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Appendix 1: Cahn–Ingold–Prelog
Rule for Stereochemical
Nomenclature
In order to establish the conWguration of a chiral centre containing four diVerent
substituents, the four substituents are ranked according to their atomic mass. The
substituent with highest atomic mass is labelled ‘1’, the next highest ‘2’, etc. (i.e.
O > N > C > H). The molecule is then drawn with substituent ‘4’ pointing away
from you, and the substituents 1, 2 and 3 connected in the direction 1 ! 2 ! 3. If
the arrows form a clockwise right-handed screw, then the centre has the R
conWguration. If the arrows form an anti-clockwise left-handed screw, then the
centre has the S conWguration. For example, the speciWcally deuterated sample of
ethanol below has four diVerent substituents (O > C > D > H), and the C-
1 centre is designated as R. This molecule is then written as 1R-[1- 2 H]-ethanol.
4
H
H 3 C
2
3
D
OH
1
1R-[1- 2 H]-ethanol
Commonly, two or more of the a-substituents have the same atomic
mass. In order to prioritise two such a-substituents the atomic mass of their
respective b-substituents is analysed in the same way, and the one with higherranking
b-substituents is given the higher ranking. For example, l-alanine
below has two carbon substituents, but the carboxyl group has oxygen
b-substituents whereas the methyl group has hydrogen b-substituents. If
an a-substituent is attached by a double bond to an atom X, then for the
purposes of this analysis the a-substituent has two X b-substituents. l-Alanine
is therefore written as 2S-alanine.
4
H
CH 3 3
H 2 N
CO 2 H
1 2
S
L-alanine = 2S-alanine
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