The R and S Naming System

According to IUPAC protocol, each molecule must have a unique, unambiguous name – even stereoisomers.

Rules for the R and S Naming System

Step 1: Assign priorities to the four atoms on the chiral center according to their atomic mass on the periodic table. 

Step 2: When there is a tie between atomic weights, compare the next set of adjacent atoms (playoffs!)

Step 3: Double bonds count twice. Triple bonds count three times

Example: Determine priorities for the following chiral center.

3m

Step 4: If the last priority group is in the back, then trace a path from highest to lowest priority

- Clockwise = R, Counterclockwise = S                            

- Always Ignore group 4

Step 5: If the last priority group is NOT in the back, swap that group with the group that is on the dash.

- Trace path as always, but this time swap the sign since you swapped groups.

  • R becomes S
  • S becomes R

Example: Provide the full name for the following molecule, taking stereochemistry into account.

7m

Example: Provide the full name for the following molecule, taking stereochemistry into account.

8m