I'm interested in two things. First, feedback on a proposed amendment to
CIP Rule 1b. Second, suggestions for how to officially propose this.
Current Rule 1:
*(1a) higher atomic number precedes lower;*
*(1b) a duplicate atom node whose corresponding nonduplicated atom node is
the root or closer to the root ranks higher than a duplicate atom node
whose corresponding nonduplicated atom node is farther from the root. *Said
differently but with the same meaning:
*(1a) higher atomic number precedes lower;**(1b) in comparing two duplicate
nodes, lower root distance precedes higher root distance, where "root
distance" for a duplicate node is defined as*
* the distance from its corresponding nonduplicated atom node to the root
node.*
Proposed amended rule:
*(1a) higher atomic number precedes lower;*
*(1b) in comparing two duplicate nodes, lower root distance preceded higher
root distance, where "root distance" is defined: (i) in the case of **a
duplicate atom for which the atomic number is averaged over two or more
atoms in applying Rule 1a, *
*the distance from the duplicate node itself to the root node; and (ii) in
all other cases, the distance of its corresponding nonduplicated atom node
to the root node.*
If that means nothing to you, ignore this. But it is a critically important
addition for any algorithm if it is to correctly assign the stereochemistry
even for very simple compounds based on CIP rules 1-5. For example, without
that modification, an algorithm following the rules in IUPAC BB 2013 will
arrive at "S" for the descriptor for in this compound:
[image: Inline image 1]
C1=CC=CC(O)=C1[C@H](C2=CC=CC=C2O)O
My second question is, having said that, how do I go about officially
stating this? Publish? Where?
Bob
--
Robert M. Hanson
Larson-Anderson Professor of Chemistry
St. Olaf College
Northfield, MN
http://www.stolaf.edu/people/hansonr
If nature does not answer first what we want,
it is better to take what answer we get.
-- Josiah Willard Gibbs, Lecture XXX, Monday, February 5, 1900
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