In the R/S stereochemical nomenclature, when we try to determine the priority of substituents around a chiral carbon, are there specific examples where the ranking cannot be definie under the CIP priority rules? (for instance, where the preference between the 2nd and 3rd positions cannot be determined)?"

My questions:

  • If the substituents are distinguishable, is it possible to prioritize them using the CIP rules?; Even if the chemical formulas are identical substituents, can they be properly ranked by the CIP rule if they are distinguishable?
  • If so, why.

At least, as an example of "isomeric" relationship between substituents, it seems that we can rank by RS ranking in the following figure.

enter image description here

Note: In the above figure, the upper left is a 2-propyl group and the upper right is a 1-propyl group. In the above figure, the 2-propyl group on the upper left and the 1-propyl group on the upper right are both represented only as C3H7- in the composition formula, but if we arrange the atoms following the carbon (deficit) directly connected to the chiral carbon atom in order of increasing atomic number;

-Three atoms, (C,C,H) in the 2-propyl group on the left side, In the 1-propyl group on the right, three atoms, (C,H,H).

The CIP rule seems to determine the higher rank of the 2-propyl group on the left.

In this sense, in the above example, the branching structure is different, so we can use the CIP rule.

However, for example what if the two of the substituents is just trans and one is cis relation? Anyway, are there any examples that, under the existing rules known as CIP, despite various efforts, we cannot rank?

References:(Written in Japanease)


1 Answer 1


Blue Various: Per your request, I offer the following examples. Consider the D-pentoses ribose 1, xylose 4 and arabinose 5. Each one has stereodescriptors in uppercase letters (R/S) at $\ce{C2, C3 and C4}$. These carbon atoms also find themselves in chiral environments. They are both stereogenic and chirotopic after Mislow and Siegel. Now imagine the reduction of these pentoses to their respective pentitols, ribitol 2, xylitol 3 and arabitol 6. In the case of ribitol, $\ce{C2 and C4}$ retain uppercase descriptors while $\ce{C3}$ has a lowercase descriptor, "s". $\ce{C3}$ remains stereogenic but it is no longer chirotopic as $\ce{H-C3-OH}$ lies in a mirror plane. Because CIP rule 1a precedes rule 5, the priorities at $\ce{C3}$ are O>R>S>H and are, necessarily, in lower case. If the stereochemistry at $\ce{C3}$ is inverted, a new stereoisomer is formed--namely, xylitol 3--that still still has $\ce{HO-C3-H}$ lying in a mirror plane. $\ce{C3}$ now has the descriptor "r" and is also stereogenic and achirotopic.

Inversion of stereochemistry at $\ce{C3}$ in arabitol 6a forms arabitol 6b. They are identical by 180o rotation about an axis perpendicular to the page. $\ce{C3}$ in arabitol 6 is non-stereogenic and chirotopic, which leads to the absence of a stereodescriptor.

If you use "Search on Chemistry" at the top of the page and enter user:55119 CIP, you will find related topics that I have previously addressed on ChemSE.

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