I have not found RS configuration in Beta-hexachlorocyclohexane(C6H6Cl6) anywhere but tried to give it, although some books say that it can not be named in this configuration. So, is it named correctly, if not what is the mistake?
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1$\begingroup$ Welcome to ChemSE. This issue has been addressed previously on this site. If this hexachloride were hexahydroxyl it would be scyllo-inositol, which has the lower case "r"-configuration at each carbon. Go to this link for an additional explanation. $\endgroup$– user55119Commented Sep 23, 2023 at 15:18
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3$\begingroup$ All the atoms have two identical substituents, so how could you get R/S? $\endgroup$– MithoronCommented Sep 23, 2023 at 18:40
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$\begingroup$ If you rotate the molecule 120 degrees in plane, it is the same molecule which means any S/R assignments must alternate. If you flip the molecule through the horizontal plan, it is the same molecule however all the S configurations would now map to R and vice versa, therefore all centres are both S and R which means they cannot be assigned. $\endgroup$– TragamorCommented Sep 24, 2023 at 16:36
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$\begingroup$ @Tragamor: Your R/S analysis is not in keeping with CIP rules. All carbons are of the r-configuration and are, therefore, assignable. Follow my links above. $\endgroup$– user55119Commented Sep 24, 2023 at 21:40
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1 Answer
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This hexachlorocyclohexane (red box) is one of nine possible stereoisomers in analogy with the inositols. The isomer in question is of the scyllo-configuration and all carbons are of the same configuration. Imagine a molecular model lying on a table. It falls off. You pick it up and place it on the table exactly as before. There are six solutions.
Configurations at $\ce{C1-C4}$, $\ce{C2-C5}$ and $\ce{C3-C6}$ are RR, SS and SR, respectively, and they are internally inconsistent because all carbons are identical. Given the planes of symmetry through $\ce{C1-C4}$, $\ce{C2-C5}$ and $\ce{C3-C6}$, each carbon is stereogenic and achirotopic. The latter condition requires r/s assignments and not R/S.
To assign the r/s configuration, one constructs a digraph (vide infra) where $\ce{C1}$ is the non-duplicate atom and C(1) is the duplicate atom, which is attached to three atoms of atomic number zero. Atoms $\ce{C2-C6}$ are given temporary (Ro/So) R/S configurations because, in the digraph, these carbons are both stereogenic and chirotopic. As an example, $\ce{C2}$ is assigned So because the priorities are Cl>long chain>short chain>H. Likewise, $\ce{C6}$ is Ro. Having made temporary assignments for $\ce{C2-C6}$ and realizing that as R>S, as is Ro>So, the scyllo-isomer reduces to Cl>$\ce{C6}$>$\ce{C2}$>H (Cl>Ro>So>H). $\ce{C1}$ and $\ce{C2-C6}$ all have the r-configuration. Other stereoisomers require more complex rules for the final assignment.
