5.2 How to Assign R and S | Absolute Configuration | Organic Chemistry

Introduction to Absolute Configurations

  • Absolute Configurations: System of designating chirality at a chiral center.

    • Two forms: R (rectus: right) and S (sinister: left).

  • Cahn-Ingold-Prelog System: The rules used to assign absolute configurations.

Chiral Centers

  • Definition: A carbon atom bonded to four different groups.

  • Example: Carbon bonded to:

    • Chlorine (1), Hydrogen (4), Methyl Group (2), Ethyl Group (3).

  • Priority assignment based on atomic number:

    • Chlorine (1) > Carbon (2 and 3) > Hydrogen (4).

Assigning Priorities

  • Tied Atoms: When there is a tie in atomic numbers, evaluate the atoms attached to the tie-breaking atoms:

    • Example: Compare the two carbons in terms of their attached atoms (H vs. C).

  • Determining Order:

    • List atoms in descending order of atomic number until a difference is found.

Determining R and S Configuration

  • Circle Method: Draw a circle from priority 1 to 2 to 3.

    • Clockwise: Right-handed turn = R.

    • Counterclockwise: Left-handed turn = S.

  • Orientation of Fourth Priority: The lowest priority must be in the back (dashed bond) when assigning R or S.

Handling Different Orientations

  1. Lowest Priority in Back: Right-handed turn = R.

  2. Lowest Priority in Front: Reverse the designation ( S becomes R, R becomes S).

  3. Lowest Priority in Plane: Make adjustments to determine if a switch of groups confirms R or S.

Example Assignments

  • Example 1: Carbon bonded to three groups and Hydrogen.

    • Assignments: Generally, Hydrogen is the lowest priority (4).

    • Check Direction: Go from 1 to 2 to 3; determine the turn direction.

  • Example 2: Complex structure.

    • Evaluate each carbon and attached atom based on bonds and priorities.

Naming Compounds with Chiral Centers

  • Single Chiral Center:

    • Name (e.g., R-2-chlorohexane).

  • Multiple Chiral Centers:

    • Use numerical designations (e.g., 2R, 3S) at the beginning of the name.

    • Follow with substituents (e.g., 2-chloro-3-methylhexane).

Conclusion and Resources

  • Ask questions in comments.

  • For practice problems and study guides, refer to online resources.