Monosaccharides - Reduction (Alditols) - Online Tutor, Practice Problems & Exam Prep

Monosaccharides can undergo reduction to form alditols, or sugar alcohols, which are no longer classified as sugars due to the absence of carbonyl groups. Common reducing agents like sodium borohydride (NaBH₄) facilitate this process, converting aldoses like glucose and mannose into their respective sugar alcohols, sorbitol and mannitol. Ketoses, such as fructose, yield a mixture of products due to racemization at the C2 position. The reduction mechanism involves nucleophilic addition of hydride to the carbonyl, followed by protonation, resulting in alcohol formation.

Monosaccharides - Reduction (Alditols)

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.

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. 

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Here’s what students ask on this topic:

What are alditols and how are they formed from monosaccharides?

Alditols, also known as sugar alcohols, are polyols formed by the reduction of monosaccharides. This process involves converting the carbonyl group (aldehyde or ketone) of the monosaccharide into an alcohol group. Common reducing agents like sodium borohydride (NaBH₄) are used to facilitate this reaction. For example, reducing glucose with NaBH₄ yields sorbitol, while reducing mannose produces mannitol. The reduction mechanism involves nucleophilic addition of a hydride ion (H^-) to the carbonyl carbon, followed by protonation to form the alcohol.

What is the role of sodium borohydride (NaBH₄) in the reduction of monosaccharides?

Sodium borohydride (NaBH₄) is a common reducing agent used in the reduction of monosaccharides to form alditols. It acts as a nucleophilic hydride donor, providing H^- ions that attack the carbonyl carbon of the monosaccharide. This nucleophilic addition converts the carbonyl group into an alcohol group. The reaction is typically followed by protonation, often using water or an acid, to complete the formation of the alcohol. NaBH₄ is particularly effective for reducing aldehydes and ketones without affecting other functional groups.

How does the reduction of ketoses differ from the reduction of aldoses?

The reduction of ketoses differs from the reduction of aldoses in that it produces a mixture of products due to racemization at the C2 position. When a ketose like fructose is reduced using NaBH₄, the hydride ion can attack the carbonyl carbon from either side, leading to the formation of two different alcohols. This results in a mixture of sugar alcohols. In contrast, the reduction of aldoses, such as glucose or mannose, typically yields a single product because the carbonyl group is at the end of the molecule, allowing for a more straightforward reduction process.

What are some common applications of alditols in the food and pharmaceutical industries?

Alditols, or sugar alcohols, have several applications in the food and pharmaceutical industries. They are commonly used as sugar substitutes due to their lower caloric value and reduced sweetness compared to regular sugars. Examples include sorbitol and mannitol, which are used in sugar-free candies, chewing gums, and diet foods. Alditols also serve as food thickeners and stabilizers. In the pharmaceutical industry, they are used as excipients in tablets and as laxatives due to their ability to retain water and promote bowel movements.

What is the mechanism of action for NaBH₄ in the reduction of monosaccharides?

The mechanism of action for NaBH₄ in the reduction of monosaccharides involves nucleophilic addition followed by protonation. NaBH₄ provides a hydride ion (H^-), which acts as a nucleophile and attacks the carbonyl carbon of the monosaccharide. This forms a tetrahedral intermediate with a negatively charged oxygen. The intermediate is then protonated, typically by water or an acid, to form the final alcohol product. This process effectively converts the carbonyl group of the monosaccharide into an alcohol group, resulting in the formation of an alditol.