Monosaccharides - Aldose-Ketose Rearrangement: Videos & Practice Problems

In basic conditions, monosaccharides undergo tautomerizations and isomerizations, notably the aldose-ketose rearrangement, also known as the enediol rearrangement. This process allows aldoses to convert into ketoses, such as D-fructose from D-glucose. The mechanism involves the removal of an alpha hydrogen, forming an enediol intermediate, which can lead to epimerization at various carbon positions. Understanding these transformations is crucial for grasping carbohydrate chemistry and the behavior of monosaccharides under different conditions.

General Reaction

On this page, I want to talk about another transformation that monosaccharides can undergo in base. So guys, in basic conditions, monosaccharides will undergo a multitude of tautomerizations and isomerizations. This is one reason that we typically don't want to expose monosaccharides to base because you get such a mixture of isomers that it just makes a big mess out of things. The most profound of these transformations that's possible is the ability of aldoses to reversibly rearrange into ketoses in the presence of base literally making brand new monosaccharides. This process is called a bunch of different things, so we have to keep our names straight. First of all, it's simply just called an aldose-ketose rearrangement. So that's the title of this page. It's also called the anediol rearrangement, where anediol is the intermediate that's going to be used for this process. If your professor says that they want you to be able to draw the mechanism for the anediol rearrangement, this is the reaction they're talking about. Finally, it's actually also called the L'Obry-Bruijn-Van Eckenstein reaction. Yes, that's the real name. If you look on Wikipedia, that is a name for this reaction. We need to keep our names straight, but at the end of the day, that's really the hardest part. The mechanism is not as difficult as remembering the name, okay?

Before I show you the detailed mechanism, I want to show you the general reaction, so you guys can get a sense of how inefficient this type of reaction is. We're going to pick on beta-D-glucopyranose because this is a monosaccharide you should know very well by now. We've already discussed how it's possible for a beta-anomer to mutarotate to a combination of alpha and beta anomers. That's called mutarotation. We've also already discussed how it's possible in base for a monosaccharide to epimerize at the C2 position. And that's what we get here in the second product where glucopyranose turns into mannopyranose because the C2 position becomes racemized, with half of it going right and half of it going left. This is called epimerization. But finally, we also have the ability to completely rearrange glucopyranose into a furanose that's made out of a ketone. So it's possible for beta-D-glucopyranose to become defructofuranose, which is just a completely different monosaccharide, a different ring, and everything's different about it, and this happens through what we call rearrangement or what's also known as the anediol rearrangement. So, in the next video, I will show you guys the whole mechanism for how we obtain this molecule. Let's move on to the next video.

Mechanism

So we already know that base is the ability to remove the alpha hydrogen of a monosaccharide and that's the same exact step for the enediol rearrangement because remember the enediol rearrangement has to get to the enediol intermediate. So that means that the first few steps of this reaction are exactly like epimerization. So my OH comes in, grabs the alpha hydrogen, forms a double bond, kicks electrons up to the O. What I wind up getting is a double bond with a negative charge where stereochemical information has been lost from carbon 2. And we know that to get to the enediol we're going to have to protonate, so let's go ahead and do that. We're going to protonate with the conjugate acid of water. And that's going to give us our enediol. Okay? And then we know beyond that that if we then just go backwards throughout the whole mechanism that we just did and deprotonate that H, we can go and epimerize C2 to get mannose. Let's show how that happens once again. You would then use OH− and just pull that H right back off to form a negative charge here, double bond, and then we would use water to protonate the alpha position because you reform the carbonyl. The double bond grabs the H and kicks off the OH and what we wind up getting is D-mannose. So this whole process that we just did was just epimerization. Okay? But it was epimerization specifically through the enediol mechanism. Okay?

Now this isn't the only thing that you can do with an enediol because what if the base instead of grabbing the top proton, what if it grabbed the second proton? What if it grabbed the proton off of carbon 2? Would that change the product? Actually, it would change it very significantly. So let's see what would happen if instead of grabbing the top one, we grab the bottom one. If I grab the bottom one and formed a negative charge, it's going to need 2 arrows. Let's just do this real quick. OH−. I grab the H and then the H gives its electron charge and an H, and guys, this is an enolate once again, but now it's the other enolate, okay? So now it's possible to just regenerate that double bond and get rid of it. So basically what I could do is I could take my water and I could use this negative charge here to make the double bond and protonate the alpha position. So basically everything that I did with this enolate would happen with this one, but now it's happening with the double bond being reformed on C2 instead of C1. Notice how in the enolate above for the epimerization, C1 reformed the carbonyl, but for my bottom enediol, C2 formed a carbonyl. So what that's going to do is it's going to give me a double bond here now, an H that's attached an extra H that's attached to here and this blue H is still here. Okay? So what we just did is we just turned D-glucose, which is an aldohexose into, we just turned it into a ketohexose, D-fructose because we were able to us

Here’s what students ask on this topic:

What is the aldose-ketose rearrangement in monosaccharides?

The aldose-ketose rearrangement, also known as the enediol rearrangement, is a process where an aldose (a monosaccharide with an aldehyde group) is converted into a ketose (a monosaccharide with a ketone group) under basic conditions. This transformation involves the removal of an alpha hydrogen, forming an enediol intermediate. For example, D-glucose (an aldose) can be rearranged into D-fructose (a ketose). This reaction is significant in carbohydrate chemistry as it demonstrates the dynamic nature of monosaccharides under different conditions.

How does the enediol intermediate form during the aldose-ketose rearrangement?

The enediol intermediate forms during the aldose-ketose rearrangement when a base removes the alpha hydrogen from the monosaccharide. This removal creates a double bond and a negative charge on the oxygen, leading to the formation of an enediol. The enediol can then undergo further transformations, such as protonation and deprotonation, to form either an epimer or a ketose. This intermediate is crucial for the rearrangement process and can lead to various isomerizations and tautomerizations in monosaccharides.

What is the significance of the enediol rearrangement in carbohydrate chemistry?

The enediol rearrangement is significant in carbohydrate chemistry because it illustrates the flexibility and reactivity of monosaccharides under basic conditions. This rearrangement allows aldoses to convert into ketoses, leading to the formation of new monosaccharides. It also highlights the potential for epimerization at different carbon positions, which can result in a variety of isomers. Understanding this process is essential for comprehending the behavior of carbohydrates and their transformations in different chemical environments.

Why are monosaccharides typically not exposed to basic conditions?

Monosaccharides are typically not exposed to basic conditions because they undergo multiple tautomerizations and isomerizations, leading to a complex mixture of isomers. This can make it challenging to isolate and identify specific products, resulting in a 'mess' of different compounds. The aldose-ketose rearrangement, epimerization, and other transformations can occur simultaneously, complicating the analysis and purification of the desired monosaccharide. Therefore, to maintain the integrity of the monosaccharide, it is generally avoided to expose them to basic conditions.

What is the mechanism of the aldose-ketose rearrangement?

The mechanism of the aldose-ketose rearrangement involves several steps. First, a base removes the alpha hydrogen from the aldose, forming a double bond and a negative charge on the oxygen, creating an enediol intermediate. This intermediate can then be protonated and deprotonated at different positions, leading to the formation of a ketose. For example, in the case of D-glucose, the enediol intermediate can rearrange to form D-fructose. This process highlights the dynamic nature of monosaccharides and their ability to undergo significant structural changes under basic conditions.