Stereocenters are labeled R or S.
The convention for naming enantiomers unambiguously was
developed by R.S. Cahn, C. Ingold, and V. Prelog.
The four substituents around
the chiral carbon must be first
ranked in order of decreasing
priority.
•a highest priority
•b second-highest priority
•c third-highest priority
•d lowest priority
When the molecule is positioned with the lowest-priority
substituent away from the viewer, the remaining three
substituents will be arranged in either a clockwise or
counterclockwise direction.
If the progression from a to b to c is clockwise, the configuration
at the stereocenter is named S (sinister) otherwise the
configuration is named R (rectus)
The R or S is added as a prefix in parentheses to the name of the
chiral compound.
(R)-2-bromobutane
(S)-2,3-dihydroxypropanal
(R,S)-bromochlorofluoromethane (a racemic mixture)
If known, the sign of rotation of plane-polarized light may also be
added, however, there is no correlation between R,S and +,-:
(R)-(+)-2,3-dihydroxypropanal.
Sequence rules assign priorities to substituents.
Rule 1:
Look first at the atoms directly attached to the stereocenter.
Precedence is in order of highest atomic number. Hydrogen is
always the lowest precedence.
A higher-mass isotope takes precedence over a lower-mass
isotope.
Rule 2:
If two atoms are of the same precedence using rule 1,
proceed along the two respective substituent chains until you
reach a point of difference.
Verify these two examples:
Rule 3:
Double and triple bonds are treated as if they are single and
the atoms in them are duplicated or triplicated at each end by
the respective atoms at the other end of the multiple bond.
Verify these assignments: