(500) Stereoisomers, Enantiomers, Meso Compounds, Diastereomers, Constitutional Isomers, Cis & Trans

Overview of Isomers

  • Isomers: Compounds with the same chemical formula but different structures.

Types of Isomers

Constitutional Isomers

  • Differ in connectivity of their atoms.

  • Example:

    • Molecule A: Alcohol is attached to Carbon 2.

    • Molecule B: Alcohol is attached to Carbon 3.

Stereoisomers

  • Atoms connected in the same way, but arranged differently in space.

  • Example:

    • Both have bromine on Carbon 2, but the spatial arrangement differs:

      • Molecule A: Bromine facing out of the page (front).

      • Molecule B: Bromine behind the page (back).

Subcategories of Stereoisomers

  • Enantiomers

    • Non-superimposable mirror images.

    • Identified by changes in chiral centers.

  • Diastereomers

    • Some but not all chiral centers change.

    • Example: Comparing R,R,R with R,S,R results in a diastereomer.

Relationships between Different Types of Isomers

Enantiomer Examples

  • If all chiral centers change and there's no plane of symmetry, they are enantiomers.

  • Example:

    • Molecule A with chiral centers in one configuration and Molecule B with all opposite configuration, demonstrating an absence of symmetry.

Meso Compounds

  • All chiral centers change their configuration but have a plane of symmetry.

  • Identical despite different configurations.

Diastereomer Examples

  • If only some chiral centers change, the compounds are diastereomers.

  • Example:

    • Molecule A: Configuration R,R

    • Molecule B: Configuration R,S, which constitutes diastereomers.

Cis-Trans Geometric Isomers

  • A specific type of diastereomer caused by restricted rotation around a double bond.

  • Example:

    • Cis-2-butene (both substituents on the same side) vs. Trans-2-butene (substituents on opposite sides).

Identical vs. Different Molecules

  • If two molecules feature a non-chiral carbon with identical substituents on both sides, they are identical molecules.

  • Differing connectivity, such as positions of bromines, leads to constitutional isomers.

Conclusion

  • Different rules apply to different types of isomers:

    • Constitutional isomers = different connectivity.

    • Stereoisomers = same connectivity but different spatial arrangements.

    • Enantiomers = all chiral centers differ.

    • Diastereomers = some chiral centers differ.

    • Meso compounds = chiral centers differ but have symmetry.

    • Cis-trans isomers = specific type of diastereomer.