Chirality
32.3.3 By absolute configuration: R- and S-
Main article: R-S System^4
The absolute configuration system stems from the Cahn-Ingold-Prelog priority rules^5 , which
allow a precise description of a stereocenter without using any reference compound. In fact
the basis is now the atomic number of the stereocenter substituents.
The R/S system is another way to name an optical isomer by its configuration, without
involving a reference molecule such as glyceraldehyde. It labels each chiral center R or S
according to a system by which its ligands are each assigned a priority, according to the
Cahn Ingold Prelog priority rules, based on atomic number.
This system labels each chiral center in a molecule (and also has an extension to chiral
molecules not involving chiral centers). It thus has greater generality than the D/L system,
and can label, for example, an (R,R) isomer versus an (R,S) — diastereomers.
The R/S system has no fixed relation to the (+)/(-) system. An R isomer can be either
dextrorotatory or levorotatory, depending on its exact ligands.
The R/S system also has no fixed relation to the D/L system. For example, one of glyc-
eraldehyde’s ligands is a hydroxy group, -OH. If a thiol group, -SH, were swapped in for
it, the D/L labeling would, by its definition, not be affected by the substitution. But this
substitution would invert the molecule’s R/S labeling, due to the fact that sulfur’s atomic
number is higher than carbon’s, whereas oxygen’s is lower. [Note: This seems incorrect.
Oxygen has a higher atomic number than carbon. Sulfur has a higher atomic number
than oxygen. The reason the assignment priorities change in this example is because the
CH2SH group gets a higher priority than the CHO, whereas in glyceraldehyde the CHO
takes priority over the CH2OH.]
For this reason, the D/L system remains in common use in certain areas, such as amino
acid and carbohydrate chemistry. It is convenient to have all of the common amino acids
of higher organisms labeled the same way. In D/L, they are all L. In R/S, they are not,
conversely, all S — most are, but cysteine, for example, is R, again because of sulfur’s higher
atomic number.
The word “racemic” is derived from the Latin word for grape; the term having its origins in
the work of Louis Pasteur who isolated racemic tartaric acid from wine.
32.4 Chiral Compounds Without Stereocenters
It is also possible for a molecule to be chiral without having actual point chirality (stere-
ocenters). Commonly encountered examples include 1,1’-bi-2-naphthol (BINOL) and 1,3-
dichloro-allene which have axial chirality, and (E)-cyclooctene which has planar chirality.
4 Chapter 41 on page 163
5 https://en.wikipedia.org/wiki/Cahn_Ingold_Prelog_priority_rules