Chem Sept. 29th

Enantiomers and Diastereomers

  • Enantiomers: Stereoisomers that are mirror images of each other.
  • Diastereomers: Stereoisomers that are not enantiomers.

Stereochemistry Basics

  • Stereochemistry is more complex than conformation analysis.
    • Includes concepts like enantiomers and diastereomers.
    • Important to distinguish between the two types of isomers.

Cis-Trans Nomenclature

  • Used for alkenes to define orientation of groups.
    • Cis: Groups on the same side.
    • Trans: Groups on opposite sides.
  • Formation of alkenes involves making a double bond; this restricts rotation:
    • Alkanes allow free rotation around single bonds.
    • Alkenes (due to pi bond) are rigid, prohibiting rotation.

Examples of Alkene Isomers

  • Cis and trans alkenes have different physical properties and are different molecules.
  • Stereoisomers:
    • Same molecular formula, same connectivity, but different spatial orientations.
    • Identified in alkenes:
      • Example 1: (Cis) groups on the same side vs. (Trans) groups on opposite sides.
  • Cannot be converted into each other without breaking a bond.

Hybridization of Alkenes

  • Hybridization state of alkenes: sp².
  • Molecular shape around the double bond is trigonal planar.

Cis-Trans Limitations

  • Works well for simple alkenes but gets complicated with more substituents.
  • Naming Complications:
    • E/Z nomenclature is used when more than two groups are present to prevent confusion.

E/Z Nomenclature

  • Based on the priority of substituent groups at each end of the double bond:
    • Rule 1: Identify and rank the substituents based on atomic mass (or atomic number).
    • Same Side: Z (Zusammen).
    • Opposite Side: E (Entgegen).
  • Example: Apply ranking rules to establish the designation.

Naming Alkenes with E/Z

  • Steps to follow:
    1. Rank the groups attached to each of the double bond carbons.
    2. Compare the highest priority groups to determine if they are cis (Z) or trans (E).
  • The priority is determined based on atomic number of the first atom in the substituent.

Stereoisomers and Constraints

  • Identical Groups: If groups are identical (e.g., both CH₃), compound cannot be classified as cis or trans.
  • Double Bonds in Cyclic Structures:
    • E/Z nomenclature applies to cyclic compounds, but smaller rings (6 carbons or less) struggle with stereoisomerism due to structural rigidity.
    • Larger rings can accommodate both E and Z forms better than smaller rings.

Summary of Key Differences

  • Cis vs Trans Properties: Different physical properties, important in drug design.
  • E/Z Nomenclature Advantages: More systematic, eliminates ambiguity especially for complex alkenes.

Practical Implications

  • When constructing organic molecules, especially drugs, knowledge of stereochemical configurations (E/Z, cis/trans) is crucial for efficacy and safety.
  • Careful consideration in synthesis routes needs to be taken to achieve the desired stereoisomer configuration.