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11.18 Polymerization of Styrene

11.18 Polymerization of Styrene. Dr. Wolf's CHM 201 & 202. 11-107. H 2 C. CH C 6 H 5. CH 2. CH. CH 2. CH. CH 2. CH. C 6 H 5. C 6 H 5. C 6 H 5. Polymerization of Styrene. polystyrene. Dr. Wolf's CHM 201 & 202. 11-108. •. •. •. H 2 C. CH C 6 H 5. Mechanism. ••. RO.

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11.18 Polymerization of Styrene

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  1. 11.18Polymerization of Styrene Dr. Wolf's CHM 201 & 202 11-107

  2. H2C CHC6H5 CH2 CH CH2 CH CH2 CH C6H5 C6H5 C6H5 Polymerization of Styrene polystyrene Dr. Wolf's CHM 201 & 202 11-108

  3. • • • H2C CHC6H5 Mechanism •• RO Dr. Wolf's CHM 201 & 202 11-109

  4. • • Mechanism •• RO H2C CHC6H5 • Dr. Wolf's CHM 201 & 202 11-110

  5. • • H2C CHC6H5 Mechanism •• RO H2C CHC6H5 • Dr. Wolf's CHM 201 & 202 11-111

  6. • • • H2C CHC6H5 Mechanism •• RO H2C CHC6H5 Dr. Wolf's CHM 201 & 202 11-112

  7. • • Mechanism •• RO H2C CHC6H5 H2C CHC6H5 • Dr. Wolf's CHM 201 & 202 11-113

  8. • • H2C CHC6H5 Mechanism •• RO H2C CHC6H5 H2C CHC6H5 • Dr. Wolf's CHM 201 & 202 11-114

  9. • • • H2C CHC6H5 Mechanism •• RO H2C CHC6H5 H2C CHC6H5 Dr. Wolf's CHM 201 & 202 11-115

  10. •• • RO • H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 Mechanism • Dr. Wolf's CHM 201 & 202 11-116

  11. •• • RO • H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 Mechanism • Dr. Wolf's CHM 201 & 202 11-117

  12. •• • RO • H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 Mechanism • Dr. Wolf's CHM 201 & 202 11-118

  13. 11.19Cyclobutadiene and Cyclooctatetraene Dr. Wolf's CHM 201 & 202 11-119

  14. Heats of Hydrogenation to give cyclohexane (kJ/mol) heat of hydrogenation of benzene is 152 kJ/mol less than 3 times heat of hydrogenation of cyclohexene 120 231 208

  15. Heats of Hydrogenation to give cyclooctane (kJ/mol) heat of hydrogenation of cyclooctatetraene is more than 4 times heat of hydrogenation of cyclooctene 97 205 303 410 Dr. Wolf's CHM 201 & 202 11-122

  16. Structure of Cyclooctatetraene cyclooctatetraene is not planar has alternating long (146 pm)and short (133 pm) bonds Dr. Wolf's CHM 201 & 202 11-124

  17. C(CH3)3 (CH3)3C 138 pm 151 pm (CH3)3C CO2CH3 Structure of Cyclobutadiene structure of a stabilized derivative is characterizedby alternating short bonds and long bonds Dr. Wolf's CHM 201 & 202 11-123

  18. Stability of Cyclobutadiene Cyclobutadiene is observed to be highly reactive, and too unstable to be isolated and stored in thecustomary way. Not only is cyclobutadiene not aromatic, it is antiaromatic. An antiaromatic substance is one that is destabilizedby cyclic conjugation.

  19. notaromatic notaromatic aromatic Antiaromatic when square Antiaromatic when planar Requirements for Aromaticity cyclic conjugation is necessary, but not sufficient

  20. Conclusion there must be some factor in additionto cyclic conjugation that determines whether a molecule is aromatic or not Dr. Wolf's CHM 201 & 202 11-125

  21. 11.20Hückel's Rule:Annulenes the additional factor that influences aromaticity is the number of p electrons Dr. Wolf's CHM 201 & 202 11-126

  22. Hückel's Rule among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 p electrons possess special stability (are aromatic) n 4n+2 0 2 1 6 2 10 3 14 4 18 Dr. Wolf's CHM 201 & 202 11-127

  23. Hückel's Rule among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 p electrons possess special stability (are aromatic) n 4n+2 0 2 1 6 benzene! 2 10 3 14 4 18 Dr. Wolf's CHM 201 & 202 11-128

  24. Hückel's Rule Hückel restricted his analysis to planar,completely conjugated, monocyclic polyenes he found that the p molecular orbitals ofthese compounds had a distinctive pattern one p orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highestand the lowest Dr. Wolf's CHM 201 & 202 11-129

  25. p-MOs of Benzene Antibonding 6 p orbitals give 6 p orbitals 3 orbitals are bonding; 3 are antibonding Benzene Bonding Dr. Wolf's CHM 201 & 202 11-130

