on the Left coming out at us and then of course we have this ch2oh down here is going away from us in space so we'll go ahead and draw that ch2oh going away from us in space like that so that would be that would be the Fischer projection translated let's go ahead and make it into an actual Fischer projection right where we just go ahead and draw straight lines and the intersection of those straight lines are where are our chirality centers are right so this would be an H this would be an OHA Savion H this would be an O H this would be our ch2oh and at the top we have our aldehyde CH 0 so this is one of the four possible stereoisomers and Fischer projections just make it much easier when you're working with carbohydrates so this is one of the four let's go ahead and redraw the one we just drew and let's get the other three to get our total of four on here so I'm going to take the one that I just drew on the right I'm going to redraw it in the draw a little bit smaller so everything will fit in here so so this is one possible stereoisomer I have my OHS on the right I have my hydrogen's I have my CH oh I have my ch2oh okay if I wanted to draw the enantiomer to this molecule I would just have to reflect it in a mirror all right so I could just do this I could reflect the molecule in a mirror and I would have the enantiomer so this would be the enantiomer to the stereo isomer that I just drew if I wanted to draw the other two right I can just go ahead and real quickly put in my Fischer projections right here all right so I have two more to go and I'm going to put the O H over here and then the H over here and then the O H over here and the H over here all right so this is yet another possible stereoisomer and I'll draw the mirror image over here on the right so I have to have a hydrogen right here and then my Oh H must be on this side and then I have an OHA have an OHA here and then a hydrogen on the other side and then a CH Oh for my aldehyde and the ch2oh so here I have my four possible stereoisomers for this carbohydrate and I'm going to go ahead and label them I'm going to label this first one here stereoisomer a stereoisomer be sterilized mercy and stereoisomer D well C and D are mirror images of each other so they are enantiomers of each other right so these are enantiomers a and B are mirror images so they are enantiomers to each other and then we talked about in the diastereomer video if I took if I took one of the ones from A and B all right so let me just go ahead and circle that if I just took a right if I took one of the ones from a and B and one of the ones from C and D and I'll just take C then a and C are diastereomers of each other they are non superimposable non mirror images of each other so those are enantiomers and diastereomers two to review what we covered in an earlier video let's do one more thing with Fischer projections let's assign absolute configurations to to one of these stereoisomers so let's just choose the first one a so we've been talking about a and let's go ahead and redraw it really fast and let's see how can we figure out the absolute configuration at my at my chirality centers from a Fischer projection so it just makes a little bit trickier than usual so here I have my Fischer projection and your aldehyde is going to get a 1 and then 2 3 4 in terms of numbering your carbon chain I want to figure out the absolute configuration at carbon 2 here so at carbon 2 what do I have I know a Fischer projection tells me that if it's a horizontal line everything's coming out at me so my OS coming out at me and my hydrogen is coming out at me let's go back up here and stare down that carbon 2 chirality Center and let's see what we would will we would actually see if we do that so here is carbon to right here I'm going to stare down right here this time right so I have my Oh H coming out at me my hydrogen coming out at me that makes this bond and this bond actually go away from me in space so the aldehyde is going to go away from me in space like that so I'm going to go ahead and draw my aldehyde now I'm actually going to go ahead and show the carbon bond to one hydrogen I know the carbons double bonded to an oxygen so I'm going to go ahead and do that that was that trick we learned in earlier video for assigning absolute configuration and then the rest of molecule is actually going down in space right so this would be a carbon here bonded to a hydrogen and this carbon is bonded to an oxygen and a carbon so what is the absolute configuration of this carbon here well if I think about this is my this is my chirality Center what are the atoms directly attached to that carbon well I have a hydrogen a carbon and oxygen and a carbon well immediately I know that my oxygen is going to win so I can go ahead and assign a number one to my oxygen right here and then I think about what's next priority right will it be carbon versus carbon so the top I have oxygen oxygen hydrogen the bottom carbon I have oxygen carbon hydrogen so we saw in the earlier video you go for first point of difference so oxygen versus oxygen no one wins then I go oxygen versus carbon and oxygen wins so this is going to number two up here and then this is going to number three for my substituent and my hydrogen will get a number four so I'm going around this way I am going around this way if I ignore my hydrogen so I'm going counterclockwise so it looks like it's s but remember the hydrogen is actually coming out at me so in the little trick I showed you in the earlier video if the hydrogen's coming out at me all you have to do is reverse it so it looks like it's s but since the hydrogen's coming out at me I can go ahead and say with certainty that it is R at that chirality Center so at carbon two at this carbon it is R so you can do the same thing with the chirality Center at the third position so you could do the same thing with this one and if you do that you will find it is also our so you could go ahead and say for this carbohydrate it is our at carbon two and it is our at carbon three so it is 2r 3r there's a 2r 3r stereoisomer and you could do that for for all four of these stereoisomers that we drew for this carbohydrate and you could then compare enantiomers and diastereomers that way as well so that's a that's a quick summary of Fischer projections practice and use your molecular model set to help you with the visualization aspect