(R) and (S) in stereochemistry

stereoisomer 除了用 D 和 L 去指出確切的立體結構還可以用 R,S去表達同一件事，A,B,C,D跟 mass 排大小，D最大，遠離自己，A,B,C順時針為R，逆時針為S。請參考下列網址。


http://en.wikipedia.org/wiki/Absolute_configuration

An absolute configuration in stereochemistry is the spatial arrangement of theatoms of a chiral molecular entity (or group) and its stereochemical description e.g. R or S.[1]




http://en.wikipedia.org/wiki/Cahn-Ingold-Prelog_priority_rule

Assignment of priorities[edit]R/S and E/Z descriptors are assigned by using a system for ranking priority of the groups attached to each stereocenter. This procedure, often known as the sequence rules, is the heart of the CIP system.1. Compare the atomic number (Z) of the atoms directly attached to the stereocenter; the group having the atom of higher atomic number receives higher priority.2. If there is a tie, we must consider the atoms at distance 2 from the stereocenter—as a list is made for each group of the atoms bonded to the one directly attached to the stereocenter. Each list is arranged in order of decreasing atomic number. Then the lists are compared atom by atom; at the earliest difference, the group containing the atom of higher atomic number receives higher priority.3. If there is still a tie, each atom in each of the two lists is replaced with a sub-list of the other atoms bonded to it (at distance 3 from the stereocenter), the sub-lists are arranged in decreasing order of atomic number, and the entire structure is again compared atom by atom. This process is repeated, each time with atoms one bond farther from the stereocenter, until the tie is broken.

2014-02-06 00:17:29 補充：
D 和 L 的應用要對照 D Lactic acid 和 L Lactic acid，在應用上有時很麻煩，但 R,S 不需再參照其他物質，使用上較方便。