Chairality

Chiral Centers in Organic Chemistry

  • Chiral centers: Carbon atoms that have four different groups attached, resulting in non-superimposable mirror images.

  • Main focus: Learn to assign absolute configurations to chiral centers using different representations.

Learning Outcomes

  • Understand differences between 3D representations and Fischer projections.

  • Use the Cahn-Ingold-Prelog rules to assign priority and absolute configuration.

3D Structural Representation

  • Use wedges and dashes to indicate orientation:

    • Wedge: coming out towards the viewer.

    • Dashed line: pointing away from the viewer.

  • Assign configurations of enantiomers (R or S) based on their 3D orientations.

Identifying Chiral Centers

  • Characteristics of chiral centers:

    • Must have tetrahedral geometry.

    • Must have four different groups attached.

  • Example of identifying chiral centers in molecules:

    • Carbons with double bonds cannot be chiral centers (not able to attach four different groups).

Cahn-Ingold-Prelog Priority Rules

  1. Locate the chiral carbon.

  2. Assign priorities to the groups based on atomic number:

    • Higher atomic number = higher priority.

    • If two atoms have the same atomic number, look at what they are attached to.

  3. Consider multiple bonds as:

    • Double bond counts as two single bonds.

    • Triple bond counts as three single bonds.

Assigning R and S Configurations

  • Orient the molecule so that the group with the lowest priority is pointing away from you.

  • Determine configuration direction:

    • Clockwise: R configuration.

    • Counterclockwise: S configuration.

  • Visualize with a concept like a steering wheel, where the lowest priority is the steering column.

Fischer Projections

  • Definition: 2D representation of 3D structures where horizontal lines point towards the viewer, and vertical lines point away.

  • To convert from 3D to Fischer projection:

    • Flatten the molecule with the chiral center at the center of a cross.

  • Different Fischer projections can represent the same molecular structure - order and orientation matter.

Manipulating Fischer Projections

  • Rotate Fischer projections by 180 degrees to retain stereochemistry.

  • To assign configuration in Fischer projections:

    1. Position the lowest priority group in a vertical position.

    2. Determine configuration by inspecting the order of 1 to 2 to 3.

    3. If going clockwise: it is R; if counterclockwise: it is S.

Practice and Application

  • Example of determining chirality and assigning R/S configurations using both methods (3D structures and Fischer projections).

  • Emphasis on understanding the manipulation of molecules in both representations, and consistent application of rules for precision in assignments.