oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic (–OH) group in glucose. CHO (CH ) CH OH Oxidation (CH ) CH OH (CH ) Oxidation Saccharic acid Gluconic acid The exact spatial arrangement of different —OH groups was given by Fischer after studying many other properties. Its configuration is correctly represented as I .
So gluconic acid is represented as II and saccharic acid as III . CHO CH OH I CH OH II III Glucose is correctly named as D(+)-glucose. ‘D’ before the name of glucose represents the configuration whereas ‘(+)’ represents dextrorotatory nature of the molecule. It should be remembered that ‘D’ and ‘L’ have no relation with the optical activity of the compound.
They are also not related to letter ‘d’ and ‘l’ (see Unit ). The meaning of D– and L– notations is as follows. The letters ‘D’ or ‘L’ before the name of any compound indicate the relative configuration of a particular stereoisomer of a compound with respect to configuration of some other compound, configuration of which is known. In the case of carbohydrates, this refers to their relation with a particular isomer of glyceraldehyde.
Glyceraldehyde contains one asymmetric carbon atom and exists in two enantiomeric forms as shown below. (+) Isomer of glyceraldehyde has ‘D’ configuration. It means that when its structural formula is written on paper following specific conventions which you will study in higher classes, the –OH group lies on right hand side in the structure. All those compounds which can be chemically correlated to D (+) isomer of glyceraldehyde are said to have D- configuration whereas those which can be correlated to ‘L’ (–) isomer of glyceraldehyde are said to have L—configuration.
In L (–) isomer –OH group is on left hand side as you can see in the structure. For assigning the configuration of monosaccharides, it is the lowest asymmetric carbon atom (as shown below) which is compared. As in (+) glucose, —OH on the