Chapter 13

1. Chapter 13 Conjugated Unsaturated Systems

2. About The Authors These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang. Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem. Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.

3. Introduction • A conjugated system involves at least one atom with a p orbital adjacent to at least one p bond • e.g.

4. Allylic Substitution and the Allyl Radical vinylic carbons (sp2) allylic carbon (sp3)

5. 2A. Allylic Chlorination(High Temperature)

6. Mechanism • Chain initiation • Chain propagation

7. Mechanism • Chain propagation • Chain termination

11. 2B. Allylic Bromination with N-Bromo-succinimide (Low Concentration of Br2) • NBS is a solid and nearly insoluble in CCl4 • Low concentration of Br•

12. Examples

13. 3. The Stability of the Allyl Radical 3A. Molecular Orbital Description of the Allyl Radical

15. 3B. Resonance Description of the Allyl Radical

16. The Allyl Cation • Relative order of Carbocation stability

17. Resonance Theory Revisited 5A. Rules for Writing Resonance Structures • Resonance structures exist only on paper. Although they have no real existence of their own, resonance structures are useful because they allow us to describe molecules, radicals, and ions for which a single Lewis structure is inadequate • We connect these structures by double-headed arrows (), and we say that the hybrid of all of them represents the real molecule, radical, or ion

18. resonance structures not resonance structures • In writing resonance structures, we are only allowed to move electrons

19. All of the structures must be proper Lewis structures X 10 electrons! not a proper Lewis structure

20. All resonance structures must have the same number of unpaired electrons X

21. All atoms that are part of the delocalized p-electron system must lie in a plane or be nearly planar no delocalization of p-electrons delocalization of p-electrons

22. The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure • Equivalent resonance structures make equal contributions to the hybrid, and a system described by them has a large resonance stabilization

23. The more stable a structure is (when taken by itself), the greater is its contribution to the hybrid

24. 5B. Estimating the Relative Stability of Resonance Structures • The more covalent bonds a structure has, the more stable it is

25. Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid this carbon has 6 electrons this carbon has 8 electrons

26. Charge separation decreases stability

27. Alkadienes and Polyunsaturated Hydrocarbons • Alkadienes (“Dienes”)

28. Alkatrienes (“Trienes”)

29. Alkadiynes (“Diynes”) • Alkenynes (“Enynes”)

30. Cumulenes enantiomers

31. Conjugated dienes • Isolated double bonds

32. 1,3-Butadiene: Electron Delocalization 7A. Bond Lengths of 1,3-Butadiene 1.47 Å 1.34 Å sp sp3 sp3 sp2 sp3 1.46 Å 1.54 Å 1.50 Å

33. 7B. Conformations of 1,3-Butadiene trans single bond single bond cis

34. 7C. Molecular Orbitalsof 1,3-Butadiene

35. The Stability of Conjugated Dienes • Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes

37. Ultraviolet–Visible Spectroscopy • The absorption of UV–Vis radiation is caused by transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals

38. 9A. The Electromagnetic Spectrum

39. 9B. UV–Vis Spectrophotometers

41. A c x ℓ or e = • Beer’s law A = absorbance e = molar absorptivity c = concentration ℓ = path length A = e x c x ℓ • e.g. 2,5-Dimethyl-2,4-hexadiene lmax(methanol) 242.5 nm (e = 13,100)

42. 9C. Absorption Maxima for Nonconjugatedand Conjugated Dienes

44. 9D. Analytical Uses of UV–Vis Spectroscopy • UV–Vis spectroscopy can be used in the structure elucidation of organic molecules to indicate whether conjugation is present in a given sample • A more widespread use of UV–Vis spectroscopy, however, has to do with determining the concentration of an unknown sample • Quantitative analysis using UV–Vis spectroscopy is routinely used in biochemical studies to measure the rates of enzymatic reactions

45. Electrophilic Attack on ConjugatedDienes: 1,4 Addition

46. Mechanism X (a) (b)

47. 10A. Kinetic Control versus Thermodynamic Control of a Chemical Reaction

50. The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes