4
$\begingroup$

The attached two compounds are diastereomers. They can be differentiated by cis/trans descriptors. Is there another way of describing them as cis/trans seems to be outdated?

enter image description here

EDIT: How to determine the r/s configuration of the pseudochiral C atoms in this case? The usual rule is that R precedes S, but here two identical groups are bounded to the same pseudochiral atom. How to assign r/s in this case?

Improve this question
$\endgroup$
1

1 Answer 1

Reset to default
4
$\begingroup$

Edit: strongly related to your previous question How to determine the configuration of C atom bounded to two groups with same configuration? and Yolo123's question that was suggested to you Number of diastereomer pairs of 1,3-dichloro-1,2,3-triphenylpropane where the nomenclature was explained in detail by Jerrepierre. The same situation applies here.

Although the cis/trans nomenclature might seem 'outdated', it is still by far the easiest way to describe disubstituted ring systems, and is commonplace in both conversation between organic chemists, and in the organic literature when such relationships are introduced into a molecule.

As these ring systems possess some symmetry, the (R) and (S) descriptions commonly used are not applicable, but (r) and (s) may be assigned to carbons ('pseudo asymmetric'), based on the IUPAC 2013 recommendations for describing achiral cyclic compounds.

I've included the assignments for the two molecules you presented in the image below. In essence (taken from March's Organic Chemistry):

enter image description here

The small letters used for the psuedoasymmetric center are assigned using established rules. [...] The descriptors “r” and “s” are used to denote such centers; they are assigned in accordance with Sequence Rule 5, taking into consideration that “(R)” has precedence over “(S)” in the order of priority.

This additional example, taken directly from the IUPAC blue book 2013 may clarify the reasoning behind assignment of (r) and (s):

enter image description here

Its worth noting that this nomenclature only applies when you can find symmetry, in many cases this is non-existent, and normal (R) and (S) nomenclature can be applied.

Improve this answer
$\endgroup$
5
  • 1
    $\begingroup$ Yes. I think, perhaps the resident IUPAC expert will correct me, but I think this situation arises because the molecules aren't chiral (their mirror images are imposable).So whereas if we had something (R,R) and flipped one of the centres we'd get (S,S), this isn't the case here because the two centres are basically labelled with respect to one another (i.e. the label given to one of them depends on the other one, and vice versa.). I added an additional example before I read your comment which may help clarify. $\endgroup$
    – NotEvans.
    Dec 13, 2015 at 23:41
  • $\begingroup$ I don't understand how do you assign r or s (R precedes S). $\endgroup$
    – EJC
    Dec 14, 2015 at 7:33
  • $\begingroup$ I edited my question. $\endgroup$
    – EJC
    Dec 14, 2015 at 18:58
  • $\begingroup$ Could you perhaps edit to show how you've tried to assign r and s so I can see where you might be going wrong? $\endgroup$
    – NotEvans.
    Dec 15, 2015 at 2:11
  • 1
    $\begingroup$ Never mind, I understood it now. I had problems with the phantom C atoms. $\endgroup$
    – EJC
    Dec 15, 2015 at 8:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.