Cahn-Ingold-Prelog priority rules
/ kahn ING-gold PREH-log (CIP) /
To say a molecule is 'right-handed' or 'left-handed', chemists first need an unambiguous way to rank the four groups around a stereocenter from highest to lowest. The Cahn-Ingold-Prelog rules are exactly that: a small, strict rulebook for deciding which group wins, agreed on worldwide so that everyone assigns the same priorities.
Rule one: at the first point of difference, the atom with the higher atomic number wins. So I beats Br beats Cl beats S beats O beats N beats C beats H. If two groups start with the same atom (say both carbons), you move outward to the next atoms and compare the sets, taking the highest in each, until you find a difference — like comparing two words letter by letter. A double or triple bond is handled by 'phantom atoms': a C=O carbon is treated as if bonded to two oxygens, and the oxygen as if bonded to two carbons. Isotopes break ties by mass (deuterium outranks ordinary hydrogen). You work through these in order and stop at the first decisive difference.
These rules are the indispensable grammar of stereochemistry. They power the R/S labels for chirality centers and the E/Z labels for double bonds, and they appear in nearly every IUPAC stereochemical name. Mastering them is mostly about patience and bookkeeping: follow each branch outward atom by atom, never jump ahead, and never add up atomic numbers — you always compare the single highest atom at the first point of difference.
Ranking the four groups on the central carbon of an amino acid like alanine: NH2 (N, atomic number 7) outranks COOH (a carbon attached to O,O,O after the phantom-atom rule), which outranks CH3 (a carbon attached to H,H,H), which outranks H (lowest). So the priority order is NH2 > COOH > CH3 > H.
Higher atomic number wins; ties are broken atom by atom moving outward.
A classic trap: priority is decided at the first point of difference, not by adding up atomic numbers. CH2Br (first atom C, then Br) outranks C(CH3)3 (first atom C, then three C's) only after you reach the second shell — and there a single Br beats three carbons. Never sum the substituents.