Question

I'm finally getting my head around the concept of chirality and stereoisomers, but I'm still really struggling with the assigning of absolute configurations. If someone could demonstrate how they would draw one of the stereoisomers of the chiral molecule, Adrenalin, and assign it the correct R or S designation, I would really appreciate it...if you could throw the enantiomer in there as well (with the opposite configuration) that would also be helpful. Thank you!

Answer 1

In the CIP (Cahn-Ingold-Prelog) system of nomenclature, each chiral center in a molecule is assigned a prefix (R or S), according to whether its configuration is right- or left-handed. No chemical reactions or interrelationship are required for this assignment. The symbol R comes from the Latin rectus for right, and S from the Latin sinister for left. The assignment of these prefixes depends on the application of two rules: The Sequence Rule and The Viewing Rule.

The Sequence Rule for Assignment of Configurations to Chiral Centers

Assign sequence priorities to the four substituents by looking at the atoms attached directly to the chiral center.

1. The higher the atomic number of the immediate substituent atom, the higher the priority. For example, H– < C– < N– < O– < Cl–. (Different isotopes of the same element are assigned a priority according to their atomic mass.)
2. If two substituents have the same immediate substituent atom, evaluate atoms progressively further away from the chiral center until a difference is found. For example, CH₃– < C₂H₅– < ClCH₂– < BrCH₂– < CH₃O–.
3. If double or triple bonded groups are encountered as substituents, they are treated as an equivalent set of single-bonded atoms. For example, C₂H₅– < CH₂=CH– < HC≡C–

The Viewing Rule

Once the relative priorities of the four substituents have been determined, the chiral center must be viewed from the side opposite the lowest priority group. If we number the substituent groups from 1 to 4, with 1 being the highest and 4 the lowest in priority sequence, the two enantiomeric configurations are shown in the following diagram along with a viewers eye on the side opposite substituent #4.