D and L Configuration of Monosaccharides

Beyond classifying a sugar as an aldose or ketose, chemists also assign it a configurational label, D or L, that describes the three-dimensional spatial arrangement of its atoms rather than any property that can be seen directly, such as the direction it rotates plane-polarised light.

The reference point for this labelling is the simplest possible chiral monosaccharide, glyceraldehyde ($2,3$-dihydroxypropanal, $\text{CHO}{-}\text{CHOH}{-}\text{CH}_2\text{OH}$), which has exactly one chiral carbon, C2. In the Fischer projection of glyceraldehyde, if the $-\text{OH}$ group on C2 is drawn on the right-hand side, the molecule is designated D-glyceraldehyde; if it is drawn on the left, it is L-glyceraldehyde.

For a larger sugar such as glucose, which has several chiral carbons, the same D/L convention is applied by looking specifically at the highest-numbered chiral carbon in the chain -- the one nearest the terminal $-\text{CH}_2\text{OH}$ group and farthest from the carbonyl group -- because that is the carbon whose configuration is directly comparable to glyceraldehyde's own single chiral centre. In D-glucose, this reference carbon is C5, and its $-\text{OH}$ group is drawn on the right in the standard Fischer projection, exactly matching the arrangement in D-glyceraldehyde -- which is why glucose is designated D-glucose. The same reasoning, applied to fructose's reference carbon (again C5), shows that the naturally occurring form of fructose is likewise D-fructose. In fact, the great majority of carbohydrates found in nature belong to the D-series; L-sugars exist but are comparatively rare. …