Skip to main content
Chemistry LibreTexts

9.2: R/S Nomenclature System (Cahn–Ingold–Prelog convention)

  • Page ID
    215736
  • The complete set of rules is given in the textbook, but here are some things to keep in mind when assigning configuration to chirality centers.

    1. Make sure you have chiral centers in the molecule. The fact that a 3-dimensional formula is given does not imply that there are chiral centers.

    2. Assign priorities to the atoms directly attached to the chirality center. The highest priority goes to the atom with the highest atomic number. In case there are isotopes, use the mass number instead, since they have the same atomic number.

    clipboard_ef7f19d35cf51f7646469db02e09a9a9c.png

    Notice that since the atomic number of hydrogen is 1, it will always be the lowest priority group, as long as it is present.

    3. If two or more of the atoms directly attached to the chiral center are of the same type, look at the next atom to break the tie. Do not do this unless there is a tie. Repeat this process until the tie is broken.

    It is important to emphasize that in trying to break ties, one looks at the atoms directly attached to the element under observation before looking at any others. Study the examples on the following page very carefully to make sure this point is clear.

    clipboard_e946e0da046dfe9b9cd0723bbef77d6ad.png

    4. If there are atoms containing double or triple bonds, count them twice or thrice respectively. This holds for each of the atoms involved in the double or triple bonding.

    clipboard_e692a8b7759c5ad30a3ff6a3b569c612e.png

    5. Although not obvious from the above examples, when duplicating the atoms involved in double or triple bonding they are also being crossed over at the same time. This only becomes apparent when the atoms involved in multiple bonding are not of the same kind, as in the examples shown on the following page.

    clipboard_e43e7a98b8373933a4ccc995a830d08c4.png

    • Was this article helpful?