Revised Cahn-Ingold-Prelog Rules - IUPAC 2013
The lower numbered rule precedes the higher.
| Rule | Description |
|---|---|
| 1a | Higher atomic number precedes lower. Br > Cl > O > N > C > H |
| 1b | Inner sphere identical atoms precede outer sphere identical atoms. |
| 2 | Higher atomic mass precedes lower. D > H; 81Br > 79Br |
| 3 | Seqcis precedes Seqtrans; Z precedes E and this precedes non-stereogenic double bonds. |
| 4a | Chiral stereogenic units precede pseudoasymmetric stereogenic units which precede non-stereogenic units. |
| 4b | Like pair R,R=lk or S,S=lk precedes unlike pair R,S=ul or S,R=ul; and M,M=lk or P,P=lk precedes M,P=ul or P,M=ul; and R,M=lk or S,P=lk precedes R,P=ul or S,M=ul; and M,R=lk and P,S=lk precedes M,S=ul or P,R=ul. |
| 4c | 'r' precedes 's'; 'm' precedes 'p'. |
| 5 | Atom or group descriptor 'R', 'M' or 'Seqcis' precede 'S', 'P' and 'Seqtrans', respectively. |