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Chapter 15 Dienes, Resonance, and Aromaticity

Chapter 15 Dienes, Resonance, and Aromaticity. Classification of Dienes. Dienes : compounds containing two double bonds Cumulenes : compounds in which one carbon participates in two double bonds Conjugated Dienes : compounds that contain two double bonds that are separated by a single bond.

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Chapter 15 Dienes, Resonance, and Aromaticity

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  1. Chapter 15Dienes, Resonance, and Aromaticity

  2. Classification of Dienes • Dienes: compounds containing two double bonds • Cumulenes: compounds in which one carbon participates in two double bonds • Conjugated Dienes: compounds that contain two double bonds that are separated by a single bond

  3. Heats of Formation 15.1 Structure and Stability of Dienes

  4. Why Are Conjugated Dienes More Stable? • Different hybridization in bonds • s bonds with more s-character are stronger • These bonding differences account for 5 - 6 kJ mol-1 of the stabilization 15.1 Structure and Stability of Dienes

  5. Stabilization through the psystem

  6. MO Picture of 1,3-Butadiene

  7. Delocalization • The p1 MO shows that the electron density is spread across the entire molecule • The electrons are said to be delocalized

  8. Conformations of Dienes 15.1 Structure and Stability of Dienes

  9. Conformations of Dienes • Newman projections: 15.1 Structure and Stability of Dienes

  10. The Diels-Alder Reaction • Discovered by Otto Diels and Kurt Alder in 1928 (shared Nobel Prize in chemistry 1950) • Involves a diene and a dienophile (alkene or alkyne)

  11. The Diels-Alder Reaction • Cycloaddition reaction: An addition reaction that results in formation of a ring • 1,4-addition reaction or a conjugate addition

  12. Pericyclic reaction: concerted reaction that involves the cyclic flow of electrons 15.3 The Diels-Alder Reaction

  13. Problems • Draw the products you get from the following Diels-Alder reactions

  14. Cyclic dienes can lead to multicyclic compounds

  15. Effect of Diene Conformation

  16. Effect of Diene Conformation 15.3 The Diels-Alder Reaction

  17. 15.3 The Diels-Alder Reaction

  18. Transition State of the Diels-Alder Reaction

  19. Stereochemistry of the Diels-Alder Reaction • The stereochemistry of the dienophile is maintained during a DA rxn • The concerted transition state results in syn-addition of the diene to the dienophile

  20. Stereochemistry of the Diels-Alder Reaction • Syn-addition is also revealed with substituents at the terminal ends of the diene 15.3 The Diels-Alder Reaction

  21. Endo and Exo Products • Endo product: The alkene substituents are cis to the outer diene substituents (Ro) • Exo product: The alkene substituents are trans to the outer diene substituents (Ro) • The endo product generally forms faster

  22. Endo vs exo products are also observed when cyclic dienes are used

  23. Problems • Predict the products for the following reactions:

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