You may remember that periodic acid has the ability to cleave vicinal diols. Sugars contain mutliple diols that can potentially be cleaved, but other functional groups can be cleaved as well. Below we will explore each of them in detail.
Hey everyone in this video we're going to talk about how you can use periodic acid to cleave or break apart a monosaccharide completely, let's take a look. So, guys in general periodic acid has this ability to cleave vicinal diols and this is not a reaction that's unique to sugars, it's actually a reaction that occurs anytime that you expose periodic acid, which I'll show you in a second to a vicinal diol, it's always going to split the diol in half leaving oxygens on both sides, okay? Now, what do we know about sugars? sugars have multiple diols or multiple alcohols and when I say diols here I'm specifically same vicinal diol. So, is the sugars have multiple alcohols that are next to each other that could potentially be cleaved, okay? So, what we're going to learn here is not a new reaction completely, what it really is, is it's an application of another reaction that already exists, that other reaction says that if you take a diol and you expose it to periodic acid it's going to cleave. So, now why don't we do that with sugars, and it turns out that the mechanism we're going to use is even identical to the general oxidative cleavage of diols with periodic acid, this is just reinforcing what I'm saying how this is not a brand new reaction, it's just an application of another reaction to sugars, cool? So guys, let's start off with the basics vicinal diol. Remember, that a vicinal diol is an alcohol or is a diol that it's, that's the two alcohols are next to each other, they're not geminal, geminal would be on the same carbon vicinal they're next to each other, okay? So, you need a vicinal diol to make this work, we need to expose to some form of periodic acid. Now here, I've gone ahead and drawn out the periodic acid for you, this is what it should look like, if you're given this form of periodic acid, which is the most common form but guys it turns out that lots of different professors and textbooks for some reason like to overcomplicate periodic acid and they like to draw it a bunch of different ways. So, here what I did is I listed out all the different ways that I've seen it written and that I want you to think of as synonyms for periodic acid. So, please don't, like I said, don't overcomplicate it, just think of them all as forms of periodic acid. So, you could see, let's just go in order, you could see periodic acid in water, same thing you could see instead of four oxygens, three oxygens, that's called ionic acid but that also functions, right? Similarly you could see it as i, o 4 negative, which is the anions, that's called periodate and then finally you could even see it with six oxygens, which is actually just another form of periodic acid. So again, don't worry too much about it just think if you see a lot of oxygens around an iodine, this is a form of periodic acid and this is going to be some form of cleavage if diols are present, okay? So, let's say that you expose the periodic acid to the diol, what you're going to wind up getting is this cyclic structure, okay? Now, you don't need to know the mechanism for this part but we are going to quickly go over the mechanism for the cyclic part because this part of the mechanism is shared with the general reaction of periodic acid cleaving diols, okay? So, first of all, let me just name this for you, this structure here is called a cyclic periodic ester, okay? The cyclic periodic ester is one of the intermediate steps of oxidative cleavage with periodic acid, okay? And at this point we formed our cyclic ester and now all we need to do is break apart the Sigma bond that's holding the two carbons together, the Sigma bond I'm talking about is this one, this is the one that we're going to try to break apart because that's what cleavage means, we're going to break carbons apart, so the mechanism for this is pretty straightforward, all it is is that you take the electrons that used to be connecting those carbons together and you make a carbonyl out of them. So, you make a carbonyl of one of them. Now, you can go either way, I could go to the left or to the right, it's going to be a cyclic mechanism and it's all concerted, meaning it all happens at the same time. Now, if I make that double bond O, I'm going to have to break upon, right? So, the way that that bond is going to break is that we're going to then take electrons and put them on the I, okay? So, because that oxygen needs to break a bond in order to not be positively charged.
