Stereochemistry: R and S (CIP) Configuration

A carbon is a stereocentre (chiral centre) when it carries four different groups; such a carbon and its mirror image are non-superimposable, giving rise to a pair of enantiomers. The Cahn-Ingold-Prelog (CIP) rules provide an unambiguous, universally applicable way of labelling which of the two mirror-image arrangements a given stereocentre actually has.

The CIP procedure:

1. Rank the four groups attached to the stereocentre by priority, using the atomic number of the atom directly attached first; the higher the atomic number, the higher the priority. If two attached atoms are identical, compare what they in turn are attached to, moving outward atom-by-atom until the first point of difference is found. Double and triple bonds are treated as if the multiply-bonded atom were duplicated.
2. Orient the molecule so the lowest-priority group (rank 4) points directly away from the viewer.
3. Trace a path from priority 1 to priority 2 to priority 3. If this path runs clockwise, the centre is labelled $R$ (from Latin rectus, right); if counterclockwise, it is labelled $S$ (sinister, left).

Worked example. In bromochlorofluoromethane, $\text{CHFClBr}$, the four substituents are $\text{Br}$ (atomic number 35), $\text{Cl}$ (17), $\text{F}$ (9) and $\text{H}$ (1) -- since all four atoms attached directly to the stereocentre are different elements, the priority order is immediate: $\text{Br} > \text{Cl} > \text{F} > \text{H}$, with no need to look further out. With hydrogen (the lowest priority) pointed away from the viewer, if the remaining path $\text{Br} \to \text{Cl} \to \text{F}$ traces clockwise, the configuration is $R$; if counterclockwise, $S$. …