Lecture 15

Lecture 15 - CHEM 2321/2324 by Dr. Robert Pankow

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Today's Lecture Outline

  • Fischer Projections
  • Stereochemistry: Enantiomers and Diastereomers
  • Stereochemistry in chair cyclohexane
  • Due Dates: Homework 5 and Quiz 5

Fischer Projections: Converting 3D to 2D

  • Fischer projections are a method for representing three-dimensional molecules in two dimensions.
    • Bonds on horizontal axis: Projected up.
    • Bonds on vertical axis: Projected down.

Example Structures

  • The following are visual representations of Fischer projections:
    • C-C-
      • CH₃
      • CH₃
      • Br
      • H
      • H
      • Br
  • Additional examples showing configurations to illustrate Fischer Projections:
    • Not a Fischer Projection:
    • Cl H
    • CH₃
    • CH₂CH₃
    • Cl
    • C
  • Important Note: When Rousseau presented the structure, it was emphasized that certain geometric configurations do not represent a Fischer projection.

Rapid Generation of Stereoisomers

  • Fischer projections facilitate quick generation of stereoisomers for a compound like 2,3-dibromopentane.
    • Molecular Structure: CH₃CHBrCHBrCH₂CH₃
    • The compound has two chiral centers denoted by asterisks (*) which calculates to four stereoisomers via the formula:
    • 2^n, where n is the number of chiral centers.
    • Diagrammatic representations of stereoisomers and enantiomers (considered pairs derived through reflection across the mirror plane).

Enantiomeric and Diastereomeric Relationships

  • When analyzing stereoisomers of 2,3-dibromopentane:
    • Enantiomers are defined based on their non-superimposable mirror image properties.
    • Example of two sets of enantiomers:
    • Set 1: CH₃CH₂CHBr-
    • Set 2: CH₃H-CHBr-
  • Diastereomers: Non-mirror image stereoisomers that differ at one or more chiral centers.
  • Meso structures are a specific case found in molecules with an internal plane of symmetry.
  • Example of meso compound:
    • CH₃-
    • H-
    • Br
    • Br
  • Note that pairs that are considered meso structures are superimposable on their mirror images indicating no enantiomers exist.

Resolution of Racemic Mixtures

  • Discusses the resolution of a racemic mixture of a carboxylic acid (R-COOH) consisting of both R and S forms.
  • One efficient method involves reacting the mixture with an enantio-pure amine, leading to diastereomers that can be separated based on differing solubilities through recrystallization methods.

Stereochemistry in Cyclohexanes

  • Importance of stereochemistry highlighted in dimethylcyclohexane configurations:
    • Cis-1,4-Dimethylcyclohexane: No chiral centers present due to the existence of a mirror plane.
    • Cis-1,3-Dimethylcyclohexane: Noted as a meso compound, having mirror planes that maintain overall molecular symmetry despite the presence of two chiral centers.
    • Ring Inversion: Characterizes how conformation and stereochemistry determine molecular behavior but does not alter configuration.
    • Examples of trans-1,3 and trans-1,2-dimethylcyclohexanes illustrate the differences in chiral interactions such as enantiomeric form not permitted through ring flips.

Prioritization in the Cahn-Ingold-Prelog System

  • Priority Assignment Rule: Determined by atomic number;
    • Bro (Br) > Cl > C > C
  • Practical exercises included determining R/S designations based on spatial orientation, involving how the lowest priority group orients with respect to the observer.

Stereochemical Designation Cases

  • Various questions provided to determine stereochemical designations:
    • Questions suggested involve recognizing configurations leading to R or S designations based on designated priority groups and their relations.
    • If lowest priority group is facing toward the observer, derived stereochemistry reverts from whatever orientation derived (R/S) to its opposite.

Achirality Identification

  • Factors leading to achirality focus on detecting planes of symmetry.
  • Examination of different compounds illustrated within the lecture notes, determining if they exhibit such symmetry.
  • Identification of chiral centers and planes, alongside rotating configurations to relent the symmetry analysis.

Summary and Conclusion

  • Lecture concluded with methods to identify stereo-specific configurations, resolution processes, and importance of conformational stability.
  • Emphasized practical understanding and recognition of meso compounds and distinction from enantiomers.
  • Clear reiteration of necessary identification strategies for problem-solving and visual comprehension within organic structures.

-** END OF LECTURE 15 **

  • Organic Chemistry – The University of Texas at El Paso