Absolute Configuration
The chemists Cahn, Ingold and Prelog put forward a specific nomenclature (R / S) for the absolute configuration of a stereocentre or stereogenic element. Prelog was awarded the Nobel Prize in 1975.
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Robert S. Cahn (1899-1981): Fellow of the Royal Institute of Chemistry, London.
Christopher Ingold (1893-1970): Professor of Chemistry at University College London.
Vladimir Prelog (1906-1998): Yugoslav-born chemist, later a Swiss citizen.
By convention, the group with the lowest priority (d) is placed pointing away from the viewer. One then follows the path from the highest priority group (a) to the lowest (c) of the remaining three groups. If the path is clockwise, the configuration is R (rectus). Otherwise it is S (sinister).
PRIORITY RULES
The atomic number of the atoms directly bonded to the stereogenic element determines their priority (for isotopes, the atomic mass). The atom with the highest atomic number has the highest priority. Hydrogen, when present, is always the atom with the lowest priority.
If there is a 'tie', i.e. two identical atoms are bonded to the stereogenic element, one must look at the next position to break it. If the 'tie' persists, look at the following position, and so on until the 'tie' is broken.
If one of the atoms bonded to the stereogenic element is part of a multiple bond, a double bond is treated as if the atom were formally bonded to two atoms, and a triple bond as if it were bonded to three. That is a clever way to break 'ties' like the one in the example.
The R/S configuration can also be defined in molecules that have no stereocentres but do have stereogenic elements, such as allenes. First, give each substituent its priority by the same rules as above. Take the one with the lowest priority and place it pointing away from the viewer. In this case there may be 'ties'; just take one of them. Look first at the front part of the molecule and assign the two substituents their priorities (1 and 2). Then look at the back part of the molecule and do the same (3 and 4). A clockwise or anticlockwise path from 1 to 2 and 3 gives the R or S configuration, respectively.
