Lecture on Meso Compounds

Introduction to Meso Compounds

• Meso compounds are achiral compounds with chiral centers.
• Example compound: 1,2-dibromocyclohexane.
  • Carbons with bromine are potential chiral centers (sp3 hybridized, 4 different groups).

Meso Compounds with Examples

1,2-Dibromocyclohexane

• Trans 1,2-dibromocyclohexane

  • Has non-superimposable mirror images (enantiomers).
  • Determining stereochemistry:
    • Left structure: 2 R centers.
    • Right structure: 2 S centers.
  • Naming:
    • Left: 1R, 2R-1,2-dibromocyclohexane.
    • Right: 1S, 2S-1,2-dibromocyclohexane.

• Cis 1,2-dibromocyclohexane

  • Mirror images are superimposable (achiral).
  • Contains an internal mirror plane of symmetry.
  • Absolute stereochemistry: Top is R, bottom is S.
  • Named as meso-1,2-dibromocyclohexane.

2,3-Dibromobutane

• Fischer Projections
  • Four orientations for chiral carbons.
  • Absolute stereochemistry:
    • First structure: Both R.
    • Second: RS.
    • Last two: One R, one S.
  • Chirality:
    • First two: Non-superimposable mirror images (enantiomers, chiral).
    • Last two: Superimposable mirror images (meso, achiral).
  • Illustrates the 2^n rule:
    • A molecule with n stereocenters can have up to 2^n stereoisomers.
    • Example: 2 stereocenters, up to 4 isomers, but symmetry reduces to 3.

2-Bromo-3-chlorobutane

• Fischer Projections
  • Two stereocenters, can have up to 4 stereoisomers.
  • Absolute stereochemistry:
    • First: 2S, 3R.
    • Second (mirror image): 2R, 3S.
    • Third: RS.
    • Fourth: RR.
  • Chirality:
    • Two pairs of enantiomers.
    • No internal symmetry, all four are chiral.
  • Relationship between molecules: Diastereomers.

Conclusion

• Understanding meso compounds involves analyzing stereochemistry and symmetry.
• Recognizing internal mirror planes helps identify meso compounds.
• Applying the 2^n rule with consideration of symmetry helps predict stereoisomer count.