Organic Chemistry

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24. Carbohydrates

Monosaccharides - Reduction (Alditols)

As polyols with carbonyls, monosaccharides can undergo a series of oxidation and reduction reactions. Reduction of a monosaccharides produces polyols known as alditols or sugar alcohols.



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So it turns out that one of the important ways that monos Ackroyd's can react is through reduction, and that's what we're gonna explore right now. So, guys, uh, mono sacha rides are essentially Polly all's meaning multi alcohol's with carbon eels. So what that means is that they can undergo a Siris of oxidation and reduction reactions. So, guys, we're not done with the reactions like we have a lot of reactions coming up because model sack rates have so many functional groups that air reactive that there's just so many different places they could react. And the one I want to talk about right now is called monounsaturated reduction, which yields compound compounds called, um, al vittles. Okay, so reduction of mono, Sack writes, produces Paulie ALS, known as Al Battles or sugar alcohols. Okay, now the term sugar alcohol is misleading because you might think that it's either a sugar or it's an alcohol. It's neither because remember that to be a sugar, you need to have at least one I h. D. And it turns out that, uh, sugar alcohols have been reduced all the way down to just a Siris of alcohols with no carbon eels, so they're no longer considered sugars their their own molecule, called either sugar alcohols or alcohols. Cool Now, Al, the tools are used industrial industrially as sugar substitutes, food thickeners and medicinal E as laxatives usually. Okay, so you might have heard of these molecules before. If you look at the back of like your food contents, if you kind of really like what your food is made out of, Ah, lot of times you're going to see molecules like sorbitol, mannitol and what these are. These are sugar alcohols that they're reduced carbohydrates that are digested by your body very similarly to alcohol. But I'm sorry, very similarly to mono sacha rides, but they usually have less caloric value. They're not a sweet they're good at, like thickening foods and stuff like that. So, guys, the reducing agent that you really need to know is N A B H. Four n A. B H four is one of the most common reducing agents that's used. Remember the N A B H four sodium borough hydrate is a week reducing agent, so it's perfectly able to reduce an alga hide to an alcohol. And that's what we see here What we see is that if I were to take de manos and reduce, it was sort of burrow hydrate and then use H 30 plus to quench the negative charge and add a proton. Basically, the the second part is just the protein nation step. What we're going to get is a molecule that looks just like de manos. Except you now have an alcohol in place of the carbon deal. So it's a sugar alcohol. Let's look at D glucose so D glucose. Remember, what's the difference between manos and glucose? Their EP immerse, right? Just at the sea to position. So if I were to go ahead and reduce d glucose, I would get a very, very similar sugar alcohol to mannitol. Notice that they look almost exactly the same again. My carbon eel gets reduced. The Onley difference is that this O. H is facing in different directions on each of these. Okay, Now the reason is because the sea to position the Chire ality isn't changed. Reduction Onley occurred here and here, So that means that this stereo chemistry of your original sugar is conserved after your reduce. Okay, so you can reduce glucose to get sorbitol, you can reduce manos to get mannitol. Now, what about the strange situation where maybe you want equal amounts of both? If you wanted that to happen, what you could do is you could actually reduce d fructose because whereas reduction of all doses produces one product, as I just showed you, those were both all doses glucose and manuals, reduction of ketosis forms to products do to see to rasam ization. So let me show you. So what you have going on here is that this is the second carbon, right? The key toast is on the second carbon and the way the mechanism works, you're basically donating in h minus. And you could donate that H minus from either side. You could either donate it from this side, which would put the alcohol to the right. Or you could donate it from this side, which would make the alcohol go to the left. So it turns out that if you're reducing a key toast with NH four, you're actually gonna get a mixture of both products. So in this case looks look at my fructose fructose. Remember, the bottom three look exactly the same as glucose. They look exactly the same as manuals. Notice three Only difference is that instead of having another Ohh, there I have a key tone. That key tone could turn into an alcohol facing writer left. It gets Rasim ized, which is why the reduction of fructose is gonna lead to two different sugar alcohols or two different alcohols. Okay, Makes sense. Now, guys, the mechanism for this follows the same exact mechanisms that we used in the reduction chapter when we first learned how to reduce with lithium aluminum hydrated, sodium, borrow, hydrate. But let me just go over it just so that you guys will remember, because I know some of you guys are gonna ask. So, guys, remember that sodium borough hydride is a good hydride donor, and it's specifically it's a nuclear filic hydrate donor. So I have be h h h h negative. And then there's just a sodium hanging around with a positive charge. Those air ironically associated Okay. And remember that these reducing agents are a good source of new Cleo Filic H minus. It's different than if I just used any H. For example, if I just used n a age this is a base. This is not a good nuclear file. Okay, but if I use N a b h four, it turns it into a nuclear file. Okay, So the way this would work is that its nuclear filic addition n a p h four acts as a hydride donor and attacks the carbon vs via nuclear like addition And then pronation occurs So my h comes in attacks The partial positive of my carbon eel kicks up electrons to the oh, What I get is my tetra hydro Intermediate with a negative here and with an h here and then Protein nation occurs where I have whatever protein ating agent It could just be water that comes in and adds a proton. So what I'm gonna get it is an alcohol plus an H, and I know that's behind me. So I'm just gonna move up. So, guys, this is the general mechanism of reduction and this is how it works. It works on the straight chain form of the of the sugar. So in this case, once you have the straight chain form, you can add the H and you can turn it into an alcohol cool. Awesome guys. So that is a little bit about Al, the tolls and or sugar alcohols. Let's move onto the next video.

Whereas reduction of aldoses produces one product, reduction of ketoses forms 2 products due to C2 racemization. 


Determine the structure of the alditol formed when b-D-xylofuranose is treated with NaBH4 and then water. Explain how NaBH4 can reduce the hemiacetal group of the furanose.