Lesson 3 of 13 · 5 min
D and L: relative configuration
NCERT §10.1.2.1
The glucose packet says D-(+)-glucose and the fructose jar says D-(–)-fructose. Meera asks how both can be D when one turns light right and the other left.
The lesson in notes
In short
The full name is D-(+)-glucose. D describes configuration; (+) says it is dextrorotatory. D and L tell you nothing about the sign of rotation, and they are not the same as the small letters d and l.
D or L relates a compound's configuration to a reference whose configuration is known. For sugars the reference is glyceraldehyde, which has one asymmetric carbon and two enantiomers.
(+)-Glyceraldehyde is D: drawn in the standard way, its –OH is on the right. (–)-Glyceraldehyde is L, with –OH on the left.
Compounds that can be chemically correlated with D-(+)-glyceraldehyde are D; those correlated with the L-(–) isomer are L.
For a monosaccharide, only the lowest asymmetric carbon is compared, with the most oxidised carbon (–CHO here) written at the top. In glucose that carbon (C5) has –OH on the right, so glucose is D. The other asymmetric carbons are ignored for this purpose.
Fructose is D as well but laevorotatory, D-(–)-fructose: proof that D does not mean (+).