Monosaccharides - Ruff Degradation

Hey guys. In this video I want to talk about a reaction, that's kind of opposite to Kiliani-Fischer change lengthening and that is the rough degradation, which is a chain shortening reaction, let's look into it. So guys, we know that Kiliani-Fischer exists to lengths and chains, we would use be cyanohydrins and then reduce them and eventually we get a new aldehyde group at the top but it turns out that reactions have also been designed to take carbons away and one of the ways that we've learned how to remove carbon in organic chemistry 2 is through reaction called decarboxylation, you guys remember decarboxylation? it was a reaction where a carboxylic acid turns into co2 gas and you lose the carbon in the process? Well, a genius, in a genius move they decided to apply that to sugars and they said, hey if we can turn the aldehyde from a sugar into a carboxylic acid then we could probably decarboxylated somehow, right? And that would shorten a chain and that's exactly, what we're going to learn how to do today. So, every time you do one of the rough degradation cycles you're going to lose one carbon and you're going to lose it because once co2 molecule is flying off into the atmosphere, okay? Now, unlike Kiliani-Fischer where remember how Kiliani-Fischer would make two different epimers because you are adding a new chiral Center and you didn't know which direction the OH would go but in a ring, I'm sorry, in a change shortening reaction, we're actually having less chiral centers so that means that we're going to have a stereo specific product, we're not going to have to worry about a mixture of epimers like, we do with Kiliani-Fischer, okay? Now, just as a just as a quick disclaimer, the c2 stereocenter is the one that's lost in a recycle. So, what we're talking about is this guy right here. Notice that right now I picked d-mannose as my original monosaccharide, which means that the OH is faced this way but afterwards that information is going to be lost because it's going to turn into an aldehyde, notice that right now it's chiral but after my reaction is going to be achiral. So, there's going to be no more stereospecific information at that position okay, cool? So, now I know you guys are ready to get into the reagents I've been talking a lot. So, what are the reagents used for a rough degradation? Well guys the first ones you already know, it's bromine water. So, remember that I said scientists were thinking hey there's got to be a way that we can do a decarboxylation, guys the easiest way that we know to turn a aldehyde into a carboxylic acid into aldonic tonic acid is just use weak oxidation with bromine water, we've done this before, you don't need to know the mechanism but you do need to know that this will oxidize to a carboxylic acid cool? that's the easy part, we already know that from before.

Now, the parts it's a little more tricky is that this is actually not the type of carboxylic acid that is easy to decarboxylate, do you guys remember? and this you can go ahead and look this up if you type in decarboxylation into the search bar clutch you'll see my whole video on this reaction but from memory, do you guys happen to remember which types of part of carboxylic acids were that easy ones to decarboxylate? it was the beta carbonyl, or the beta keto carboxylic acid. So, remember that it would always help that if this is your alpha carbon you want to have like a carbonyl next to it and that would make the whole mechanism go quickly and you'll be able to easily be decarboxylate, okay? But, we don't have that, in fact we don't have no other carbonyls. So, technically this shouldn't really decarboxylate that easy and that's why we're going to need very special reagents to do the next step, the next step is actually not going to proceed through the same decarboxylation mechanism you learned in the past, it's going to proceed through a new mechanism, that's actually mostly unknown all we really know is that it's a radical mechanism because it uses hydrogen peroxide, which is a radical initiator and then and iron sulfate complex, these two things together, what they're going to do is not only are they going to use radicals to decarboxylate but they're also going to oxidize, okay? So, they're going to do all that, they're going to use radicals to decarboxylate, take it off, and then to oxidize the final alcohol that's left. So, what you need to know here is not really the mechanism but what happens and how it works. So, remember that in the carboxylation you cleave off whatever carboxylic acid you have and the C and two O's becomes co2. So, that's where, this is going to go, it's going to become co2 gas, okay? Now, these three sugar, I'm sorry, not three sugar, these three hydroxyl groups are the same as the three hydroxyl groups over there, the only difference is that now my c2 position right here, like I told you it is going to become an aldehyde. So, it's going to basically turn into a double bond, I have it drawn to the left over here, here I have it form to the right it doesn't matter because it's trigonal planar. So, there's free rotation around that bond, it doesn't matter which structure you draw it in they're the same thing, okay? And again, I'm not going to show you the whole mechanism but you should just know that this is what this step does the radical decarboxylation step takes off the aldehyde and oxidizes that c2 alcohol into an aldehyde, okay? And then notice guys, what we're left over with is only the last three hydroxyl in the same place everything else above those last three hydroxyls got chopped off or changed, okay? And that is how we get d-arabinose in this case, which is the degradation products of d-mannose, cool? Awesome guys. so hopefully that makes sense, let's go ahead and move on to a practice problem