Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Babcock_Hall on October 13, 2017, 06:19:39 PM
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Suppose one is looking at stereoisomers of this compound. How does one pick which carbon is carbon-1? Does it depend on the absolute configuration of carbon 2? In other words do we pick the numbering system on the basis of the R or S configuration of carbon-2? I am helping a student with an assignment on Fischer projections, and this question came up?
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Maybe feed this into chemdraw and see what pops out?
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Sorry about being slow to get back to this. My student put it into ChemDraw, but that is what prompted the question. ChemDraw named the molecule as (2R,3R,4R,5S)-hexane-2,3,4,5-tetraol. My student named it as (2S, 3R,4R,5R)-hexane-2,3,4,5-tetraol. I don't know of a rule that tells us which carbon should be carbon-1 in this situation. I can post a drawing if that would help.
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From that chemdraw name, I suspect it tries to get the lowest # for the R centers, which is probably just a totally arbitrary choice by IUPAC.
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ChemDraw is IUPACly correct because R- has priority over its enantiomorph S-.
Consequently, the lowest numbering for the R- centers, must be preffered.
Preferred IUPAC Names, Chapter 9, P-91.1.1.2, The ‘Sequence Rules’, Rule 5, p.7, (2004)
http://old.iupac.org/reports/provisional/abstract04/BB-prs310305/Chapter9.pdf
Also:
Z- has priority over E- and cis- has priority over trans-.
Rules for the Nomenclature of Organic Chemistry-Recommendations 1974, Section E: Stereochemistry, Rule E-2.2.3, Pure & Appl. Chem., (1976), 11-30, p.17
https://www.iupac.org/publications/pac/1976/pdf/4501x0011.pdf