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Conjugated Unsaturated Systems

Chapter 13. allylic substitution & allylic radicals allylic bromination sabitility of allylic radicals allylic cations resonance theory - detailed (recall chapter 1 info) alkadienes, polyunsaturated hydrocarbons 1,3-butadiene, resonance delocalization stability of conjugated dienes

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Conjugated Unsaturated Systems

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  1. Chapter 13 allylic substitution & allylic radicals allylic bromination sabitility of allylic radicals allylic cations resonance theory - detailed (recall chapter 1 info) alkadienes, polyunsaturated hydrocarbons 1,3-butadiene, resonance delocalization stability of conjugated dienes electronic attack on conjugated dienes, 1,4-addition diels-alder rx, 1,4-cycloaddition • Conjugated Unsaturated Systems 46 Modified from sides of William Tam & Phillis Chang

  2. Introduction conjugated system at least one p orbital adjacent to one (or more) π bond e.g.

  3. Allylic Substitution vs Allyl Radical vinylic carbons (sp2) allylic carbon (sp3) mechanisms?

  4. Radical chain reaction Chain propagation (r.d.s.) Addition rx

  5. Allylic Bromination with N-Bromo-Succinimide (NBS) NBS (a solid insoluble in CCl4) low concentration of Br•

  6. Examples

  7. Resonance of Allyl Radicals

  8. Allyl Cation (recall SN1 intermediate) Relative order of carbocation stability

  9. Resonance theory Rules for Writing Resonance Structures Resonance structures don’t exist But structures allow predictive description of molecules, radicals, & ions for which a single Lewis structure is inadequate Connect resonance structures by ↔ The hybrid (combined “weighted” avg.) of all resonance structures represents the real substance

  10. resonance structures not resonance structures writing resonance structures move only electrons

  11. All structures must be proper Lewis structures X 10 electrons! not a proper Lewis structure

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

  13. All delocalized atoms of the π-electron system must lie roughly in a plane

  14. The energy of the hybrid is lower than the energy estimated for any contributing structure A system described by equivalent resonance structures has a large resonance stabilization

  15. The more stable a contributing structure the greater its contribution to the hybrid

  16. Estimating Relative Stability of Resonance Structures The more covalent bonds a structure has, the more stable it is

  17. Structures in which all of the atoms have a complete valence shell of electrons (“octets”) make larger contributions to the hybrid this carbon has 6 electrons this carbon has 8 electrons

  18. Charge separation decreases stability

  19. Alkadienes and Polyunsaturated Hydrocarbons Alkadienes (“Dienes”)

  20. Alkatrienes (“Trienes”)

  21. Alkadiynes (“Diynes”) Alkenynes (“Enynes”)

  22. Cumulenes

  23. Conjugated dienes Isolated double bonds

  24. 1,3-Butadiene: Electron Delocalization 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

  25. Conformations of 1,3-Butadiene trans single bond single bond cis

  26. The Stability of Conjugated Dienes Conjugated dienes are thermodynamically more stable than isomeric isolated alkadienes

  27. Electrophilic Attack on Conjugated Dienes: 1,4 Addition

  28. Mechanism (a) (b)

  29. Kinetic versus Thermodynamic Control of a Chemical Reaction

  30. Diels–Alder Reaction: 1,4 Cycloaddition Reaction of Dienes

  31. e.g. diene dieophile adduct

  32. Factors Favoring the Diels–Alder RX Types A and B are normal Diels-Alder rxs

  33. Types C and D are Inverse Demand Diels-Alder rxs

  34. Relative rate

  35. Steric effects

  36. Stereochemistry of the Diels–Alder RX Stereospecific: syn addition and the dienophile configuration is retainedin the product

  37. The diene reacts in the s-cis conformation (s-trans can’t cycloadd) X

  38. e.g. (diene locked s-cis)

  39. Cyclic dienes with the double bonds s-cis are usually highly reactive in the Diels–Alder rx, e.g.

  40. R is exo longest bridge R is endo DA rx can form bridged structures The Diels–Alder rx occurs primarily in an endo fashion

  41. endo Alder-Endo Rule For dienophiles with activating groups having π bonds, the ENDOorientation in the t.s. is preferred exo

  42. e.g. = =

  43. Stereospecific reaction

  44. Stereospecific reaction

  45. Sterics Diene A reacts 103 times faster than diene B

  46. Examples Rate of Diene C > Diene D (27 times) tBu group electron donating group and favors s-cisdiene end

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