5.2: General class names and Common names of monosaccharides
- Assign and interpret class names of monosaccharides.
- Assign D/L stereodescriptors to the common names and define epimers.
- Draw structure from the name and vice versa for important monosaccharides, including D-ribose, D-glucose, D-mannose, and D-galactose.
General class names of monosaccharides
Monosaccharides are either polyhydroxy aldehydes that take aldo as prefixes or polyhydroxy ketones that take keto as prefixes in their general name. The general formula of monosaccharides is \(\ce{C_{n}H_{2n}O_{n}}\) where n can be 3, 4, 5, 6, 7, or 8 representing triose, tetrose, pentose, hexose, heptose, or octose, respectively, in the general name. For example, D-glucose belongs to aldohexose, where the aldo- prefix tells it is an aldehyde, -hex- in the middle of the name tells it has six \(\ce{C's}\), and -ose suffix denotes it is a carbohydrate. D-fructose belongs to ketohexose, i.e., it is a monosaccharide with a ketone group and six \(\ce{C's}\). D-glyceraldehyde is an aldotriose, i.e., a monosaccharide with an aldehyde group and three \(\ce{C's}\).
Common names of monosaccharides
Common names are specific for each monosaccharide. All \(\ce{C's}\) in a monosaccharide are chiral centers except the two terminal \(\ce{C's}\) and the \(\ce{C}\) of ketone group if it is a ketose. The absolute configuration of the penultimate \(\ce{C}\), i.e., the second-last \(\ce{C}\), is explicitly expressed by D- or L-stereochemical descriptors and the absolute configuration of all other chiral centers is implicit in the common name of the monosaccharide. There is one set of common names for all D-isomers, and their mirror images (enantiomers) have the same common name with D- replaced with L-. For example, D-glyceraldehyde and L-glyceraldehyde, D-glucose, and L-glucose are enantiomer pairs shown in Figure \(\PageIndex{1}\).
If the \(\ce{-OH}\) group on the second-last carbon (penultimate \(\ce{C}\) or the second \(\ce{C}\) from the bottom end) in Fisher projection of a monosaccharide is on the right side, it is assigned D- and if it is on the left side, it is assigned L-configuration. Monosaccharides that are enantiomer pairs have the same common name, but D- is replaced with L- or vice versa. For example, D-glyceraldehyde and L-glyceraldehyde, D-glucose and L-glucose enantiomer pairs are shown in Figure \(\PageIndex{1}\). with the penultimate \(\ce{C}\) defining D- or L-configuration shown in red color.
D-configuration of monosaccharides is commonly found in nature. The D/L stereodescriptors do not indicate the rotation of the plane polarized light, i.e., the enantiomer's dextro/levo rotatory nature. However, if one enantiomer is dextro (d-), the other is levo (l-) to the same degree, and vice versa.
The structures as Fisher projections and common names of D-aldoses are shown in Figure \(\PageIndex{2}\). and those of D-ketoses are shown in Figure \(\PageIndex{3}\). These are the most common monosaccharides found in nature.
Some important monosaccharides
D-Glucose is the most abundant monosaccharide in nature. Plants produce it in a photosynthesis process. D-Galactose and D-mannose are two important diastereomers of D-glucose that differ from D-glucose in the configuration of only one chiral center.
Epimers are diastereomers that differ in absolute configuration of only one chiral center. For example, D-galactose configuration is different from D-glucose only at \(\ce{C}\)#4, i.e., D-galactose is a \(\ce{C}\)4-epimer of D-glucose. Similarly, D-mannose is \(\ce{C}\)2-epimer of D-glucose.
D-Fructose is another important monosaccharide that differs at \(\ce{C}\)#1 and \(\ce{C}\)#2 from glucose. That is, the \(\ce{C=O}\) is an aldehyde group at \(\ce{C}\)#1 in D-glucose, but it is a ketone at \(\ce{C}\)#2 in D-fructose. Fisher projections of D-glucose, D-galactose, D-mannose, and D-fructose are shown in Figure \(\PageIndex{4}\). with the differences from D-glucose highlighted by red-color fonts. D-ribose is another important monosaccharide present in RNA.
Drawing structures of important monosaccharides
D-ribose is aldopentose, i.e., an aldehyde with five \(\ce{C's}\). All chiral \(\ce{C's}\) have \(\ce{-OH}\) groups oriented towards the right in the Fisher projection. D-allose is aldohexose with the same structural features as D-ribose, i.e., all chiral \(\ce{C's}\) have \(\ce{-OH}\) groups oriented towards the right in the Fisher projection. D-glucose is \(\ce{C}\)3 epimer of D-allose. The other three important monosaccharides can be drawn by relating them to D-glucose, i.e., D-galactose is a \(\ce{C}\)4 epimer, D-mannose is a \(\ce{C}\)2 epimer of D-glucose, and D-fructose has a ketone group at \(\ce{C}\)2 in the place of aldehyde group of D-glucose at \(\ce{C}\)1.