1. "You cannot reason a person out of a position he did not reason himself into in the first place." Jonathan Swift

2. Chapter 16 Conjugated double bonds 2,6,10,14,19,23,27,31-Octamethyl-dotriaconta-2,6,8,10,12,14,16,18,20,22,24,26,30-tridecaene

3. 3 Conjugated Alkenes Conjugated Not Conjugated Red Shift in electronic spectra More stable than corresponding non-conjugated molecules sp 2 -sp 2  bonds are shorter and stronger than sp 3 -sp 2 bonds 5.5 kJ/mole more stable than linolenic acid (below)

4. 4 Identifying Conjugated Organics

5. Conjugated C=C Bonds and Electronic Spectra

6. Conjugated Alkenes Crocetin (from Saffron; 422 nm) Lycopene (505 nm) Indigo Blue (605 nm) 430 nm 500 nm 560 nm 580 nm 400 nm 420 nm 465nm 530 nm 600 nm 620 nm 640 nm 680 nm 720 nm Chlorophyll a (675 nm)

7. Structure Determination in Conjugated Systems: UV Spectroscopy The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to the lowest unoccupied molecular orbital (LUMO)

8. Conjugated Alkenes 430 nm 500 nm 560 nm 580 nm 400 nm 420 nm 465nm 530 nm 600 nm 620 nm 640 nm 680 nm 720 nm

9. Butadiene: Molecular Orbitals 4 p-orbitals

10. Conjugated C=C Bonds and Electronic Spectra

11. 430 nm 500 nm 560 nm 580 nm 400 nm 420 nm 465nm 530 nm 600 nm 620 nm 640 nm 680 nm 720 nm

12. Conjugation, Color and the Chemistry of Vision  -Carotene is converted to Vitamin A, which is converted to 11-cis-retinal:

13. Conjugation, Color and the Chemistry of Vision 11-cis-retinal is converted to rhodopsin in the rod cells of the retina. Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain

14. SUNSCREENS

15. Special Properties of Conjugated Dienes Length of the central single bond is shorter than non- conjugated similar molecule Comparison of 1,3-Butadiene and Butane with C-13 NMR H2C=CH-CH=CH2CH3-CH2-CH2-CH3 148 pm153 pm 1,3-ButadieneButane Shorter Bond

16. Why more stable (thermodynamics) than non- conjugated? p-p overlap is better than hyperconjugation

17. More reactive (kinetics) than non-conjugated LUMO is lower – more electrophilic HOMO is higher– more nucleophilic

18. Faster reactions, less ΔG for reactions (start lower)

20. Diene Preparation Cyclohexene3- Bromocyclohexene 1,3- Cyclohexadiene Based Induced Elimination of HX

21. Electrophilic Additions to Conjugated Dienes: Allylic Carbocations Electrophilic addition to 1,3-Butadiene yields a mixture of two products: 1,2 addition 1,4 addition

22. Non-conjugated alkene addition reactions 2- Methylpropene Tertiary Carbocation 2-Chloro-2- methylpropane

23. At room temperature, electrophilic addition to a conjugated diene leads to a product mixture where the 1,2 adduct predominates over the 1,4 adduct. At high temperatures, the product ratio changes and the 1,4 adduct predominates 1,2 adduct1,4 adduct At 0 o C:71% 29% At 40 o C:15% 85%

24. Reacting Butadiene with Electrophiles

25. Allyl Cations

26. Allylic and Benzylic Cations

27. Diene polymerizations

29. Vulcanization:

30. Diels-Alder Reaction

31. Dienes Dienophiles

32. Diels-Alder Reaction:Diene Reactivity Generally more reactive than acyclic dienes

33. Diels-Alder Reaction No way of telling which approach is used with this system

34. Diels-Alder Reaction

35. Stability of exo product is greater Higher temperature is required to reach product for exo path Running reaction at higher temp gives thermodynamic product (exo) Running reaction at room temp gives kinetic product (endo)

36. Diels-Alder Reactions

37. Diels Alder in Synthesis Look for cyclohexene = product of DA

38. 38 The key Diels–Alder reactions used to prepare the C ring of estrone and the B ring of cortisone are as follows: Steroid Synthesis

39. Diels-Alder Reactions

43. NMR of Dienes

45. Need to know Diene nomenclature Recognize conjugated versus non-conjugated systems Know reaction and mechanism: -addition of Br 2 or Cl 2 to dienes -addition of HCl or HBr to dienes -Diels Alder cycloaddition (exo and endo) Understand kinetic versus thermodynamics products for diene reactions and Diels Alder cycloaddition reactions Be able to synthesize dienes from alkanes through bromination and elimination reactions. Extra special: predict regiochemistry of Diels Alder cycloadditions 45