5.5 Relative configuration
5.5 Relative configuration
As mentioned before, lactic acid in muscle is dextrorotatory, i.e., (+)-lactic acid. This can be determined by experiment. However, based on this experimental fact, will it be possible to decide which configuration, 17 or 18, (+)-lactic acid has? It is impossible unless one can see the molecule! The pioneers of stereochemistry built up a logical system of relative configuration by making a few simple hypothesis. The system of relative configuration can be summarized as below.
1) Of the enantiomer pair ofglyceraldehydes HOCH2C*H(OH)CHO, the dextrorotatory isomer is assigned to have the configuration 21, and the levorotatory isomer the configuration 22. (Note that there was no experimental support for this assignment)
2) Compound which can be derived from either 21 or 22 without changing the configuration of the asymmetric carbon atom belong toD- and L-series, respectively. 3) The stereochemistry of a chiral compound is named by a combination of two terms, one (D or L) defines the relative configuration, and the other (+ or -) indicates the experimentally determined rotation.