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11.19 Cyclobutadiene and Cyclooctatetraene

11.19 Cyclobutadiene and Cyclooctatetraene. 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. Heats of Hydrogenation. To give cyclooctane (kJ/mol):.

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11.19 Cyclobutadiene and Cyclooctatetraene

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  1. 11.19Cyclobutadiene and Cyclooctatetraene

  2. 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

  3. 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

  4. Structure of Cyclooctatetraene Cyclooctatetraene is not planar; has alternating long (146 pm)and short (133 pm) bonds.

  5. Structure of Cyclobutadiene MO calculations give alternating short and longbonds for cyclobutadiene. H H 135 pm 156 pm H H

  6. 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.

  7. 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.

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

  9. Conclusion There must be some factor in additionto cyclic conjugation that determines whether or not a molecule is aromatic.

  10. 11.20Hückel's Rule: The additional factor that influences aromaticity is the number of  electrons.

  11. Hückel's Rule Among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2  electrons possess special stability (are aromatic). n 4n+2 0 2 1 6 Benzene! 2 10 3 14 4 18

  12. Hückel's Rule Hückel restricted his analysis to planar,completely conjugated, monocyclic polyenes. He found that the  molecular orbitals ofthese compounds had a distinctive pattern. One  orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highestand the lowest.

  13. Hückel's Rule Frost's circle is a mnemonic that allows us to draw a diagram showing the relative energies ofthe p orbitals of a cyclic conjugated system. 1) Draw a circle. 2) Inscribe a regular polygon inside the circle so that one of its corners is at the bottom. 3) Every point where a corner of the polygon touches the circle corresponds to a p electron energy level. 4) The middle of the circle separates bonding and antibonding orbitals.

  14. Antibonding Bonding p MOs of Benzene Frost's Circle

  15. -MOs of Benzene Antibonding 6 p orbitals give 6  orbitals. 3 orbitals are bonding; 3 are antibonding. Benzene Bonding

  16. -MOs of Benzene Antibonding 6  electrons fill all of the bonding orbitals. All  antibonding orbitals are empty. Benzene Bonding

  17. -MOs of Cyclobutadiene(Square Planar) Antibonding 4 p orbitals give 4 orbitals. 1 orbital is bonding, one is antibonding, and 2 are nonbonding. Cyclo-butadiene Bonding

  18. -MOs of Cyclobutadiene(Square Planar) Antibonding 4  electrons; bonding orbital is filled; other 2 electrons singly occupy two nonbonding orbitals. Cyclo-butadiene Bonding

  19. -MOs of Cyclooctatetraene(Planar) Antibonding 8 p orbitals give 8  orbitals. 3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding. Cyclo-octatetraene Bonding

  20. -MOs of Cyclooctatetraene(Planar) Antibonding 8  electrons; 3 bonding orbitals are filled; 2nonbonding orbitals are each half-filled. Cyclo-octatetraene Bonding

  21. -Electron Requirement for Aromaticity 6  electrons 4  electrons 8  electrons notaromatic notaromatic aromatic

  22. H H Completely Conjugated Polyenes 6  electrons;not completelyconjugated 6  electrons;completely conjugated notaromatic aromatic

  23. 11.21Annulenes

  24. Annulenes Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule.

  25. [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°.

  26. van der Waalsstrain betweenthese two hydrogens [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.

  27. [14]Annulene 14  electrons satisfies Hückel's rule. van der Waals strain between hydrogens insidethe ring. H H H H

  28. [16]Annulene 16  electrons does not satisfy Hückel's rule. Alternating short (134 pm) and long (146 pm) bonds. Is an antiaromatic 4np-electron system.

  29. H H H H H H [18]Annulene 18  electrons satisfies Hückel's rule. Resonance energy = 418 kJ/mol. Bond distances range between 137-143 pm.

  30. 11.22Aromatic Ions

  31. H H H H + H H H Cycloheptatrienyl Cation 6  electrons delocalizedover 7 carbons positive charge dispersedover 7 carbons very stable carbocation also called tropylium cation

  32. H H H H H H H H + + H H H H H H Cycloheptatrienyl Cation

  33. + 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

  34. H H •• H H – H Cyclopentadienide Anion 6  electrons delocalizedover 5 carbons negative charge dispersedover 5 carbons stabilized anion

  35. H H H H – •• H H H H – H H Cyclopentadienide Anion

  36. 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

  37. H H H H – •• H H H H H H H H H H H H – •• – • – • • • H H H H H H H H H Electron Delocalization in Cyclopentadienide Anion •• –

  38. H H H H H H H H H H H H H H H Compare Acidities ofCyclopentadiene and Cycloheptatriene pKa = 16 Ka = 10-16 pKa = 36 Ka = 10-36

  39. H H H H H H •• H H – •• H H H H Compare Acidities ofCyclopentadiene and Cycloheptatriene – Aromatic anion 6  electrons Antiaromatic anion8  electrons

  40. H H H H + + H H Cyclopropenyl Cation n = 0 4n +2 = 2  electrons also written as

  41. Cyclooctatetraene Dianion H H H H n = 2 4n +2 = 10 electrons – H H H H •• alsowritten as 2– •• H H H H – H H H H

  42. 11.23Heterocyclic Aromatic Compounds

  43. •• •• •• O S N N •• •• •• H Examples Pyridine Pyrrole Furan Thiophene

  44. • N • N •• Examples Quinoline Isoquinoline

  45. 11.24Heterocyclic Aromatic CompoundsandHückel's Rule

  46. N •• Pyridine 6  electrons in ring. Lone pair on nitrogen is in ansp2 hybridized orbital;not part of system of ring.

  47. •• N H Pyrrole Lone pair on nitrogen must be part of ring  system if ring is to have6  electrons. Lone pair must be in a p orbitalin order to overlap with ring system.

  48. •• O •• Furan Two lone pairs on oxygen. One pair is in a p orbital and is partof ring  system; other is in an sp2 hybridized orbital and is notpart of ring  system.

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