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Chapter 13

Chapter 13. Conjugated Pi Systems. Introduction. A conjugated system involves at least one atom with a p orbital adjacent to at least one p bond. e.g. Allylic Substitution and the Allyl Radical. vinylic carbons (sp 2 ). allylic carbon (sp 3 ).

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Chapter 13

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  1. Chapter 13 Conjugated Pi Systems

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

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

  4. 2A. Allylic Chlorination(High Temperature)

  5. Mechanism • Chain initiation: • Chain propagation:

  6. Mechanism • Chain propagation: • Chain termination:

  7. Allylic vs vinyl bond energies:

  8. Allylic vs vinyl activation energies:

  9. Radical stabilities:

  10. 2B. Allylic Bromination with N-Bromo-succinimide (Low Concentration of Br2) • NBS is a solid and nearly insoluble in CCl4. • Low concentration of Br•

  11. Examples:

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

  13. Molecular orbitals:

  14. 3B. Resonance Description of the Allyl Radical

  15. The Allyl Cation • Relative order of Carbocation stability.

  16. 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.

  17. resonance structures not resonance structures • In writing resonance structures, one may only move electrons.

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

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

  20. 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

  21. 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.

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

  23. 5B. Estimating the Relative Stability of Resonance Structures • The more covalent bonds a structure has, the more stable it is.

  24. 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

  25. Charge separation decreases stability.

  26. Alkadienes and Polyunsaturated Hydrocarbons • Alkadienes (“Dienes”):

  27. Alkatrienes (“Trienes”):

  28. Alkadiynes (“Diynes”): • Alkenynes (“Enynes”):

  29. Cumulenes: enantiomers

  30. Conjugated dienes: • Isolated double bonds:

  31. 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

  32. 7B. Conformations of 1,3-Butadiene trans single bond single bond cis

  33. 7C. Molecular Orbitals of 1,3-Butadiene

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

  35. Stability due to conjugation:

  36. 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.

  37. 9A. The Electromagnetic Spectrum

  38. 9B. UV–Vis Spectrophotometers

  39. 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)

  40. 9C. Absorption Maxima for Nonconjugatedand Conjugated Dienes

  41. 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, 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.

  42. Electrophilic Attack on ConjugatedDienes: 1,4 Addition

  43. Mechanism: X (a) (b)

  44. 10A. Kinetic Control versus Thermodynamic Control of a Chemical Reaction

  45. The 1,4-product is thermodynamically more stable.

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

  47. e.g.

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