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Current time:0:00Total duration:14:56

Video transcript

the last video we saw two different ways to form an epoxide from an alkene the first way was to add a peroxy acid the second way was to first form a halo hydron using bromine water and then using sodium hydroxide to start an intramolecular williamson ether synthesis to form our epoxide in this video we'll look at the stereochemistry of epoxide formation for either of these two reactions and if we start with a a sis a lien all right so our hydrogen's on the same side of the double bond and or you can think about the R groups as being on the same side for the product the R groups are still going to be on the same side of where the double bond used to be if you're looking at a trans alkene so I know this is trans it's my hydrogen's are on opposite sides all right you think about the R groups being on opposite sides of the double bond and then the product the R groups are still going to be on opposite sides of where the double bond used to be so let's look at a reaction involving stereochemistry so if I if I start out with an alkene and I'm going to I'm going to put a methyl group here and I'm going to put an ethyl group over here and then I'm going to have my hydrogen's like that and if I react this alkene with either of those two an either of those two reagents right so I'll go ahead and put ditto marks again so either of those two are going to give me the same products if I think about the formation of an epoxide alright so I know I'm going to make an epoxide and I know I need to have those R groups on opposite sides of where the double bond used to be so one possible product right would be to have the ethyl group coming out at me in space and then the methyl group would therefore need to be going away from me in space like that so if that's my product at this carbon over here the left I must have my hydrogen going away from me in space and at this carbon over here on the right I must have my hydrogen coming out at me in space like that so that's one possible product I could actually draw another product where my R groups are on opposite sides of where the double bond used to be right so I could have I could have my my ethyl group going away from me in space at the carbon on the left and I could therefore have my methyl group coming out at me on the carbon on the right like that and so this would have a hydrogen coming out at me and then this carbon have a hydrogen going away from me like that so I have two possible products and these are actually enantiomers of each other okay so you're actually going to create a racemic mixture 50% of one and antemer and 50% of the other one as well because this reaction creates two new chirality centers right so if I take a look at this molecule right these two carbons are chirality centers so there's stereochemistry at both of those carbons so when you're drawing the the innate the enantiomers right if I start with that molecule on the left I have my fol at me all I have to do is reverse that absolute configuration to make the other group going away from me for the enantiomer on the right if I'm looking at if I'm looking at this chirality Center over here I have my my methyl group going away from me right I like to do is reverse that absolute configuration had the methyl group coming out at me so these two are enantiomers of each other let's look at the details of the formation of these two enantiomers and then we'll also name them using stereochemistry so if we start with the alkene over here on the left right so for starting with that alkene and I think about these two carbons right these two carbons are sp2 hybridized meaning the molecule will be planar at those two carbons so I could I could think about that alkene as being planar at those two carbons so if I'm going to go ahead and draw draw that molecule here okay so I'm going to put my methyl group over here on the left and hydrogen over here and then I'm going to have a hydrogen over here and then an ethyl group like that so it's the same molecule the same molecules this one over here on the left it's just rotated a little bit and I know that those two carbons are on the either side of my double bond or sp2 hybridized meaning that that portion of the molecule is planar right so I can think about that portion of the molecule is planar and I could think therefore about the oxygen adding from either side of that plane right the oxygen could add from the top of the mein or the oxygen could add below the plane so let's go ahead and draw the results of those two different ways of adding the oxygen so over here on the Left I'm going to think about what happens to the molecule when the oxygen adds to the top of that plane so if the oxygen adds to the top of the plane all right I'm going to go ahead and keep I'm going to go ahead and keep my methyl group and my ethyl group on opposite sides of where the double bond used to be and therefore the two hydrogen's are also going to be on opposite sides of where the double bond used to be and I'm going to show the oxygen adding to the top of the plane so therefore my epoxide would look like that so that's one possible way to form your epoxide so let's go ahead and think about what would the epoxide look like if the oxygen added from below that plane all right so the oxygen added from the below the plane once again and we need to think about keeping our keeping our methyl group and our ethyl group on opposite sides of where the double bond used to be like that and the hydrogen's are also going to be on opposite sides of where the double bond used to be like that and then we're going to show our oxygen adding to form our epoxide from below the plane so our POC side might look like that so these are my two possible products and these are enantiomers of each other but it's kind of hard to see that as as as we've drawn them right here so let's let's see if we can get a different vantage point on our epoxide product so I'm going to put my eye right here and I'm going to stare at this Epoque side like that okay so if I'm if I'm staring at that epoxide right I could I could redraw my epoxide here so I'm going to to put that portion of the molecule like that now if I'm staring at it that way all right then I'm going to first focus in on the carbon on my left so that would be this carbon down here and I can see that there's an ethyl group coming out at me so I'm going to go ahead and draw an ethyl group coming out at me in space and I can see that there's a hydrogen going away from me in space right hopefully it's obvious this hydrogen's going away from me in space so I could I can represent that so I'll put my dash in here to show my hydrogen going away and now we're going to take a look at the carbon on the right side so this carbon the other side right here this time I could see that the hydrogen is coming out at me in space so I'm going to go ahead and draw the hydrogen coming out at me and space like that and therefore this methyl group back here this methyl group is going away from me in space right so I can go ahead and show that methyl group going away like that so that's that's one enantiomer let's go ahead and try to redraw the one on the right here