In order to assign the configuration as R or S:

 

• Identify each of the chirality centers (most commonly an sp³ C with 4 different groups attached)
  
  

Then at each chirality center....

• Assign the priority (high = 1 to low = 4) to each group attached to the chirality center based on atomic number


• Reposition the molecule so that the lowest priority group is away from you as if you were looking along the C-(4) σ bond. If you are using a model, grasp the group in your fist.


• Determine the relative direction of the priority order of the three higher priority groups (1 to 2 to 3)
  
  


• If this is clockwise then it is the R-stereoisomer (Latin; rectus = right handed) 


• If this is counter-clockwise then it is the S-stereoisomer (Latin; sinister = left handed)
  
  


• If there is more than one stereocenter, then the location needs to be included with the locant, e.g. (2R)-
  
  

Subrules:

• Isotopes: H vs D ? Still use atomic number, so D > H


• If the same atom is attached, then look for the first point of difference by moving out one atom at a time, locate the first point of difference and apply rules there.


• If a multiple bond is encountered, treat it as if the atoms are attached by the same number of single bonds e.g. C=C is treated a 2 C-C and C=O is 2 C-O
  
  


• Example:  chlorofluoroiodomethane
  
  
  
  
  
  
  
  
  
  
  
  

  The chirality center should be easy to spot, and the four attached groups are in priority order, highest to to lowest: I , Cl , F and H Rotate the image on the left so the you are looking along the C-H bond and the H is away from you, then determine the direction of high to low priority.  It decreases clockwise, so this is the R enantiomer.

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  identify the chirality center(s)	assign the relative priorities	rotate the low priority group away	determine the sense of groups 1 -> 3 clockwise = R

  
  
  
  Example : cyclohex-2-enol
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  identify the chirality center(s)	assign the relative priorities.... here we have two C groups	so list the groups the C are attached to in atomic number order (note multiple bond)	first point of difference is the C > H	determine the sense of groups 1 -> 3 counterclockwise = S

  
  
  
  
  
  plzzz...rate!!!