So, we're going to take those electrons away put them on the I and now what we have to do is we have to basically keep the formal charge of the I the same. So, we're going to break up on, on the I and we're going to take these electrons and make a carbonyl down here. So, as you can see, we're just redistributing the same electrons that we're already there and what we're going to get at the end is now two carbonyls with this bond breaking, right? That bond is gone and now we're going to get two carbonyls. So, let's draw, what that would look like. So, what this would look like is on the left side I would have O, double bond, whatever that group was on the left I'm just going to keep it at a stick because, I don't know what that is and then on the right side, I'm going to have an h. Now, this h is this h right here, okay? And I mean just to be super clear this stick is this stick right here, cool. And then on the other side I would just get the same exact thing just flipped around, what I would get is then I have that stick there and this h here, that's that and, oops, almost perfect, there we go. So, look, what we've accomplished and then you get your ionic acid as a byproduct, okay? So. Notice, what we're accomplishing, we're cleaving because we're taking two carbons and we're separating them and we're also oxidizing, that's why this is called oxidative cleavage because I start off with alcohols and I'm ending up here with aldehydes, okay? So, guys, that's the general mechanism, your professor may or may not want you to know that part of the reaction but now in the next video what I'm going to do is, I'm going to show you, what you actually need to know the four cleavage patterns for oxidative cleavage, okay? So, let's go that video.
So, this is the part that can get a little bit tricky if you don't have everything mapped out already, so it turns out that different types of alcohols and carbonyls on sugars respond differently to periodic acid, it's not like a one-size-fits-all that it's always an aldehyde, it's actually not, you have to kind of do a little bit of memorization here. So, what I've done is I'm trying to organize it for you the easiest way possible so that you're going to just be able to look at this chart and know exactly what to do, just to repeat, you can't just guess what the product will be the product is very specific depending on, what type of alcohol or carbonyl you're starting with. So, what I'd like to start with is basically the difference between an aldose and a ketose, okay? So, remember that most, I mean, sugars always come in either the form aldose or ketose, okay? Remember, that an aldose at the top is going to have an aldehyde. Remember, that a ketose at the top is going to have a ketone and that's what I've just drawn here, okay? Well, that oxygen that's either in the aldose or the ketose will react with periodic acid. Now, this isn't the same exact reaction as the vicinal diol z' reaction because this is not an alcohol, it's a carbonyl but it still is going to oxidize that carbonyl, okay? So, how would we oxidise these carbonyls? Well, if you're starting off with an aldehyde, what you're going to wind up getting at the end is you're going to wind up getting, and I'm going to say this a lot, formic acid, okay? Now, formic acid is the common name for this molecule, you could also call it methanoic acid, which would be the iupac name but most commonly is called formic acid and that's just the simplest carboxylic acid possible, it's a carboxylic acid that only has one carbon. So, if you have an aldehyde at a terminal end, well, aldehydes are always terminal you're going to get formic acid as your oxidation product. Now, if we start up with a ketone for a ketose at the top then you're not going to get formic acid, you're actually going to get co2. So, I'm just going to put your co2, we all know that's carbon dioxide. So, you can see that in both cases we're oxidizing, we're adding more bonds to O but so the exact products are a little bit different. So, everyone got that so far, we've got formic acid, we got co2, but we have these. Now, we have to look at the alcohols, those are the carbonyls but what about the alcohols? Well, if you have an alcohol that is an internal alcohol, an internal alcohol, what do I mean by internal? there are things on both sides so it has something on the top and something on the bottom, what you're going to wind up getting from that is also formic acid, okay? You're also going to get one equivalent of formic acid for every internal alcohol that you have okay, cool? And, if you, let's say you have a terminal alcohol, if you have a terminal alcohol. Now, by the way, this terminal alcohol I drew as if it was the bottom but we know that terminal alcohols could also exist on the top because maybe you have a ketone in the top. So, then you have an alcohol there, that's fine, whenever you have a terminal alcohol your product is going to be this, which is the the condensed formula for it, is Ch2O, which is also known as formaldehyde, oops, let's try that again, formaldehyde and we know that formaldehyde is the simplest aldehyde, okay? So, you're either going to get the smallest aldehyde or the smallest carboxylic acid or co2, carbon dioxide, okay? So, that's what you need to know and if you know these four cleavage patterns then you should be able to take any molecule, react any sugar, sorry, any monosaccharides react it with periodic acid and predict exactly what products you're going to get, okay? So, why don't we go ahead and do a example of this, together, to get our, to get some practice figuring out what the products would look like.
These are the 4 cleavage patterns of monosaccharides you should memorize:
Example: Predict the products of the following oxidative cleavage5m
Example: Predict the structure of the glycoside products13m