Chirality

Cahn-Ingold-Prelog System

The Cahn-Ingold-Prelog system is used to identify enantiomeric structures by assigning a priority based on the atomic number to each atom attached to a single tetrahedral atom with four different groups.

Stereoisomers have the same connectivity, but differ in the arrangement of their atoms in space. Therefore, using only the IUPAC naming system will produce the same name for a pair of stereoisomers. It is problematic for two reasons:

  • Each molecule should have its own distinct name.
  • A chemist who understands the naming system should be able to produce a structure for the molecule based solely upon the name.

Using the Cahn-Ingold-Prelog system (or R,S-system), named for organic chemists Robert Sidney Cahn, Christopher Kelk Ingold, and Vladimir Prelog, in addition to the IUPAC system solves these problems. The Cahn-Ingold-Prelog system (or CIP) is a system of nomenclature used with the IUPAC system to identify enantiomeric structures by name alone. The rules for the Cahn-Ingold-Prelog system are:

  • Each of the four groups attached to a chirality center is assigned a priority (1 to 4). A chirality center (or stereogenic center or chiral center) is a single tetrahedral atom with four different groups.
  • Priority is first assigned based on the atomic number of the atom that is directly attached to the chirality center.
  • The group with the lowest atomic number is given the lowest priority (4).
  • The group with the next higher atomic number is given the next higher priority (3), and so on.
Groups attached to the chirality center are assigned a priority based on atomic number. The group with the lowest atomic number receives the lowest priority.
  • When a priority cannot be assigned based on the atomic number of the atoms that are directly attached to the chirality center, the next set of atoms in the unassigned groups are examined, and priority is assigned at the first point of difference.
If two identical atoms are attached to a chirality center, the atoms attached to those atoms are prioritized by atomic number to determine the overall priority. For example, if a CH3 and a CH2CH3 are attached to a chirality center (both having a C attached to the chirality center), the CH2CH3 would have priority since it has a CHH attached to the carbon, which is attached to the chirality center versus HHH for the CH3.
  • After prioritization, the formula can be rearranged (rotated) so that the group with the lowest priority is directed away from the viewer and a path is traced from priority atom one to priority atom two to priority atom three.
To determine the chirality center designation of 1-chloroethanol, trace a path from 1 (chlorine) to 2 (alcohol) to 3 (methyl). When the path goes in a clockwise direction, the enantiomer is designated R. If it goes in a counterclockwise direction, the enantiomer is designated S.
  • If the path goes in a clockwise direction, the enantiomer is designated R (based on the Latin word rectus, which means "right-handed").
  • If it goes in a counterclockwise direction, the enantiomer is designated S (based on the Latin word sinister, which means "left-handed").
  • If there is more than one chiral center, the location must be included inside of parenthesis, for example, (2R,3S).
When determining the name of a structure using a combination of the IUPAC naming system and the Cahn-Ingold-Prelog system, the R and S are placed at the beginning of the name of the compound. Examples of names generated using the combination of the two systems are (R)-2-butanol and (S)-1-chloroethanol.
IUPAC nomenclature gives the base names (2-butanol and 1-chloroethanol). Once R or S is determined, the R or S is added in parentheses as a prefix in front of the molecules.