Decalin and polycyclic molecules showcase the fascinating world of conformational isomerism in fused ring systems. These structures demonstrate how the arrangement of atoms in space impacts molecular shape, stability, and reactivity.

Understanding conformations in complex molecules is crucial for predicting their behavior. From the bent cis-decalin to the rigid norbornane, these structures illustrate how ring fusion and bridging influence molecular geometry and strain.

Conformations of Decalin and Polycyclic Molecules

Cis vs trans decalin structures

• Decalin is a bicyclic molecule composed of two fused cyclohexane rings
  • Bridgehead carbons are the two carbons shared by both rings at the point of fusion
• cis-Decalin has both bridgehead hydrogens on the same side of the molecule
  • Bridgehead hydrogens are cis to each other resulting in a bent structure
  • Both cyclohexane rings adopt chair conformations for stability
  • Rings are fused in a cis manner creating a V-shaped molecule (cis-decahydronaphthalene)
• trans-Decalin has bridgehead hydrogens on opposite sides of the molecule
  • Bridgehead hydrogens are trans to each other leading to a more linear structure
  • Both cyclohexane rings assume chair conformations to minimize strain
  • Rings are fused in a trans fashion producing a more extended shape (trans-decahydronaphthalene)
• The cis and trans forms of decalin are examples of conformational isomerism in fused ring systems

Conformational analysis in polycyclic compounds

• Steroids are polycyclic compounds with multiple fused rings
  • Typically have four fused rings: three six-membered rings (cyclohexane) and one five-membered ring (cyclopentane)
• Conformational analysis principles for cyclohexane rings apply to the rings in steroids
  • Rings prefer to adopt chair conformations to minimize angle strain and torsional strain
  • Substituents favor equatorial positions over axial positions to reduce 1,3-diaxial interactions
• Steroid rings are often represented as planar for simplicity, but they adopt puckered conformations
  • Puckering minimizes eclipsing interactions and 1,3-diaxial interactions between substituents
• Conformational flexibility of steroid rings is restricted by ring fusion
  • Ring fusion limits the conformations accessible to each individual ring due to shared atoms
  • Conformational changes in one ring affect the conformations of fused rings (concerted changes)

Structure of norbornane

• Norbornane is a bicyclic molecule with the molecular formula $C_7H_{12}$
  • Consists of a cyclohexane ring fused to a methylene bridge ($CH_2$) at carbons 1 and 4
• Norbornane contains a conformationally locked boat cyclohexane ring
  • Boat conformation is imposed by the methylene bridge connecting C1 and C4
  • Boat conformation is more stable than in isolated cyclohexane due to reduced flagpole interactions
• Norbornane has two bridge positions: C1 and C4
  • C1 and C4 are the bridgehead carbons shared by the cyclohexane ring and methylene bridge
  • C1 and C4 are sp3 hybridized carbons with reduced bond angles (~98°)
• Norbornane is a rigid molecule with limited conformational flexibility
  • Rigidity stems from the locked boat cyclohexane ring and bridge connections
  • Limited rotation about C-C bonds in the bicyclic framework (bicyclo[2.2.1]heptane)

Ring strain in polycyclic molecules

• Ring strain is a key factor in the stability and reactivity of polycyclic molecules
• Factors contributing to ring strain include:
  • Angle strain: deviation from ideal bond angles
  • Torsional strain: unfavorable eclipsing interactions between adjacent bonds
  • Transannular strain: interactions between non-bonded atoms across a ring
• Fused ring systems often experience increased ring strain due to conformational restrictions
• Chair-chair interconversion in fused cyclohexane rings is typically more energetically demanding than in isolated cyclohexane rings