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Current time:0:00Total duration:10:42

Carbohydrate - Glycoside formation hydrolysis

Video transcript

so the natural progression at least in my mind when I'm thinking about you know the cyclic nature of monosaccharides and their hemiacetal structure is how do these molecules progress to complete acet Al's remember that one o H in 100 our group is a hemiacetal so if we have a carbon chain and we have one o age group in 100 our group that's a hemiacetal so hemiacetal and if we have two O R groups on that carbon chain it's a it's a full-blown acetal so two O R groups or two alkoxy groups is an acetal and this carbohydrate right here is a hemiacetal you can see this is kind of the carbon that would be in the center and we've got one o age group and one RoR group over here and we want to know how it progresses to an asset al and well it actually progresses just like it would if this wasn't a cyclic carbohydrate so the anomeric carbon remember that's the carbon right here that's the carbon that was formerly the carbonyl carbon before the ring closed now it's the anomeric carbon and it's attacked by another nucleophilic alcohol so another nucleophilic alcohol comes in and the carbon is oxidized and loses this hydroxyl group right here in the form of water in order to make room for the for the Ord pour the alkoxy group so we have loss of water and we've made room now for the alkoxy group and the product then is an acetone kind of looks like this so now the carbon has two O r groups and when this happens in the context of a carbohydrate we call it a glycoside so that's kind of a special kind of name a glycoside and and what happens is we're able to add a functional group onto the carbohydrate this would be the functional functional group and we say that it's it's bonded by a glycosidic linkage so that linkage right there would be the glycosidic linkage really that's that's the big picture idea but what I want to do in this video is I want to show you the actual mechanism of course this is an organic chemistry type idea so we have to go into the the mechanisms you know but I want to show you how this actually plays out so let me kind of clear some room here and I'll tell you I went ahead and I pre drew kind of the backbone for this mechanism just to kind of it kind of save you the endurance of having me draw out all these little diagrams but I'll try to walk us through and I'll show us kind of the functional steps for what's going on here so right here we have beta-d-glucose so we've got the beta anamur of d-glucose and so this is a this is a hemiacetal we've got the a our group in the OAH creep and what happens in the presence of kind of a mild acid is this O H group right here on the anomeric carbon gets protonated so it's protonated and the the chloride atom is is good at accepting those extra electrons from its bond and what happened is so that Oh H group lost electrons to the to the proton and now we have a a cation right here and what happens is we have water loss so h2o Lassa this this group right here will leave we've got h2o loss and then we have from the previous step this additional chloride chloride anion so we've got that chloride anion and you can see that when when the water when we lose that water we end up with a carbo cation right here because you know these electrons left we've got this cation right here and it happens to be as you can see in this image resonance stabilized and that's important this product right here is resonance stabilized stabilized so it's resonance stabilized and we have the the planar nature that is going to be important in such 3 & 4 so kind of pull those in we've got such three and four here so we start with this planar carbo-cation and it's the planar nature of this carbo-cation that's going to allow the the next alkoxy group the next and the nucleophilic alcohol to attack from above or below so we've got the above attack kind of on this top shelf and then we've got the below attack on the bottom shelf of the mechanism and so that alcohol group has nucleophilically attacked and now the oxygen atom right here has a has a positive charge on it but those chloride anions that we lost in the previous set come back into play here and accept that extra that extra hydrogen back allowing the electrons from that bond to go to the oxygen same thing down here and that oxygen loses its positive charge and we end up with our products now that again the planar carbo cation nature of that intermediate product allowed the the alcohol group to attack from above or below and and what that means is that our glycoside just like our our carbohydrate formed two different animals that beta anamur and the alpha anamur are glycoside does the same thing so if the additional oru right here is sis with respect to that last carbon we end up with the beta glycoside the beta glycoside and again if it's trans we end up with the alpha glycoside and these glycosides they actually occur naturally and a few of them are quite significant the one that really stands out to me is digoxin it's a it's a medication that's given for four different cardiac or heart complications I draw a little heart here and it's actually still extracted from the foxglove plant in the Netherlands for use as a medication stands out to me because as a as a nurse I've given this medication several times but and anyway it may have occurred to you at some point over the last few minutes as we've kind of gone through this mechanism that if an alcohol can join onto a monosaccharide and remember that's that's essentially what we're doing in the form nation of this glycoside that the additional alcohol could be another amount of saccharide you know remember that monosaccharides have plenty of hydroxyl groups and that this reaction could be a means of joining carbohydrates together and if that's the observation that you made that's a really really a super astute observation this is indeed than the method by which carbohydrate joined together and we call these linkages glycosidic linkages and of course we'll talk at length and in the future about the polysaccharides that are produced I'm going to do a whole video on polysaccharides but this is the means this glycosidic formation by which these chains are produced but Before we jump away from the actual glycosidic linkage subject I want to point out that this whole process of forming glycosides can happen in Reverse so these linkages can actually be broken down as well and remember that with this bond formation we lost a water molecule so this was a dehydration reaction so in Reverse what's going to happen is we're going to add water back in a process called hydration and so because we see a hydration reaction that breaks down these these poly unit molecules we call it and a hydrolysis reaction this is glycoside hydrolysis the lysis for the breaking down and then the Hydra as a reference to the water molecule coming in so what happens at the beginning is that the alkoxy group that we added before that oh our group is protonated and after protonation of that of that asset al so our group happens we lose it in the form of an alcohol so we lose that asset al alkoxy group we've kind of formed some water right here right here as we lost this extra proton on that hydronium anion and again what what happens just like in that previous step we form a resonance stabilized carbo cation again it's like we're forming that middle product again so this right here is resonance stabilized resonance stabilized so again the beginning of steps 3 through for with glycoside hydrolysis is going to be a planar carbo cation so again we've got a planar carbo cation right here and again what what that means is that the water that comes in now is going to be able to nucleophilically attack above or below this molecule so again we've got the above and the below and the below and we'll see what happens we've got an h2o that comes in right here and the extra the extra electrons on that water molecules oxygen are going to come in and they're going to nab that proton and these extra electrons from this bond are going to come back in and they're going to stabilize that hydroxyl group and the same thing would happen in the in the below situation we got h2o that extra electrons here nab the hydrogen NAB that proton the electrons from the bond shift down and stabilize that hydroxyl group and what we've done because of that above or below attack is again we've formed the the beta anamur of d-glucose right here because this hydroxyl group is sis with respect to that last carbon so beta-d-glucose and we've we've formed the alpha anamur so alpha D glucose and if if the terms beta and an alpha and anamur are confusing i covered those in a previous video on this on the cyclic formation of monosaccharides but that's that's kind of where you'll find that terminology and i guess again right here we've we've reformed that hydronium because now the the water molecule has picked up that extra proton