Chemistry Reference
In-Depth Information
Appendix 2
Assignment of Chiral Centre
Handedness using
Cahn-Ingold-Prelog Rules
A centre of symmetry at a four-coordinate centre can be defined as left or right handed
using a convention introduced by Cahn, Ingold and Prelog. The method is based on assign-
ing a priority to each of the groups attached to the chiral centre. Figure A2.1 shows a
generic tetrahedral centre with four attached groups
a
-
d
and illustrates the process of
assigning the
R
(rectus, right) and
S
(sinister, left) labels to each of the two enantiomers.
The molecule is classified as
R
or
S
according to the following steps:
1. Number the groups according to their priority, 1 for highest down to 4 for low-
est priority; in the examples in Figure A2.1 we have assumed that the priority runs
a
d
. There is more detail on assigning priorities to groups below.
2. Orientate the molecule so that the highest priority groups are toward you and the
molecule is viewed down the bond from the chiral centre to the lowest priority group.
3. The handedness is assigned according to the order of the priority read around the chiral
centre in this orientation. If the 1-3 priority atoms are arranged in a clockwise fashion,
then the chiral centre is
R
; if they are read in order in an anticlockwise fashion, then the
centre is
S
.
>
b
>
c
>
The most difficult part of the process is the assignment of the group priorities, for which
the following rules are employed:
1. Assign the priority of the atoms directly bonded to the chiral centre based on their
atomic numbers, with atoms further down the periodic table having the higher priority.
If a lone pair forms one of the groups, then it has the lowest priority. For unambiguous
