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.

For more on this topic see the nomenclature site section on naming stereochemistry.

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 -CH₃ and 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.

NB: The term stereogenic has superceded the term chiral.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 JSMOL 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.

© Dr. Ian Hunt, Department of Chemistry