so again if I'm if I'm staring right if I'm staring at this epoxide like that all right what do I see well I will see the POC side as being upside down right so the oxygen will be down here and I can see that if I'm looking at the carbon on the left side of my vantage point right so that would be this carbon down here I can see there's an ethyl group coming out at me in space so here is my ethyl group coming out at me in space and this hydrogen would be going away from me so I'll represent that hydrogen going away from me like that and then if we move to the carbon on the right side here I can see that this hydrogen is coming out at me in space so I can go ahead and draw the hydrogen coming out at me in space and the methyl group back here would be going away from me in space so I can have the methyl group as a dash like that so this is one of my products and again it's still not obvious that these are enantiomers so this is where the the model set comes in handy so if you if you make this molecule on the right and then you you hold you hold this oxygen and you rotate this oxygen up right much easier to see with the model set right in front of you you will find you will find that this molecule is the exact same molecule as the one that we're going to draw right here so if we rotate it so the oxygen is now pointing upwards that's actually going to take this hydrogen back here and move it to the front okay so when you're when you're drawing that molecule rotated that hydrogen is going to move to the front which pushes the ethyl group to the back so the ethyl group is actually going to back here same thing with this methyl group this methyl group was in the back when you rotate it that way the methyl group is going to end up coming out at you like that and then that means the hydrogen is going to go away from you in space so now we can see hopefully that these two are enantiomers to each other and let's go ahead and think about naming this product right so if I were going to name these these two these two molecules I have to think about how to do it all right so I need to find my longest carbon chain and I want to give my epoxy substituent the lowest number possible so if I'm going to number my carbon chain I would make this carbon number one this carbon number two three four and five like that so I'm going to name it as a pentane base right so this would be pentane like that and I can see my epoxy uh occurs between carbons 2 & 3 so it'd be 2 3 epoxy pentane like that so this in answer would be 2 3 epoxy pentane this one would be 2 so I can go ahead and write 2 3 epoxy epoxy pentane like that but now I have to think about the stereochemistry at carbons 2 & 3 so that makes things a little bit trickier so I'm going to I'm going to look at the enantiomer on the left and I'm going to redraw the enantiomer on the left so I'm going to redraw this one right down here and let's see if we can start to assign some stereochemistry so I have my hydrogen coming out at me I have my methyl group going away from me like that this ethyl group is coming out at me this hydrogen is going away from me so let's let's start with let's start with this carbon if I wanted to figure out the absolute configuration that that carbon I have to think about what atoms are directly connected to that carbon right so let me let me go ahead and draw a carbon in here right so that is one of the atoms directly attached to that carbon bluh so so the carbon and blue is directly attached to this carbon this oxygen this hydrogen and this carbon right down here so those would be the four atoms so if I'm trying to determine priority I think about the atomic number and I know that oxygen has the highest atomic number out of those four atoms so the oxygen is going to get a number one hydrogen has the lowest priority so hydrogen gets a number four so now I have to think about these two carbons it's that there's at I have to think about what those two carbons are attached to well the carbon on the right so so this carbon right here this carbon this carbon right here is attached to an oxygen alright so let me go ahead and write this here the carbon on the right is directly attached to an oxygen it's directly attached over here this oxygen is directly attached to a carbon and it's directly attached to a hydrogen so oxygen carbon hydrogen let's go ahead and look at the carbon on the left right so this carbon right here what is that carbon attached to it's attached to another carbon over here and two hydrogen's so CH H so the oxygen is going to beat the carbon in terms of atomic number so this carbon on the right is going to get highest priority so this carbon is going to get a number two here and that means this carbon is going to get a number three so for absolute configuration my lowest priority group is going away from me and I'm traveling around this way alright so I'm going clockwise so it's an AR absolute configuration at that carbon okay let's let's go ahead and since that drawing is really busy let's go ahead and draw it one more time and we'll figure out the absolute configuration at the other carbon okay so let's see we still have my ethyl group coming out at me still have the hydrogen going away from me still have this hydrogen coming out at me still have this methyl group going away from me like that so if I'm trying to find the absolute configuration for this carbon we're going to approach it the same way look at the atom directly attached to that carbon so the oxygen carbon carbon hydrogen so priorities right the oxygen gets highest priority the hydrogen gets lowest priority and then this carbon over here is directly attached to an oxygen so the is going to get second highest priority and that makes this methyl group over here the the third highest priority so the the one-two-three is going around this way which is counterclockwise which makes you think it might be s but number that trick that I told you about an earlier video it looks s but this hydrogen is coming out at me so all I have to do is switch that and that takes care of the fact that my lowest priority group is not pointing away from me so it looks s but since the hydrogen is coming out at me I can say with confidence that is R for an absolute configuration so we can go ahead and finalize the name right so at carbon two and at carbon three we have an AR absolute configuration so the name of this enantiomer would be 2 R 3 R 2 3 e poxy pentane and for this and an see mirror over here on the right I know that that it's the enantiomer so I just have to switch the absolute configuration so this one is going to be 2s and 3's 2 3 poxy pentane and we don't have time in this video to go ahead and assign absolute configuration to the enhancement on the right but you can go ahead and do so for practice and you should get 2 s 3 S 4 its absolute configuration now this is a problem because if you're forming a racemic mixture of your epoxide that's not always what you want to do in organic chemistry so there are ways to use chiral catalyst that allow you to select out for one of these enantiomers and we won't cover it in any of these videos since these are more intro organic chemistry videos but just be aware that it is possible to be selective in which enantiomer you produce