So a carbon atom is chiral if it has 4 different group attached to it, and it is pseudo chiral if it has 3 different groups of atoms attached to it, where the 2 similar groups have different configuration (such as R/S or cis/trans). For the pseudo chiral carbon atom, it must have 2 and only 2 similar groups on it. (according to the IUPAC Goldbook) But what if the carbon atom has only 2 different group, but both with different configuration (like 2 similar groups of cis/trans and 2 similar groups of R/S) like this one : enter image description here

This carbon atom can't be pseudo chiral as it does not have "only 2" similar groups on it. But it is not chiral because it does not have 4 different substituents. This atom does not have a plane of symmetry or centre of symmetry or alternate axis of symmetry (correct me if I am wrong). So this molecule is chiral. But is the carbon atom chiral or pseudo chiral or some other terminology is used in this case(the middle one, with 4 different groups).

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    $\begingroup$ The question, I think, is pretty well presented, but the title would benefit from an improvement, e.g. see I have a doubt about "doubt" in doubt titles. $\endgroup$
    – andselisk
    May 15, 2023 at 15:32
  • $\begingroup$ I don't actually have anything to put in the title, so if you could suggest something, it would be helpful $\endgroup$
    – Ham Lemon
    May 15, 2023 at 17:09

1 Answer 1


I have simplified your structure to the dienes 1 and 2. The "pseudo-asymmetric" carbon in diene 1 is stereogenic and achirotopic and labeled with a lower case descriptor. The CIP rules dictate that Z>E and R/S and that Z/E>R/S. The priorities in 1 are Z>E>R>S. The stereochemistry at the "pseudo-asymmetric" carbon is "s". There is a plane of symmetry passing through Z-s-E and R mirrors S. The compound is optically inactive.
The central carbon in diene 2 is non-stereogenic (no descriptor) and chirotopic. The S,S-enantiomer is optically active as is its enantiomer.
For related posts on this topic on ChemSE go here and here.

  • $\begingroup$ So if I understand correctly, you are saying, the carbon labelled s in 1 can be optically active when the S and R enantiomers become S and S (it will be non stereo genic then). So what would the terminology of that carbon be? Like it can't be "pseuodochiral" because it does not have "only two" same groups with different configuration (and not chiral because it does not have 4 different groups), right? $\endgroup$
    – Ham Lemon
    May 15, 2023 at 7:34
  • $\begingroup$ Atoms are not chiral, their environments are. Hypothetically, if all of the bonds were removed and the atoms were held in place, the carbon atom (a sphere?) in question in structure 2 would find itself in a chiral environment. I avoid using the term "pseudo-whatever" and prefer the nomenclature of Mislow and Siegel. If you switch the attachment of the E and Z groups to the central carbon in 2 and rotate this structure by 180 degrees about a vertical axis, you will still have the original structure. continued $\endgroup$
    – user55119
    May 15, 2023 at 14:26
  • $\begingroup$ ...There is no stereochemical change but there is a chiral environment owing to the atoms with the S-configuration. If you do the same group switch in structure 1, you obtain a diastereomer that is also achiral. Read this. $\endgroup$
    – user55119
    May 15, 2023 at 14:36
  • $\begingroup$ Thank you for the explanation and the reference, I understood what you tried to say, but I still actually have a doubt pending, according to iupac, what would the central carbon atom be called? I understand that you avoid the iupac nomenclature here, but my original doubt remains, so sorry if I can't verify your answer $\endgroup$
    – Ham Lemon
    May 15, 2023 at 17:08

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