How To Assign R or S Configuration (Absolute Configuration/The Cahn-Ingold-Prelog Rules)

Whenever we have a chiral carbon, there is a configuration that can be assigned to it, describing the spatial orientation of the groups attached to this carbon.

  1. Identify a chiral center (carbon with four different groups) and groups attached to it. If the carbon has three bonds only, show a hydrogen.

  2. Assign priority from 1 to 4 to the atoms bonded to the chiral center where 1 is the biggest atomic number and 4 is the smallest atomic number.

  3. If you can't assign priority right away because atoms are the same, you have to continue going atom by atom until the first point of difference.

  4. Atoms that have a double of triple bond are considered to be bonded to 2 or 3 other atoms. For example C=O is considered as O-C-O

  5. Connect 1,2,3. If the arrow is going clockwise, it is R (right) and counterclockwise is S. However, group number 4 must be dashed (going into the plane for this to happen).

  6. If group number 4 is not dashed, switch 4 with the group that is dashed. Then, determine R/S configuration and say the OPPOSITE! (This trick was taken from David Klein book Orgo As A Second Language).


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