  26. p-MOs of Benzene Antibonding 6 p electrons fill all of the bonding orbitals all p antibonding orbitals are empty Benzene Bonding Dr. Wolf's CHM 201 & 202 11-131

  27. p-MOs of Cyclobutadiene(square planar) Antibonding 4 p orbitals give 4p orbitals 1 orbital is bonding, one is antibonding, and 2 are nonbonding Cyclo-butadiene Bonding Dr. Wolf's CHM 201 & 202 11-132

  28. p-MOs of Cyclobutadiene(square planar) Antibonding 4 p electrons; bonding orbital is filled; other 2p electrons singly occupy two nonbonding orbitals Cyclo-butadiene Bonding Dr. Wolf's CHM 201 & 202 11-133

  29. p-MOs of Cyclooctatetraene(square planar) Antibonding 8 p orbitals give 8 p orbitals 3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding Cyclo-octatetraene Bonding Dr. Wolf's CHM 201 & 202 11-134

  30. p-MOs of Cyclooctatetraene(square planar) Antibonding 8 p electrons; 3 bonding orbitals are filled; 2nonbonding orbitals are each half-filled Cyclo-octatetraene Bonding Dr. Wolf's CHM 201 & 202 11-135

  31. p-Electron Requirement for Aromaticity 6 p electrons 4 p electrons 8 p electrons notaromatic notaromatic aromatic Dr. Wolf's CHM 201 & 202 11-136

  32. H H Completely Conjugated Polyenes 6 p electrons;not completelyconjugated 6 p electrons;completely conjugated notaromatic aromatic Dr. Wolf's CHM 201 & 202 11-137

  33. 11.21Annulenes

  34. Annulenes Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule. Dr. Wolf's CHM 201 & 202 11-138

  35. [10]Annulene predicted to be aromatic by Hückel's rule,but too much angle strain when planar and all double bonds are cis 10-sided regular polygon has angles of 144° Dr. Wolf's CHM 201 & 202 11-139

  36. [10]Annulene incorporating two trans double bonds intothe ring relieves angle strain but introducesvan der Waals strain into the structure andcauses the ring to be distorted from planarity Dr. Wolf's CHM 201 & 202 11-140

  37. [10]Annulene van der Waalsstrain betweenthese two hydrogens incorporating two trans double bonds intothe ring relieves angle strain but introducesvan der Waals strain into the structure andcauses the ring to be distorted from planarity Dr. Wolf's CHM 201 & 202 11-141

  38. [14]Annulene 14 p electrons satisfies Hückel's rule van der Waals strain between hydrogens insidethe ring H H H H Dr. Wolf's CHM 201 & 202 11-142

  39. [16]Annulene 16 p electrons does not satisfy Hückel's rule alternating short (134 pm) and long (146 pm) bonds not aromatic Dr. Wolf's CHM 201 & 202 11-143

  40. H H H H H H [18]Annulene 18 p electrons satisfies Hückel's rule resonance energy = 418 kJ/mol bond distances range between 137-143 pm Dr. Wolf's CHM 201 & 202 11-144

  41. 11.22Aromatic Ions Dr. Wolf's CHM 201 & 202 11-145

  42. H H H H + H H H Cycloheptatrienyl Cation 6 p electrons delocalizedover 7 carbons positive charge dispersedover 7 carbons very stable carbocation also called tropylium cation Dr. Wolf's CHM 201 & 202 11-146

  43. H H H H H H H H + + H H H H H H Cycloheptatrienyl Cation Dr. Wolf's CHM 201 & 202 11-147

  44. + H Br Cycloheptatrienyl Cation Tropylium cation is so stable that tropyliumbromide is ionic rather than covalent. mp 203 °C; soluble in water; insoluble indiethyl ether Br– Ionic Covalent Dr. Wolf's CHM 201 & 202 11-148

  45. H H •• H H – H Cyclopentadienide Anion 6 p electrons delocalizedover 5 carbons negative charge dispersedover 5 carbons stabilized anion Dr. Wolf's CHM 201 & 202 11-149

  46. H H H H – •• H H H H – H H Cyclopentadienide Anion Dr. Wolf's CHM 201 & 202 11-150

  47. H H H H + H+ •• H H H H – H H H Acidity of Cyclopentadiene Cyclopentadiene is unusually acidic for a hydrocarbon. Increased acidity is due to stability of cyclopentadienide anion. pKa = 16 Ka = 10-16 Dr. Wolf's CHM 201 & 202 11-151

  48. H H H H H Electron Delocalization in Cyclopentadienide Anion •• – Dr. Wolf's CHM 201 & 202 11-152

  49. H H H H – •• H H H H H H Electron Delocalization in Cyclopentadienide Anion •• – Dr. Wolf's CHM 201 & 202 11-153

  50. H H H H – •• H H H H H H H H • • H H H Electron Delocalization in Cyclopentadienide Anion •• – – Dr. Wolf's CHM 201 & 202 11-154

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