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I was reading about stereoisomers and I came across a term pseudo chiral centre. I looked up on the net for its definition and found a weird formula for calculating no. of isomers if the compound has a pseudo chiral centre.

I can't understand how to deduce the formula and how to do analytically instead of using the formula in case of complex molecules say of more than 3 chiral carbon atoms.

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    $\begingroup$ Suppose you have the following compound,4-Methylhepta-2,5-diene.The central carbon atom-C 4 is pseudo chiral because it is chiral only when the stereo centers at C2-C3 and C5-C6 are of different configuration other wise it is not chiral $\endgroup$
    – Adithya
    Commented Aug 19, 2020 at 11:06
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    $\begingroup$ See the gold book definition of pseudo-asymmetric carbon atom. $\endgroup$ Commented Aug 19, 2020 at 12:23
  • $\begingroup$ Yes I saw the definition but how to deduce the formula $\endgroup$
    – Charlie
    Commented Aug 22, 2020 at 19:18
  • $\begingroup$ It's something dependent on number of stereocentes $\endgroup$
    – Charlie
    Commented Aug 22, 2020 at 19:18

2 Answers 2

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Consider this molecule

Carbon number 4 is a pseudo chiral center because its chirality depends on the chirality of the second and sixth carbons. For example, if the second carbon is S and the sixth carbon is R, then carbon number 4 can be r or s, but if the chirality of the second and sixth carbons are the same, then carbon number 4 is not the chiral center, because it doesn't have 4 different groups around itself.

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    $\begingroup$ When C2/C6 are of different configurations (R/S), C4 will be stereogenic and achirotopic (r/s). When C2/C6 are of the same configurations (R/R or S/S), C4 will be chirotopic and non-stereogenic. There is no r/s assignment. See: ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/isomers/… $\endgroup$
    – user55119
    Commented Oct 8, 2020 at 16:25
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Pseudoassymetric center is the carbon that show chirality and achirality simultaneously. The r/s (not R/S) nomenclature of carbon 4 depend on R/S configuration of directly attached carbon atom. In the given molecule the carbon 4 is attached with constituitionaly same but configurationaly different group so it is a chiral center (note: R configuration has priority over S configuration). if we imagine it has a plane of symmetry and that make it achiral.

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    $\begingroup$ What is the "given molecule" that you are referring to? $\endgroup$ Commented Jan 12, 2022 at 20:05
  • $\begingroup$ It is good that (interpretation reading your answer) that you discern chirality at the level of an atom, and its neighbours from chirality of the molecule. The answer would improve if there were illustrations to support the argumentation. But then, if you focus on the lower level, i.e. environment around an atom, it is more than puzzling than atom should simultaneously experience an spatial arrangement both chiral and achiral when the leading «a» in «achiral» denotes the absence of chirality; the two are mutually exclusive. Like e.g., is an information public .or. private. $\endgroup$
    – Buttonwood
    Commented Jan 12, 2022 at 22:14

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