Lecture on Classifying Compounds

Key Concepts

• Isomers: Molecules with the same chemical formula but different structures.
• Categories of Isomers:
  • Constitutional Isomers: Differ in connectivity.
  • Stereoisomers: Differ in spatial arrangement but have the same connectivity.
  • Enantiomers: A type of stereoisomer, non-superimposable mirror images, all chiral centers change.
  • Diastereomers: A type of stereoisomer, some but not all chiral centers change.
  • Meso Compounds: Have a plane of symmetry, appear different but are identical.

Distinguishing Between Isomers

• Constitutional Isomers:

  • Example: Alcohol on carbon 2 in one molecule and on carbon 3 in another.
  • Differ in connectivity despite having the same chemical formula.

• Stereoisomers:

  • Same connectivity but different spatial arrangement.
  • Example: Bromine atoms on the same carbon but oriented differently in space.

Types of Stereoisomers

Enantiomers

• Characteristics:
  • Non-superimposable mirror images.
  • All chiral centers change configuration.
• Example:
  • If all centers change from RRR to SSS without symmetry, we have enantiomers.

Diastereomers

• Characteristics:
  • Some chiral centers change, not all.
  • Includes cis-trans geometric isomers.
  • Example:
    • RRR vs. RSR would be diastereomers.
    • Cis-trans isomers are always diastereomers but not vice versa.

Meso Compounds

• Characteristics:
  • Have a plane of symmetry.
  • Appear different but are identical.
  • Example:
    • All chiral centers change yet there is a line of symmetry, indicating meso.

Classifying Examples

• Example 1:

  • Identical molecules without chiral carbons.
  • Example: Molecules with non-chiral carbon, no front/back distinction needed.

• Example 2:

  • Constitutional Isomers
  • Example: Different connectivity of bromine atoms (Carbons 1, 3 vs. 1, 2).

• Example 3:

  • Meso Compounds
  • Example: Chiral centers change, but a plane of symmetry exists, indicating they are identical.

Additional Notes

• Cis-Trans Isomers:
  • These are a subtype of diastereomers.
  • Example: Cis isomer vs. Trans isomer.
  • Trans butene and cis butene are examples of geometric isomers.

This lecture provides a detailed overview of different types of isomers and how to distinguish between them based on connectivity and spatial arrangement.