Absolute Configuration

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Chapter: Organic Chemistry : Stereochemical and Conformational Isomerism

The observation of optical activity and the measurement of optical rotation distinguish one enantiomer (+) from the other (−).


ABSOLUTE CONFIGURATION

The observation of optical activity and the measurement of optical rotation distinguish one enantiomer (+) from the other (). The sign of rotation (+ or ) is thus an experimental way to differentiate enantiomers. A second way to des-ignate these stereoisomers is to assign the configurations as R or S based on stereostructures (for enantiomers which contain a single chiral center). The R and S configurations are relative configurations based on three-dimensional structures which are drawn on paper. However, it is not possible to predict a priori whether the R enantiomer (for example) will be dextrorotatory (+) or levorotatory ().

The absolute configuration of an enantiomer is determined only when the optical rotation of an enantiomer (+ or ) can be matched with its configuration (R or S). For example, the absolute configuration of lactic acid has been found to be R-() in that the R enantiomer is levorotatory.


Conversion to the methyl ester does not change the configuration of the stere-ocenter, which remains R. However, the rotation is found to be positive so the absolute configuration is R-(+). This illustrates that while the relative config-uration (R,S) can be used to show the structures of stereocenters, the absolute configuration must be known to show the changes in configuration that occur dur-ing a chemical sequence. That is, just knowing that a chemical reaction changes the optical rotation in one direction or another is not sufficient to indicate whether a change in configuration at a chiral center has occurred.

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