Question

Sugar Alcohols In the monosaccharide derivatives known as sugar alcohols, the carbonyl oxygen is reduced to a hydroxyl group. For example, \(\mathrm{D}\)-glyceraldehyde can be reduced to glycerol. Why can the sugar alcohol glycerol no longer be designated D or L?

Short answer

Glycerol lacks a chiral center after reduction, so it can't be designated D or L.

Step-by-step solution

Understanding Sugar Alcohols

Sugar alcohols are formed by the reduction of a carbonyl group in a monosaccharide into a hydroxyl group. This process converts the carbonyl carbon into an alcohol carbon.

Analyzing Glyceraldehyde

The structure of \(\text{D-glyceraldehyde}\) includes a stereocenter where the \(\text{D or L configuration}\) is determined by the orientation of the hydroxyl group on the last stereocenter (farthest chiral carbon from the aldehyde group).

Reduction Process

In the reduction process, the carbonyl group of \(\text{D-glyceraldehyde}\) is converted into a secondary alcohol, forming glycerol. This process involves the loss of the object's stereochemistry at the previous aldehyde carbon, as it becomes saturated.

Chirality and Resolution

Glycerol, resulting from the reduction, lacks a chiral center because all of its carbons are bonded to either identical groups (hydroxyls) or hydrogen atoms, which means the molecule is now achiral.

Conclusion on D or L Designation

Since \(\text{glycerol}\) does not have a chiral center to determine the orientation of D or L, it cannot be designated as such. The lack of stereocenters means glycerol is achiral.

Key concepts

Monosaccharide Derivatives

Monosaccharides are simple sugars, the fundamental building blocks of carbohydrates. Sometimes, these sugars undergo chemical changes to form derivatives. One such derivative is sugar alcohols. In sugar alcohols, the carbonyl oxygen (found in the aldehyde or ketone form of a sugar) gets reduced to a hydroxyl group. This conversion changes the chemical behavior and physical properties of the sugar.
Monosaccharide derivatives are crucial in various biological functions, often influencing the flavor and digestion properties of sugars. Sugar alcohols, in particular, are used as sweeteners in food because they provide sweetness with fewer calories. Understanding monosaccharide derivatives helps in identifying how sugars can interact differently in chemical processes and within the body.

Chirality

Chirality is an interesting concept in chemistry. It refers to a molecule's mirror-image properties, meaning that the molecule and its mirror image cannot be superimposed. Think of your left and right hands; they are the same yet not identical when placed over each other. This non-superimposable nature is due to the presence of a chiral center, a carbon atom bonded to four different groups.
In the context of sugar alcohols derived from monosaccharides, chirality plays a pivotal role. Initially, monosaccharides like D-glyceraldehyde have chiral centers, which give rise to D or L configurations—these denote the spatial arrangement around the chiral center that dictates whether the molecule is left or right-oriented. But as we see with the reduction of D-glyceraldehyde to glycerol, this chirality can be lost, leading to non-chiral, or achiral, products.

Glycerol

Glycerol is an organic compound originally derived from glyceraldehyde through a reduction process. It is a simple sugar alcohol. Interestingly, glycerol has lost the chiral centers it once had as a monosaccharide. This results in a molecule that does not have any stereocenters left.
Glycerol is colorless, odorless, and a sweet-tasting liquid. It has a wide range of applications, from being used in the pharmaceutical industry as a humectant to being an ingredient in skincare products due to its moisturizing properties. The non-toxic and easily digestible nature of glycerol also makes it valuable in food science. Understanding glycerol's structure and its transformation from glyceraldehyde helps in grasping why it cannot be classified under the D or L configuration given its lack of chirality.

Reduction Process

The reduction process is a chemical reaction where a compound undergoes gain of electrons. In sugar chemistry, this process often reduces carbonyl groups to alcohol groups, turning monosaccharides into sugar alcohols.
To transform a carbonyl-centered molecule, like D-glyceraldehyde, into glycerol, the carbonyl group is targeted and reduced to a hydroxyl group. This process not only changes the type of the functional group (from a reactive carbonyl to a more stable alcohol) but also impacts the stereochemistry of the molecule. The original chiral center of glyceraldehyde is affected, and the conversion results in the generation of molecules that lack chiral centers. Therefore, the product of this reduction—glycerol—becomes an achiral compound, meaning it can no longer possess the D or L designation typically associated with sugars.