Priority Rules for Naming Chiral Centers - The R,S System
1. Prioritize the four atoms, or groups of atoms, attached to the chiral center based on the
atomic number of the atom that is bonded directly to the chiral center. The higher the atomic
number, the higher the priority.
!Number the four atoms, or groups of atoms, such that “1” has the highest priority and
“4” has the lowest priority.
2. If two or more of the atoms that are bonded directly to the chiral center are the same, then
prioritize these groups based on the next set of atoms (i.e., atoms adjacent to the directly-
bonded atoms). Continue until priorities can be assigned. Priority is assigned at the first
point of difference.
!If two atoms have substituents of the same priority, higher priority is assigned to the
atom with more of these substituents.
!A larger group (i.e., more atoms) may not necessarily have a higher priority over
another (smaller) group.
-CH2Cl has higher priority than -CH2CH2CH2CH3
H
C
Cl
CH3
Br
atomic numbers
H
C
Cl
Br
1
6
17
35
H(4)
C
Cl(2)
CH3(3)
Br(1)
chiral center
prioritize
H
C
Cl
CH3
CH2CH3
atomic numbers
H
C
Cl
1
6
17
H(4)
C
Cl(1)
CH3(3)
CH2CH3(2)
chiral center
prioritize
C
H
Cl Cl C
H
HCl
(-CHCl2)(-CH2Cl)
has higher priorit y than
