1. Chapter 15 Dienes, 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 kJ mol-1 of the stabilization
15.1 Structure and Stability of Dienes

5. Stabilization through the p system

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

9. Conformations of Dienes
15.1 Structure and Stability of Dienes

10. Conformations of Dienes
Newman projections:
15.1 Structure and Stability of Dienes

11. 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)

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

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

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

15. Cyclic dienes can lead to multicyclic compounds

16. Effect of Diene Conformation

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

19. 15.3 The Diels-Alder Reaction

20. Transition State of the Diels-Alder Reaction

21. 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

22. 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

23. 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

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

25. Problems
Predict the products for the following reactions: