Relative and Absolute Configurations or D-L Nomenclature Preparation and Properties
→ Two configurations are possible for a compound having one asymmetric carbon atom which represents two optical isomers. One of them is dextro rotatory and other one is laevo rotatory. E. Fischer suggested that optical activity is an experimental property.
→ In other words, we need a system of nomenclature that indicates the configuration (arrangement) of the atoms or groups about the asymmetric carbon. Some rules are given by Fischer to determine the configuration of asymmetric carbon atom.
→ This is a system of assigning relative configuration to a molecule by establishing configurational relationship among the members of a family of compounds (i.e., carbohydrates and amino acids). Glyceraldehyde is arbitrarily chosen as standard.
→ Most of the sugars are genetically related to either D or L-glyceraldehyde and thus their configurations are defined by D or L According to Fischer projection formula, the main carbon chain of glyceraldehyde is kept vertically in which —CHO group is at the top and CH2OH group is at the bottom.
→ Now, if —OH group of the chiral carbon atom is placed right side, it is D-configuration and if —OH is placed on the left side, it is L – configuration. Note that D and L have no relation with optical rotation of a molecule.
Similarly, end of sugars can be shown by D and L configurations.
→ D and L nomenclature is referred as the relative configuration because the molecule to be assigned D- or L-notation is being related to another molecule D- or L-glyceraldehydes accepted as a standard.
→ The compounds which can be obtained by D-glyceraldehyde or can be converted to D-glyceraldehyde are referred to as compounds having D-configuration whereas the compounds which can be obtained by L-glyceraldehyde or can be converted to L-glyceraldehyde are referred to as compounds having L-configuration. This is relative configuration. For example, the compounds obtained by D-glyceraldehyde have D-configuration as :
→ The D-notation may be given to both dextro rotatory [d or (+)] or laevo rotatory molecule [l or (-)]. Same is true for L-notation. Hence, in the carbohydrates, to avoid the confusion (in the spoken language) between D- and d (+) and L- and l -; the most commonly used term to denote dextro rotatory compounds is (+) and not d. Simi1rly, most commonly used term to denote laevo rotatory compounds is (-) and not l.
→ In 1950, when X-ray crystallographic techniques are developed then Bijvoet found that Rosen hoff was considered that form of glyceraldehyde as D-configuration volumterically, it was actually D-form. Some other examples: