Draw a theoretical acid-catalyzed Fischer Glycosidation mechan...

Question

Draw a theoretical acid-catalyzed Fischer Glycosidation mechanism for the condensation of D-Glucose into cellubiose. (Note:This reaction would lead to very poor yields of cellubiose- can you hypothesize why?)

Fischer glycosidation mechanism showing D-Glucose condensation to form cellobiose.

Accepted Answer

Draw a theoretical acid catalyzed Fischer Glycosidation mechanism for the condensation of D glucose, that's 2 d glucose molecules into cellubios. Note this reaction would lead to actually poor yields of cellulose. Can you hypothesize why? Cool. So guys first things first, let's just get this mechanism down on paper and then we can talk about the theory of why this wouldn't work so well. Okay. What do we know about Fischer Glycosidation? It's an acid catalyzed mechanism. So So let's just go ahead and stick some acid in here. I'm gonna go ahead and use H3O plus. And what we know is that what we're gonna try to do is we're gonna try to get rid of this Oh. Remember that according to the mechanism, we're trying to get rid of that Oh and get the new OR to attach. Now that new OR group usually is just like a methanol, but in this case, it's going to be an entire sugar. So I'm going to get that whole sugar to attack. Let's go ahead and draw out step by step what the mechanism would be and remember that all this can be found in your o glycosidation videos. So oxygen grabs the h. We're gonna protonate and let's go ahead and draw this out. And if you guys don't mind, I am going to copy this so that the video goes a little bit faster onto my clipboard so that I can use this sugar the rest of the video. You guys can pause if you are having a hard time following along. Cool, awesome. Cool, so I don't have to keep drawing that sugar over and over again. Great, So we know that here I would have a proton and a plus charge and this is where I can now make my I have a good leaving group and I can kick it out using my O. And what that's going to give me is a new sugar. Well, a new sugar derivative and it's actually my intermediate. It's the most important intermediate probably of this chapter because what I wind up getting, this is called my oxocarbenium Okay? The oxocarbenium, remember, it's the reason that we've only react at the anomeric position and no other positions because we make this resonance stabilized intermediate with the O. Cool. So then I'm just going to draw that the resonance structure would be either plus on the o or plus on the carbon. Let's go ahead and just bracket that off. Cool. So that's my oxocarbenium and now what we know is that we want instead of just methanol or some random alcohol to attack, what we actually want to attack is the four position of my second sugar. And remember the reason is because Cellubios is what kind of linkage. If you don't have this memorized, it's fine. Remember that cell ubiose would be a beta 14 linkage. So I'm looking for the 4 position of my second sugar to come in and attack. Cool. So let's go ahead and fix this up. I'm gonna use this version of the intermediate of the intermediate which is the plus here because it's just it's the easiest one to work with and it's easy to visualize that this O can now come from the top and attack that positive. Now could it also have come from the bottom? Totally, but that would not be beta 14 anymore. If it came from the bottom, what would that be? Alpha 14 because it would be coming opposite to the stereo descriptor. It would then be going this way. But in this case, I'm trying to make the beta 14, so let's go ahead and draw that the arrow is coming from the top. Cool? So now what I'm going to get oh, I should have used for this whole time. Make sure that we're using the correct arrows, guys. These are equilibrium arrows because this is all reversible. It's all acid catalyzed. So now what I'm gonna get is this and actually this is gonna work out nicely. I'm just gonna erase this one here, erase this H. Remember that that's my water right? So I'm actually drawing this is the condensation mechanism. I'm losing water and now we can just reposition this guy. I know you wish you could do all these things. It's okay though. You can pause. Cool. How awesome is that? So we just lost our water and we just made our beta 14 linkage but remember that we're gonna have an extra H. Oh I already put that we lost our water. So when you draw that you lose your water like that, that's not really a mechanism. It's more formula. It's just showing you the progress. I'm gonna go ahead and erase this because we haven't lost our water completely yet. What we actually have lost is a water and now we're about to lose an H to regenerate the acid. Okay. So then finally, I have water here. By the way, this is the water that we lost earlier and now I'm going to attack that h And now I'm gonna get my product and my product would be the Solubios. Now guys, just I don't think I was very clear with what I just said. And what I was just saying is that if your professor wants every single arrow, you can't just put lose water and condensate. Okay? Because that's not providing every single arrow. You would have to do it the way that I just did it now which would give you your final product here, okay. But a lot of times professors and textbooks and homework problems take shortcuts and they don't draw the whole mechanism. They'll just draw like, oh, these 2 sugars come together, you lose water and you get the final answer here which is also acceptable but it's just like a shortcut to the mechanism. So in this case, I didn't want to take the shortcut. I want to do it the legitimate way so that you guys, in case you have to draw it this way, now you know how. Cool? So there is our our entire, mechanism. Let's just draw this reversible arrow as well. And now let's go back to the other question which is why do you think this would lead to a really poor yield of solubios? And guys, it's actually pretty obvious. Poor yields due to what do you guys think? I hear you saying it, you're not using the same words I'm gonna say but I think you got the point. Mixture of linkages. Right? You'd get a mixture of linkages because we just drew the beta 14, but you would get a mixture let me just put, like, 1 for, like, point number 1. You'd get a mixture of alpha and beta, which means that if you were just to link at the 14 position, that would be a mixture of, I think, of maltose and cellulose. So already you're at like at half and half right there, right? But then you'd also get a mixture of, positions. Right? So right now I just got a 14, but it could have also I could have gotten a 12 linkage, right, Or a 13 linkage or a 1 or a 1 6 linkage. All of these can be seen in different sugars. So right now we drew this happened to be the beta 14 because I was asking for solubiles, but it's very, very, if I were to just react this in acid, all of these different permutations would happen, which would mean that I would have almost no solubiles and I would just have a big conglomeration of disaccharides and trisaccharides and they would all just be kind of like intermingling and it would just not be a great synthesis. So just so you guys know, you might be saying, well then how do you make disaccharides? Guys, that's for a different video. The proper synthesis of disaccharides in the lab. If you're a sugar chemist, you're going to have to use protecting groups, you're going to have to use very special reagents that make sure that everything goes where you want. If you're, if it's in the human body, you have enzymes to make sure that you do that. You have specific enzymes to make sure that you make exactly the right disaccharides. But if you're in the lab, you can't just throw this with some acid and expect to get cellulose, you're going to get a mixture of everything. Okay. So guys I hope that made sense, let's go ahead and move on to the next page.

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Draw a theoretical acid-catalyzed Fischer Glycosidation mechanism for the condensation of D-Glucose into cellubiose. (Note:This reaction would lead to very poor yields of cellubiose- can you hypothesize why?)

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