Step 1: rank by atomic number
At the stereocenter, look at the atom directly bonded. Higher atomic number = higher priority. Br > Cl > F > O > N > C > H. For carbon-rich groups that tie at the first atom, go to the second atom outward.
Step 2: break ties at the next atom
If both first atoms are C, look at the atoms those carbons are bonded to. A carbon bonded to (O, O, H) beats a carbon bonded to (C, C, H). A double bond counts as two bonds to that atom — a C=O is treated as a C bonded to two O's. Go further out atom by atom until you find a difference.
Step 3: orient with #4 in back
Rotate the molecule (mentally, or on paper) so the lowest-priority group points away from you. If the lowest-priority group is already on a dash (going into the page), you can just read the rotation directly off the page. If it's on a wedge (coming out of the page), determine the rotation as drawn and then flip it.
Common worked examples
(R)-2-bromobutane: priorities Br > CH₂CH₃ > CH₃ > H. (S)-glyceraldehyde: priorities OH > CHO > CH₂OH > H. For amino acids, the L-form is (S) for everything except cysteine — where the sulfur in CH₂SH outranks the carboxyl, making L-cysteine (R).
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.