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stereodescriptor for symmetrically substituted compounds

We assign the stereodescriptors R and S for chiral centres - carbons and other atoms, as per the set of sequence rules (CIP rules). The pseudochiral centres are given the descriptors (r) & (s) whenever there are stereoisomeric groups present on a pseudochiral centre with different configurations R/S or E/Z.

In this compound, how are the priority 2 & 3 being decided for the tertiary carbon of the cyclohexane ring?

[The stereochemistry has been software generated using ChemDraw]

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  • $\begingroup$ In "Search on Chemistry" at the top of your screen, type "user:55119 descriptors". You will find useful links here on ChemSE. Also, look here and here. That is, E>R>S>H = r. $\endgroup$
    – user55119
    Jan 17 at 17:19

1 Answer 1

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The diene 1a not only has a plane of symmetry passing through $\ce{C1-C4}$ but it also has a two-fold axis of symmetry passing through the bonds $\ce{C2-C3}$ and $\ce{C5-C6}$. What is true for $\ce{C1}$ is also true for $\ce{C4}$.

The first step is to modify the double bonds and add the "phantom" carbon atoms shown in red in 1b. Clearly, the branched side chain has top priority while the hydrogen has the lowest priority. But what are the priorities of the methylene groups $\ce{C2 and C6}$? To answer this question, one turns to diagram 1c.

The compound is deconstructed into two pathways around the ring: $\ce{C1 -> C2 -> -> (C1)}$ and $\ce{C1 -> C6 -> -> (C1)}$. The red dots, $\ce{(C1)}$, are carbons of atomic number six and they are bound to three atoms of atomic number zero. The temporary assignments R0 and S0 at $\ce{C4}$ are made as you would for any stereogenic center. Because R precedes S (CIP Rule 5), the priorities at $\ce{C1}$ are (E)-chain>R0 >S0 >H. The configurations at $\ce{C1}$ and $\ce{C4}$ are "r".

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  • $\begingroup$ That was too deep to realize that even achiral compounds (at least in cyclic rings) would get the stereo-descriptors (r and s). So, in the process we have treated the cyclohexyl ring as the two separate groups to decide its descriptor. In that sense, should the tertiary carbons be called pseudo-chiral or not? $\endgroup$
    – Che Mistry
    Jan 18 at 17:42
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    $\begingroup$ I avoid "pseudo" terminology. I prefer the terms of Mislow and Siegel. For a link and example go here. R/S, stereogenic and chirotopic; r/s, stereogenic and achirotopic; no descriptor, non-stereogenic and chirotopic. If you are compelled to use "pseudo", use it. $\endgroup$
    – user55119
    Jan 18 at 19:13

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