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🥼organic chemistry review

key term - Conjugated Dienes

Citation:

Definition

Conjugated dienes are organic compounds with two carbon-carbon double bonds that are separated by a single carbon-carbon bond. This arrangement of alternating double and single bonds creates a system of delocalized pi electrons, which gives conjugated dienes unique stability and reactivity properties.

5 Must Know Facts For Your Next Test

  1. Conjugated dienes exhibit enhanced stability compared to isolated dienes due to the delocalization of pi electrons across the entire system.
  2. The stability of conjugated dienes can be explained using molecular orbital theory, which shows that the pi-bonding and pi-antibonding orbitals are closer in energy, resulting in greater electron delocalization.
  3. Electrophilic additions to conjugated dienes often proceed through the formation of allylic carbocations, which are stabilized by the delocalization of the positive charge.
  4. Reactions involving conjugated dienes can exhibit either kinetic or thermodynamic control, depending on the specific reaction conditions and the relative stabilities of the possible products.
  5. Conjugated dienes are important intermediates in the Wittig reaction, a nucleophilic addition reaction involving phosphorus ylides, which is used to synthesize alkenes.

Review Questions

  • Explain how the stability of conjugated dienes can be understood using molecular orbital theory.
    • According to molecular orbital theory, the pi-bonding and pi-antibonding orbitals in conjugated dienes are closer in energy compared to isolated dienes. This allows for greater delocalization of the pi electrons across the entire system, resulting in enhanced stability. The overlap of the p orbitals in the conjugated system creates a more extensive network of pi bonds, which stabilizes the molecule and makes it less reactive than isolated dienes.
  • Describe the role of allylic carbocations in the electrophilic additions to conjugated dienes.
    • Electrophilic additions to conjugated dienes often proceed through the formation of allylic carbocations as intermediates. These carbocations are stabilized by the delocalization of the positive charge across the three-carbon allylic system. The presence of the nearby double bond allows for the pi electrons to participate in the stabilization of the carbocation, making the intermediate more stable and the reaction more favorable. The formation and stabilization of allylic carbocations is a key factor in determining the products of electrophilic additions to conjugated dienes.
  • Discuss how the concepts of kinetic and thermodynamic control can be applied to reactions involving conjugated dienes, and explain the significance of these principles in the context of the Wittig reaction.
    • Reactions involving conjugated dienes can exhibit either kinetic or thermodynamic control, depending on the specific reaction conditions and the relative stabilities of the possible products. Kinetic control favors the formation of the kinetically preferred product, which is the product formed more rapidly, while thermodynamic control leads to the formation of the thermodynamically more stable product. In the Wittig reaction, which involves the nucleophilic addition of phosphorus ylides to conjugated dienes, the choice between kinetic or thermodynamic control can be an important factor in determining the stereochemistry of the alkene product. Understanding the principles of kinetic versus thermodynamic control is crucial for predicting and controlling the outcomes of reactions involving conjugated dienes, such as the Wittig reaction.