Question

What does R/S stereochemistry indicate?

Answer 1

R-and S- nomenclature of chirality centers

The Cahn-Ingold-Prelog priority rules are used for naming chirality centers and geometric isomers (e.g. E- or Z-alkenes)

These rules are used to establish the priority of the groups attached to the chirality center and are based on atomic number.

In the simplest and most common case, a chirality center is characterised by an atom that has four different groups bonded to it in such a manner (e.g. tetrahedral) that it has a non-superimposable mirror image. Terms such as an asymmetric, stereogenic or chiral center have been used in the past.

In order to assign the configuration as R or S:

• Identify each of the chirality centers (most commonly an sp³ C with 4 different groups attached)

Then at each chirality center....

• Assign the priority (high = 1 to low = 4) to each group attached to the chirality center based on atomic number.
• Reposition the molecule so that the lowest priority group is away from you as if you were looking along the C-(4) σ bond. If you are using a model, grasp the lowest priority group in your fist.
• Determine the relative direction of the priority order of the three higher priority groups (1 to 2 to 3)
• If this is clockwise then it is the R-stereoisomer (Latin; rectus = right handed)
• If this is counter-clockwise then it is the S-stereoisomer (Latin; sinister = left handed)
• If there is more than one stereocenter, then the location needs to be included with the locant, e.g. (2R)-