Conjugated Systems Study Notes

Chapter 15: Conjugated Systems

Conjugated Dienes and Their Stability

  • Dienes and π Bonds:

    • Dienes with conjugated π bonds can interact with each other, leading to electron delocalization.
    • Examples of structures:
    • Isolated Diene: C=C−C=CC=C-C=C.
    • Conjugated Diene: C=C=CC=C=C.

  • Stability Analysis Through Heat of Hydrogenation:

    • Heat of hydrogenation data gives insights into stability.
    • Isolated Diene's heat of reaction: ( ext{ΔH}^{ ext{°}} = -126 ext{kJ/mol} )
    • Conjugated Diene's heat of reaction: ( ext{ΔH}^{ ext{°}} = -225 ext{kJ/mol} )
      • Conjugated is more stable by approximately 14extkJ/mol14 ext{kJ/mol} compared to isolated.
    • Cumulated Diene (example not specified): ( ext{ΔH}^{ ext{°}} = -292 ext{kJ/mol} )
      • Less stable by about 40extkJ/mol40 ext{kJ/mol} than conjugated.

  • Explanatory Theories:

    • Stability differences are explained through resonance theory and molecular orbital (MO) theory.

Resonance Perspective

  • Resonance in Dienes:
    • Various resonance structures depict the delocalization of electrons across the diene system.
    • Comparison with normal C-C bonds, which have a bond length of approximately 1.54 Å.

Huckel Molecular Orbital Theory

  • Theory Focus:

    • Hückel MO theory specifically examines the π-bond system in conjugated systems.

  • Dienes Examples:

    • Ethylene:
    • Structure: CH<em>2=CH</em>2CH<em>2=CH</em>2
    • Molecular Orbitals: (2 ext{ AOs}
      ightarrow 2 ext{ MOs} )
    • Butadiene:
    • Structure: CH<em>2=CH−CH=CH</em>2CH<em>2=CH-CH=CH</em>2
    • Molecular Orbitals: (4 ext{ AOs} ightarrow 4 ext{ MOs} )
      • Total of 4 π electrons.

Molecular Orbital Energy Levels

  • MO Energy and Nodal Planes:
    • As energy increases, a nodal plane is introduced, increasing orbital symmetry and overlap.
    • Bonding MOs are denoted such as MO1 and MO2, indicative of constructive overlap.
    • Key MOs:
    • HOMO: Highest Occupied Molecular Orbital
    • LUMO: Lowest Unoccupied Molecular Orbital

Reactions of Conjugated Dienes

  • Hydrobromination Example:

    • Reaction scheme:
    • Starting material: ( CH2=CH-CH=CH2 )
    • Addition of HBr can yield both 1,2- and 1,4-products.
      • Major control through conditions:
      • Low temperatures (e.g., -80°C) favor kinetic control with 80% 1,2-product.
      • Moderate temperatures (e.g., 40°C) favor thermodynamic control with 85% 1,4-product.

  • Kinetic vs. Thermodynamic Control:

    • Kinetic Control: Lower energy barrier for faster reaction paths.
    • Activation Energy (Ea) for 1,2-product is less than for 1,4-product.
    • Thermodynamic Control: Higher energy barriers; products favor stability.
    • The enthalpy change for the formation of 1,4-product is more exothermic than 1,2-product.

Substituted Dienes

  • Effect of Substituents:
    • Substituted dienes favor reactions that lead to the more electron-rich alkene.

Allylic Bromination Mechanism

  • Free Radical Mechanism:
    • Initiation: Formation of bromine radicals from Br-Br bonds.
    • Propagation: Series of steps utilizing the allylic radical intermediate.
    • Example reaction:
    • Using N-bromosuccinimide (NBS) with HBr to generate bromine without direct addition to the alkene.

Diels-Alder Cycloaddition

  • Reactions Between Dienes and Dienophiles:
    • Approach is characterized as a [4+2] cycloaddition.
    • Driving Force: Conversion of two π bonds to new σ bonds.
    • The mechanism is concerted, preserving the configuration of starting materials.

Regio- and Stereochemical Observations

  • Electronic Effects:

    • Electron Donating Groups (EDGs) stabilize developing positive charge; Electron Withdrawing Groups (EWGs) destabilize it.
    • Diene typically electron-rich; dienophile typically electron-poor for optimal reaction.

  • Stereochemical Requirements:

    • Dienes must adopt an s-cis conformation to undergo cycloaddition.
    • Cycloaddition occurs through the ‘syn’ addition pathway.

  • Endo Rule:

    • In bicyclic products, the endo product is preferred due to secondary orbital overlap with π bonds from electron-withdrawing groups on the dienophile.

Summary of Key Terms and Concepts

  • Conjugation: Interaction of π bonds in a molecule leading to stabilized structure.
  • Electron Delocalization: Distribution of electrons across adjacent π systems.
  • Kinetic Control: Reaction pathways favored under low temperatures for quick formation; typically involves less stable products.
  • Thermodynamic Control: Reaction pathways maximized stability leading to more stable products at high temperatures.
  • Diels-Alder Reaction: A specific cycloaddition reaction efficient for creating cyclic structures via diene and dienophile interactions.