14.1 Stability of Conjugated Dienes: Molecular Orbital Theory

Conjugated dienes are special molecules with overlapping p orbitals that form a continuous π system. This unique structure allows electrons to spread out, making the molecule more stable than its non-conjugated counterparts.

Molecular orbital theory helps explain this stability by showing how atomic orbitals combine to form molecular orbitals. The resulting energy levels and electron behavior give conjugated dienes their distinctive properties and reactivity.

Molecular Orbital Theory and Conjugated Dienes

Molecular orbital theory for conjugated dienes

Top images from around the web for Molecular orbital theory for conjugated dienes

• 

  Molecular Orbital Theory | Chemistry: Atoms First View original

  Is this image relevant?

• 

  Molecular Orbital Theory | Boundless Chemistry View original

  Is this image relevant?

• 

  Molecular Orbital Theory | General Chemistry View original

  Is this image relevant?

• 

  Molecular Orbital Theory | Chemistry: Atoms First View original

  Is this image relevant?

• 

  Molecular Orbital Theory | Boundless Chemistry View original

  Is this image relevant?

1 of 3

Top images from around the web for Molecular orbital theory for conjugated dienes

• 

  Molecular Orbital Theory | Chemistry: Atoms First View original

  Is this image relevant?

• 

  Molecular Orbital Theory | Boundless Chemistry View original

  Is this image relevant?

• 

  Molecular Orbital Theory | General Chemistry View original

  Is this image relevant?

• 

  Molecular Orbital Theory | Chemistry: Atoms First View original

  Is this image relevant?

• 

  Molecular Orbital Theory | Boundless Chemistry View original

  Is this image relevant?

1 of 3

• Molecular orbital theory uses wave functions and energy levels to describe electron behavior in molecules
  • Combines atomic orbitals into molecular orbitals (linear combination of atomic orbitals, LCAO)
  • Bonding molecular orbitals have lower energy and stabilize the molecule (σ and π bonding orbitals)
  • Antibonding molecular orbitals have higher energy and destabilize the molecule (σ* and π* antibonding orbitals)
• Conjugated dienes have overlapping p orbitals that form a conjugated π system
  • Allows electron delocalization over the entire conjugated system
  • Delocalization lowers the molecule's overall energy, increasing stability (1,3-butadiene)
• Conjugated π system forms bonding and antibonding π molecular orbitals
  • Bonding π molecular orbitals have lower energy than the original atomic p orbitals
  • Antibonding π molecular orbitals have higher energy than the original atomic p orbitals
• Occupied bonding π molecular orbitals being lower in energy than the atomic p orbitals results in net stabilization of conjugated dienes

Conjugated vs nonconjugated diene properties

• Conjugated dienes have more uniform bond lengths compared to nonconjugated dienes
  • Conjugated dienes have shorter single bonds and longer double bonds
  • Due to electron delocalization in the conjugated π system
  • Partial double bond character shortens single bonds in conjugated dienes (1,3-butadiene)
  • Partial single bond character lengthens double bonds in conjugated dienes (1,3-butadiene)
• Conjugated dienes have lower heats of hydrogenation than nonconjugated dienes
  • Heat of hydrogenation: energy released when a compound is completely hydrogenated
  • Lower heat of hydrogenation in conjugated dienes indicates greater stability (1,3-butadiene vs 1,4-pentadiene)
  • Stability attributed to electron delocalization in the conjugated π system

Electron delocalization in conjugated systems

• Delocalization: spreading of electrons over multiple atoms in a molecule
• Conjugated systems have overlapping p orbitals that allow electron delocalization
  • Creates a conjugated π system extending over the entire conjugated region
  • Electrons in the conjugated π system are spread out, not confined to a single bond (1,3,5-hexatriene)
• Electron delocalization in conjugated systems increases molecular stability
  • Delocalized electrons occupy lower-energy bonding π molecular orbitals
  • Lowers energy and stabilizes the molecule compared to a nonconjugated counterpart (1,3-butadiene vs 1,4-pentadiene)
  • More extensive conjugation leads to greater delocalization and stability (1,3,5,7-octatetraene)
• Resonance structures represent electron delocalization in conjugated systems
  • Each resonance structure shows a different π electron arrangement
  • Actual structure is a hybrid of all resonance structures with delocalized electrons (benzene)

Molecular Orbital Energy Levels and Hybridization

• Energy level diagrams illustrate the relative energies of molecular orbitals
  • HOMO (Highest Occupied Molecular Orbital) is the highest energy occupied orbital
  • LUMO (Lowest Unoccupied Molecular Orbital) is the lowest energy unoccupied orbital
• Nodes in molecular orbitals indicate regions of zero electron density
• Hybridization of atomic orbitals affects the overall molecular orbital structure and energy levels