So it turns out that one of the important ways that monosaccharides can react is through reduction, and that's what we're going to explore right now. Monosaccharides are essentially polyols, meaning multi-alcohols with carbonyls. This means that they can undergo a series of oxidation and reduction reactions. We have a lot of reactions coming up because monosaccharides have so many functional groups that are reactive. There are just so many different places they could react. The one I want to talk about right now is called monosaccharide reduction, which yields compounds called aldotols.
Reduction of monosaccharides produces polyols known as aldotols or sugar alcohols. The term sugar alcohol is misleading because you might think that it's either a sugar or an alcohol. It's neither. To be considered a sugar, you need to have at least 1 IHD. Sugar alcohols have been reduced all the way down to just a series of alcohols with no carbonyls, so they're no longer considered sugars; they're their own molecule called either sugar alcohols or aldotols.
Aldotols are used industrially as sugar substitutes, food thickeners, and medicinally as laxatives. You might have heard of these molecules before. When you look at the back of your food contents, you might see molecules like sorbitol and mannitol. These are sugar alcohols that are reduced carbohydrates, digested by your body similarly to monosaccharides, but they usually have less caloric value, are not as sweet, and are good at thickening foods.
The reducing agent that you really need to know is NaBH4. NaBH4 (Sodium Borohydride) is a weak reducing agent, perfectly able to reduce an aldehyde to an alcohol. If I were to take D-Mannose and reduce it with Sodium Borohydride and then use H3O+ to quench the negative charge and to add a proton, what we're going to get is a molecule that looks just like D-Mannose, except it now has an alcohol group. Reduction maintains the stereochemistry of your original sugar molecule.
You can reduce glucose to get sorbitol, you can reduce mannose to get mannitol. Now, if you wanted to have equal amounts of both, you could reduce D-fructose. Reduction of aldoses produces one product, as glucose and mannose are both aldoses. Reduction of ketoses forms two products due to C2 racemization. When reducing a ketose with NaBH4, you're going to get a mixture of both products. This is due to the ketose's C2 position being able to receive the hydride from either side, leading to the production of two different aldotols.
The mechanism for this follows the same procedure used in the reduction chapter where we learned how to reduce with lithium aluminium hydride and sodium borohydride. Recall that sodium borohydride is a good hydride donor. It is a nucleophilic hydride donor, with Na+ ionically associated to BH4−. Sodium borohydride acts as a hydride donor and attacks the carbonyl via nucleophilic addition. The protonation step then occurs, generally using water as the protonating agent, finalizing the formation of an alcohol. This process preserves the straight chain form of the sugar, transforming it into an alcohol. Awesome, that is a little bit about aldotols and sugar alcohols. Let's move on to the next video.
