Advices For Studying Organic Chemistry

5.6A DESIGNATION OF STEREOCENTER

1. 2-Butanol (sec-Butyl alcohol):

HO C

CH 3

H

CH 2

CH (^3)
H C
CH 3
OH
CH 2
CH (^3)
I II

1. R. S. Cahn (England), C. K. Ingold (England), and V. Prelog (Switzerland)
   devised the (R–S) system (Sequence rule) for designating the configuration of
   chiral carbon atoms.


2. (R) and (S) are from the Latin words rectus and sinister:
   i) R configuration: clockwise (rectus, “right”)
   ii) S configuration: counterclockwise (sinister, “left”)

2. Configuration: the absolute stereochemistry of a stereocenter.

5.6B THE (R-S) SYSTEM: (CAHN-INGOLD-PRELOG SYSTEM)

1. Each of the four groups attached to the stereocenter is assigned a priority.
   
   
   1. Priority is first assigned on the basis of the atomic number of the atom that is
      directly attached to the stereocenter.
   
   
   2. The group with the lowest atomic number is given the lowest priority, 4 ; the
      group with next higher atomic number is given the next higher priority, 3 ;
      and so on.
   
   
   3. In the case of isotopes, the isotope of greatest atomic mass has highest priority.
   
   
   
   


2. Assign a priority at the first point of difference.
   
   
   1. When a priority cannot be assigned on the basis of the atomic number of the
      atoms that are diredtly attached to the stereocenter, then the next set of atoms in
      the unassigned groups are examined.