Can pseudochirality be considered when three or four groups on the carbon atom are constitutionally the same, but configurationally different?

For example, when the C atom is bonded to one H, and then three more structurally identical groups with 2 chiral C each - in configurations R,R and S,S and R,S respectively - then by assigning each of the 4 groups a priority value (since they are all different - though 3 have the same structure) we can assign a configuration to this C atom.

  • $\begingroup$ See this earlier answer and also follow the link describing "prochirality". That web page also discusses proprochirality. If that doesn't answer your question, post back with a bit more detail in your question (in these cases a drawing of the molecule would be very helpful) $\endgroup$
    – ron
    Commented Apr 20, 2015 at 15:37

2 Answers 2


Ah, now I think I understand what you're asking. You're saying that we have a central carbon to which is attached a hydrogen and 3 groups with the same structure. Each of these 3 groups contains 2 chiral carbons. One group is R,R one is R,S and one is S,S.

So yes, the chirality of the central carbon (it is chiral, it is not meso or prochiral - there is no plane of symmetry bisecting the central carbon) can be assigned using the Cahn–Ingold–Prelog priority rules where "R" has higher priority than "S". Therefore the priority ranking around the central carbon in your molecule would be R,R > R,S > S,S > H.

  • $\begingroup$ Yes, that's exactly what I meant. Although I think this carbon cannot rotate light in its path, and isn't optically active - I think its configuration will be designated using symbols r and s rather than R and S. $\endgroup$
    – Charles
    Commented Apr 21, 2015 at 15:22
  • $\begingroup$ Your molecule is chiral, there is no symmetry element present. It will rotate the plane of polarized light.. The enantiomer would be R,R S,R S,S. R and S will still be used to denote the configurations at the 6 chiral carbons. r and s are only used with prochiral carbons (see my link in my comment up above). There are no prochiral carbons in your molecule. $\endgroup$
    – ron
    Commented Apr 21, 2015 at 15:26
  • $\begingroup$ Oh - a C which has H, OH, and a group in R, R and R, S configurations (the group has 2 chiral C) attached is optically active? A non superimposable mirror image does form about this C - the configurations of the groups attached become S,S and R,S. $\endgroup$
    – Charles
    Commented Apr 24, 2015 at 15:56
  • $\begingroup$ "a compound in which a C has H, OH, and a group ", what is the fourth group attached to C? $\endgroup$
    – ron
    Commented Apr 24, 2015 at 15:58
  • $\begingroup$ Sir, what about Cis trans or E Z in case of pseudo chiral center, if one attached is Z and other is E, which is given more priority in these cases? $\endgroup$ Commented Oct 18, 2019 at 3:36

I have belatedly come across this inquiry by Charles (unseen by ChemSE in 3 years) and answered by @ron. If Ron's interpretion of what Charles intended is correct, then I presume that the hexaol (arbitrary numbering) shown below would fit into the conversation. If not, my apologies.

The central carbon $\ce{C2}$ bears, in addition to hydrogen, three groups that have the same constitution but differ configurationally. They are identified as RS ($\ce{C5-C4-C6}$), RR ($\ce{C11-C1-C16}$) and SS ($\ce{C10-C3-C19}$). The pseudo-asymmetric carbon $\ce{C4}$ is readily assigned the configuration $\ce{C4-s}$ according to CIP rule 5 (IUPAC Blue Book 2013). Carbons $\ce{C1 and C3}$ are non-stereogenic (ns) and are designated as such. The priorities are $\ce{C4-s > C1-ns = C3-ns}$ as prescribed by CIP rule 4a. The impasse between $\ce{C1 and C3}$ is resolved by CIP rule 5 where R > S. Thus, the central carbon is $\ce{C2-s}$ because the priorities are $\ce{C4-s > C1-(R) > C3-(S) > H}$. Carbons $\ce{C2 and C4}$ are also described as stereogenic and achirotopic. The hexaol has a plane of symmetry (meso) and would not rotate plane polarized light. The meso isomer $\ce{C2-s,C4-s}$ is but one of four meso stereoisomers. Again, if this structural interpretation is not what Ron gleaned from the OP's inquiry, my apologies.

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