Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Synthetic Polymers and Biopolymers
20.14 Monosaccharides, Monomers of Carbohydrates

Carbohydrates are another type of biopolymers. The monomers of carbohydrates are called monosaccharides, and they are the building blocks of more complex sugars such as disaccharides and polysaccharides. Monosaccharides have different names, and the type of reactions that monosaccharides undergo can be determined by the name of the monosaccharide. If the name of a monosaccharide ends with “OSE,” then that monosaccharide is a reducing sugar. That is, present on the molecule is a functionality that has a carbonyl group or an OH group, which can be oxidized and hence serve as a reducing agent. An aldose has an aldehyde present in the molecule, and a ketose has a ketone present. The number of carbons present in a monosaccharide is represented within the type name. For example, if the monosaccharide has six carbons, it is called an aldohexose. Shown below are examples of aldohexoses.

From the above monosaccharides, you will notice that they all have more than one stereogenic center, and hence several stereoisomers. It was observed that natural occurring monosaccharides have the same configuration as D-(+) glyceraldehyde, which was discussed in Section 20.8.

Since monosaccharides have more than one stereogenic center, monosaccharides with the stereogenic center farthest from carbon 1 (or the carbon of the aldehyde functionality) is used to assign a D or L configuration. For a particular monosaccharide, if the OH group that is furthest from the aldehyde is on the right, the monosaccharide is assigned a D configuration. On the other hand, if the OH group is on the left, the monosaccharide is assigned the L configuration as shown in the examples below.

Epimers are diastereomers that differ in configuration at only one of the chiral carbon atoms. Thus, D-glucose and D-galactose are epimers since at carbon 4 and only at that carbon, there is a difference in stereochemistry.

Problem 20.14

What are the structures of L-glucose, L-galactose, and D-mannose?