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The R/S System: A New and Simple Approach to Determining Ligand Priority and a Unified Method for the Assignment and Correlation of Stereogenic Center Configuration in Diverse Stereoformulas
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Abstract
A new approach providing an "at-a-glance" priority order of ligands attached to a stereogenic center in organic molecules and a unified method for assigning and correlating stereogenic center absolute configuration in diverse stereochemical representations is presented. The approach to assigning ligand priority uses ligands in their proper bonding connectivity and proposes the following "application rules": (i) carbon-oxygen/carbon-nitrogen bond: higher number of bonds gives higher priority; (ii) hydrogen atoms: the fewer the number of H atoms linked to the first or outward atom, the higher is the precedence; (iii) chain propagation vs chain termination: in case of a tie in the number of H atoms, chain propagation (presence of next outward atom) precedes chain termination (absence of next outward atom). For assigning absolute configuration, the procedure is as follows. For a stereogenic center, any two ligands in a plane are chosen and then linked by a clockwise arc taking the stereogenic atom as the geometric center. The stereoformula is then transformed into a Fischer projection in which the ligands at the initial and final positions of the clockwise arc occupy the vertical top and vertical bottom, and those in front and in the rear take up the horizontal right and horizontal left positions, respectively. The Fischer projection drawn is then used to determine the R/S configuration. The correlation of absolute configuration between varied representations can be made in terms of a clockwise arc and a front (or rear) ligand defined for a stereogenic center. These procedures are illustrated with suitable examples.
Keywords (Audience):
Second-Year UndergraduateKeywords (Domain):
Organic ChemistryKeywords (Subject):
Molecular Properties / StructureCiting Articles
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This article has been cited by 5 ACS Journal articles (5 most recent appear below).
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Michael Abraham, Valsamma Varghese, and Hui TangJournal of Chemical Education2010 87 (12), 1425-1429Stereochemistry is an important topic in organic chemistry. It is also a difficult topic for students to learn. This study investigated the relative effectiveness on students’ understanding of three kinds of molecular representations of stereochemistry ...
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The Use of Stick Figures to Visualize Fischer Projections
Laurie S. StarkeyJournal of Chemical Education2001 78 (11), 1486The Use of Stick Figures to Visualize Fischer Projections
Laurie S. StarkeyJournal of Chemical Education2001 78 (11), 1486This paper presents a very simple method for interconverting line drawings and Fischer projections. The technique uses stick figures to help students view these structures from the proper point of view.
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- Received: August 03, 2009
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