Monosaccharides - Osazones

Concept: Concept

11m
Video Transcript

What's up, in this video we're going to talk about a carbonyl side reaction of monosaccharides called osazones. So guys, one of the reasons that you've got to love monosaccharides is because they have so many functional groups, they have so many alcohols, they have carbonyls and in this video we're going to talk about a specific reaction that happens to the carbonyl of a monosaccharide and it turns out that al aldose aldehydes so the aldehydes of the top of an aldose are susceptible to the same exact nucleophilic addition reactions that we learned back for carbonyl chemistry, okay? So, you can't forget those reactions those reactions are very relevant to monosaccharides because monosaccharides will have carbonyls that can react in those reactions. So, do you guys remember, what would happen, when you expose a ketone or an aldehyde to a primary amine in acid, do you guys remember what happen? what you would get is an ammine, you would get a functional group called an ammine that had a double bond n to the carbon okay? Well, remember that similarly, if you exposed a carbonyl to a primary amine derivative, meaning it's not exactly ammonia it's not exactly a primary amine maybe it has some Z group or x group on it like alcohol or a or a benzene ring or something like that does that benzene ring would stick off of the n but you still get amines, that product would be called an imine derivative and guys, if you're getting a little bit lost but what I'm saying maybe you've just started clutch and you just started watching this chapter, I have a whole section in carbonyl chemistry talking about amines and amine derivatives, okay? So, you can go back and watch all those videos, anyway, what I'm trying to do is to remind you of what we already know. So, we can move on and understand osazones. So, when the specific amine, primary amine derivative phenylhydrazine is used the product is referred to as a phenylhydrazone and these are again things you've already discussed from the carbonyl chemistry chapter. So, when I talk about phenylhydrazine, what I'm talking about is this molecule right here, Now this is called a primary amine derivative why? Because notice, I have a primary amine here, this is a nitrogen with two H's and one R group technically but the reason that it's not an amine and it's an amine derivative is because instead of using an R group I'm actually using what I call in my videos as a C group, okay? Which is basically something electronegative or something that's different from R. So, in this case phenylhydrazine is a primary amine derivative because it has the NH2 and the benzene ring as a c group, okay? But you could actually, if you wanted to just write the general formula of this it would be NH2 Z, with something on the side that is attached to the end, cool? Well guys, what we learned and this is, right now this is just recollection, I'm not teaching you anything new, if you're to react a primary amine derivative with the carbonyl and acid, what you're supposed to get is that n double bonded to the carbon with the Z coming off. So, guys don't actually write this whole thing out just let me write it and then I'm going to erase and show you. So, what we would expect according to our rules of using an imine derivative is we should expect a double bond n with a Z, okay? And you might be saying, we'll Johnny, what happened to the H's? that's part of the whole mechanism them in the mechanism they come off but what you get is this group here and that's called an amine. Now, the reason that I'm saying it's an amine derivative is because there's a Z group? Well, what is the Z group? it's the N in the benzene ring. So, let's go ahead and plug that in and this is what I want you guys to write down I want you guys to write down that you expect to get a double bond end with another n with a benzene ring and this end, the red end does require an h to complete three necessary bonds so I'm justo going to put h, double bond, double bond, double bond and there you have it, this is what we would expect just even if i hadn't taught you anything new, if you had just seen this reagent with acid and a carbonyl you should know from carbonyl chemistry that the aldehyde would make a phenylhydrazone or a phenylhydrazone derivative because in this case it's not just the phenylhydrazone that has a sugar attached to it. So, that's why i'm calling it a phenylhydrazone derivative, okay? So guys, so this is the part that you should already know and that's why I keep saying like, remember, recall because it's something that we've already learned from before but now this is where things get weird because surprisingly when exposed to additional equivalents of phenylhydrazine meaning that instead of just using a 1 to 1 mole ratio what if we use 3 moles or X's moles of phenylhydrazine, your amine derivatives or basically this thing right here, the phenylhydrazine derivative is going to continue to tautomerize and react, and the resulting product is going to be something utterly new, completely new that you've never learned before and that's why we need to learn it here and that is going to be a c1-c2 diphenylhydrazone derivative that we call an osazone, okay? So, I know that was really big term c1-c2 diphenylhydrazone but i think that the name there speaks for itself, this is C1, this is C2 and basically what ends up happening is that through a mechanism that we're not going to go through here, we're not going to go through the whole mechanism, it's actually, if your professor makes you draw the entire mechanism for drawing osazone product, your professor hates you.

So, I'm going to go ahead and skip that mechanism here, if you really need to learn that whole mechanism I'm sure you can find it somewhere on Wikipedia or something but what I really want you guys to know is just the general product, the general idea of this and hopefully, really praying for you guys that you do not have to draw the whole mechanism because it's very intense, but what ends up happening is that tautomerization occurs at this O. So, like kind of like the double bond ends up moving over here at some point and then what ends up happening is that two more equivalents of your phenylhydrazine are going to react at this position and according to what I'm stating that the product is, that means that we should draw another imine derivative or another phenylhydrazone at the two position. So, let's go ahead and do that now, where the final product is going to look like is n with another double bond with another NH and with another benzene ring, cool? And that is your final product that is an osazone. So, a few takeaways here, first note that you always have to use at least three equivalents of the federal hydrazine to produce this and even though you use three equivalents you're only going to get two phenylhydrazones in your products. So, it's kind of just something to keep in mind, another thing that I want to say that you guys should all be aware of, is that usually in a question a professor is always going to want you to go all the way to the osazone, the only reason that I split this page up into step one, for the first one, and step two for the second one was for teaching purposes only because they want you to recollect what we already learned but your professor is never going to ask you to just draw the first phenylhydrazone product, okay? What they want to see, if they, if your professor or your textbook or your online homework ever reacts a saccharide with the phenylhydrazine and acid they expect you to go all the way to the osazone and they expect you, you can skip the middle step, you can just draw the ozone because that's exactly what they're looking for they're always going to assume that you have enough phenylhydrazine present to make the osazone does that make sense? Cool. So, don't don't get too carried away saying but how many equivalents are there are there one or two, don't worry about that, I just did that for teaching purposes you can understand the reaction better but really in actuality you're always have enough phenylhydrazine to complete the osazone, okay? Lastly I just want to give you a historical point of why this reaction is important why a lot of professors and textbooks care to teach it, this was a very important reaction for Emil Fischer who's really the father of carbohydrate chemistry, in 1888 he used osazone to prove that glucose and mannose were epimers of each other and basically osazones were actually very toxic, he ended up dying because he was exposed to so much phenylhydrazine and osazone in these experiment that he got I think like cancer all over the body, it's a really sad story, it doesn't end there, it gets even more sad, but just letting you know that he's heralded as one of like the best chemist ever and he is the father of sugar chemistry because he was able to use osazones which crystallize out of solution very easily to determine that two different type of sugars were epimers of each other and then eventually he was able to build this entire beautiful proof called the Fischer proof that showed what all of the structures of monosaccharides were. So, what I'm trying to say is that we don't really use osazones that much now, but the reason that they continue to be taught in textbooks is because they were very important back in the day to prove what the structure of monosaccharides was before we knew what they looked like. Awesome guys, so hopefully this made sense, let's go ahead and move on to the next video.