Priority Rules

Useful Concepts

Priority Rules for Nomenclature

The Cahn-Ingold-Prelog priority rules are used for naming both geometric isomers (e.g. E- or Z-alkenes) and configurational isomers (e.g. R- or S- for enantiomers and diastereomers). The rules are based on atomic number, and the first point of difference.
Naming Alkenes

1. Imagine the alkene as 2 pieces each containing one of the sp² C
2. Assign the priority (high = 1, low = 2) to each group on each sp² C based on atomic number
3. Determine the relative position of the 2 higher priority groups
4. If they are on the same side then it is a Z-alkene (German; zusammen = together)
5. If they are on opposite sides then it is an E-alkene (German; entgegen = opposite)

Example: 2-butene

The two substituents are -CH₃and H, so since the atomic numbers C > H then the -CH₃ group is higher priority. Therefore the two high priority groups are on the opposite side, then this is E-2-butene.

Subrules:

Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H
Same atom attached ? By moving out one unit at a time, locate the first point of difference and apply rules there.

Naming Enantiomers

NB: The term stereogenic has superceded the term chiral.

1. Identify the stereogenic centers (most commonly an sp³ C with 4 different groups attached)
2. Assign the priority to each group (high = 1, low = 4) based on the atomic number of the atom attached to the stereogenic center
3. Position the lowest priority group away as if you were looking along the C-(4) sigma bond. If you are using a model, grasp the lowest priority group in your fist
4. For the other 3 groups, determine the direction of high to low priority (1 to 3)
5. If this is clockwise, then the center is R (Latin: rectus = right)
6. If this is counter clockwise, then it is S (Latin: sinister= left)

Example: chlorofluoroiodomethane

The chirality center is easy to spot, and the four attached groups are I (purple), Cl (green), F(brown) and H (white), listed in priority order, highest to to lowest. Rotate the JMOL image below so the you are looking along the C-H bond and the H is away from you, then determine the direction of high to low priority. It decreases clockwise, so this is the R enantiomer.

Subrules:

Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H
Same atom attached ? By moving out one unit at a time, locate the first point of difference and apply rules there.