Stereocenter Configurations

Overview

• Learn to assign R and S configurations to stereocenters
• Learn to assign Z and E configurations to double bonds

Assigning R and S Configurations

Basic Steps

1. Prioritize the Four Groups
   • Based on atomic mass from the periodic table.
   • Example: Cl (35.45) > O (16) > N (14) > H (1).
2. Orient the Molecule
   • Lowest priority group (often hydrogen) must point away from you.
   • Draw arrows through numbers 1 to 3.
3. Determine Configuration
   • Counterclockwise (numbers 1, 2, 3) = S configuration.
   • Clockwise (numbers 1, 2, 3) = R configuration.

Common Mistakes and Fixes

• Incorrect prioritization leads to wrong configuration assignment.
• Ensure you follow Step 2 (orient the lowest priority away).
• Use three methods if the lowest priority isn't easily orientable:
  1. Rotating the Molecule
  2. Opposite Trick: Only if the lowest priority is a wedge.
  3. Switcheroo Trick: Switch the lowest priority group with the dash group.

Naming Molecules

• Include R or S in the name depending on the configuration.
• Example: R-1-bromo-1-chloroethane.
• For molecules with multiple stereocenters, include every configuration:
  • Example: (1R, 4S)-1-bromo-4-fluoro-1,4-dichlorobutane.

Advanced Topics

Multiple Stereocenters

• Treat each stereocenter independently.
• Apply the previous steps to assign R/S to every stereocenter.
• Example molecule with two stereocenters: Assign configurations independently for both.

Breaking Ties

• When identical atoms are directly attached to the stereocenter, look one atom further away.
  • Compare atoms directly attached along each path.
  • Identify and prioritize based on first point of difference.
  • Continue comparing until you find a difference.
  • Example: Tiebreakers between two carbon groups differentially attached to other atoms help determine configuration.

Alkenes: Assigning Z and E Configurations

Basic Definitions

• Z (Zusammen): Highest priority groups are on the same side of the double bond.
• E (Entgegen): Highest priority groups are on opposite sides of the double bond.

Steps for Assignment

1. Determine Priorities on each end of the double bond using atomic mass.
2. Compare Priorities
   • If same side = Z
   • If opposite sides = E

Example and Naming

• Correctly name molecules with Z/E assignment at the beginning of the name.
  • Example: (Z)-2-pentene vs (E)-2-pentene.
• For multiple double bonds, include position numbers and configurations of each.
  • Example: (1E, 3Z)-1,3-pentadiene.

Fischer Projections

• Identify stereocenters on Fischer projections.
• Low priority groups should ideally be in the back (- position).
  • For wedges (↑), use the opposite configuration technique.
  • Orient without flipping.

Identifying Diastereomers and Enantiomers

1. Naming and Configurations: Compare configurations after naming (R/S) for both molecules.
   • Completely opposite configurations indicate enantiomers.
   • Some configurations the same and others different indicate diastereomers.
2. Three Methods: Rotate, name, or compare configurations.

Practice and Summary

• Practice: Apply tricks and methods to various molecules.
• Stay Basic: Always fall back to basic steps when complex molecules are daunting.
• Reach Out: Contact for further guidance and clarification.