5.6 Absolute configuration一R ，S 一convention
5.6 Absolute configuration一R ，S 一convention
the system of relative configuration is a self-consistent system except the arbitrary assumption give one of the enantiomer of glyceraldehyde a certain configuration. In practice, the system was effective and a very important field of organic chemistry, i.e., sugar chemistry, and natural product chemistry in general, was constructed on it. Chemists were, however, anxious to know the fact: what configuration has D(+)-glyceraldehyde? Is it really 21, or 22 instead? In 1951, Bijvoet et al. successfully determined the true configuration, i.e., absolute configuration, of a chiral molecule rubidium sodium salt of tartaric acid by abnormal scattering of X-ray. Very fortunately, the absolute configuration did coincide with the relative configuration. The hypothesis was correct! Today the absolute configuration of many compounds has been decided.
There were several difficulties in practicing the relative configuration when a variety of chiral compounds were treated. Sometimes it was impossible to relate a complex molecule to glyceraldehyde. Under such a circumstance, a new stereochemical nomenclature, R, S nomenclature, was proposed. The idea was that the system should be general and be related to the sequence rule which is the ground of systematic nomenclature. The essence of R, S nomenclature is as follows. For each center of symmetry (compounds with many centers should be considered), 1) The priority of four ligands is decided by the sequence rule. 2) An alphabet is assigned to each ligand according to the decreasing order of priority; L > M > S > s 3) We suppose looking down the bond from the asymmetric carbon atom toward the ligand of lowest priority (s). The other three ligands (L, M and S) will face you. Connect these three ligands with an arrow running from highest to lowest priority (L > M > S). 4) If this arrow runs clockwise, the enantiomer is called R (Latin, rectus (meaning right)). If it runs counterclockwise, the enantiomer is called S (Latin, sinister (meaning left)). 5) The label (R ) or (S) is added to the name.
▶go to S5.9 How to decide the configuration by R, S nomenclature