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Chapter 6

Chapter 6. Alkenes - C n H 2n - Naming. Degree of unsaturation. Stereoisomers - cis, trans, E, & Z. Alkene stability. Electrophilic Addition. The Hammond Postulate. Alkene - C n H 2n. ethene or ethylene. propene or propylene. Alkene - C n H 2n. trans-2-butene. cis-2-butene.

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Chapter 6

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  1. Chapter 6 • Alkenes - CnH2n - Naming. • Degree of unsaturation. • Stereoisomers - cis, trans, E, & Z. • Alkene stability. • Electrophilic Addition. • The Hammond Postulate. Chapter 6 - alkene

  2. Alkene - CnH2n ethene or ethylene propene or propylene Chapter 6 - alkene

  3. Alkene - CnH2n trans-2-butene cis-2-butene 2-Methlypropene Chapter 6 - alkene

  4. Alkene - CnH2n 1-Pentene ?- 2-Pentene ?- 2-Pentene 2-Methyl-1-butene Chapter 6 - alkene

  5. 6 3 7 2 8 1 1 8 7 2 6 3 4 5 4 5 Alkene - CnH2n 4,7-Dimethyl-2,5-octadiene 3-Methylcyclohexene 3,4-Diisopropyl-2,5-dimethyl-3--hexene Chapter 6 - alkene

  6. Alkene - CnH2n 2,3-Dimethylcyclopentene 4,4-Dimethylheptene Chapter 6 - alkene

  7. Chapter 6 • Alkenes - CnH2n - Naming. • Degree of unsaturation. • Stereoisomers - cis, trans, E, & Z. • Alkene stability. • Electrophilic Addition. • The Hammond Postulate. Chapter 6 - alkene

  8. Degree of Unsaturation. Page 270, problem 7.44 Compound A has the formula C8H8. It reacts rapidly with KMnO4 to give CO2 and a carboxylic acid, B (C7H6O2), but reacts with only 1 molar equivalent of H2 on catalytic hydrogenation over a palladium catalyst. On hydrogenation under conditions that reduce aromatic rings, 4 equivalents of H2 are taken up, and hydrocarbon C (C8H16) is produced. What are the structures of A, B, and C? Write the reactions. Chapter 6 - alkene

  9. CnH2n+2 — Saturation. CnH2n — Unsaturation. CnH2n-2 — Unsaturation. Degree of Unsaturation. Chapter 6 - alkene

  10. Rules for Degree of Unsaturation. See page 191. Chapter 6 - alkene

  11. Chapter 6 - alkene

  12. C8H14 C6H5N C6H5NO2 C20H22ClN Chapter 6 - alkene

  13. etc. Chapter 6 - alkene

  14. Given the oUS — many possibilities. Given the oUS & composition — possibilities reduced. Given the oUS, composition, & chemical information — often compounds identity can be determined. Chapter 6 - alkene

  15. Degree of Unsaturation. Page 270, problem 7.44 Compound A has the formula C8H8. It reacts rapidly with KMnO4 to give CO2 and a carboxylic acid, B (C7H6O2), but reacts with only 1 molar equivalent of H2 on catalytic hydrogenation over a palladium catalyst. On hydrogenation under conditions that reduce aromatic rings, 4 equivalents of H2 are taken up, and hydrocarbon C (C8H16) is produced. What are the structures of A, B, and C? Write the reactions. Chapter 6 - alkene

  16. Chapter 6 • Alkenes - CnH2n - Naming. • Degree of unsaturation. • Stereoisomers - cis, trans, E, & Z. • Alkene stability. • Electrophilic Addition. • The Hammond Postulate. Chapter 6 - alkene

  17. Disubstituted alkenes, Stereoisomers - cis, trans p 196. “Disustituted means two substituents other than hydrogen that are bonded to the double-bond carbons.” trans cis neither Chapter 6 - alkene

  18. Stereoisomers - E, & Z. Trisubstituted Tetrasubstituted Chapter 6 - alkene

  19. Stereoisomers - E, & Z. high high low high low low low high E Z “ze zame zide” (Z)-3-Methyl-2-pentene (E)-2,2,3,4-Tetramethyl-3-hexene Chapter 6 - alkene

  20. Stereoisomers - E, & Z. Assign E or Z configuration: low high high low high low Z low high Z Chapter 6 - alkene

  21. Chapter 6 • Alkenes - CnH2n - Naming. • Degree of unsaturation. • Stereoisomers - cis, trans, E, & Z. • Alkene stability. • Electrophilic Addition. • The Hammond Postulate. Chapter 6 - alkene

  22. mono E di tri tetra Alkene Stability Evidence: heats of hydrogenation Reasons: hyperconjugation bond strength Chapter 6 - alkene

  23. Hyperconjugation p 205. “ … interaction between the unfilled antibonding C = C bond orbital and a filled C - H  bond orbital on a neighboring substituent...” “ The more subtituents that are present, the more opportunities exist for hyperconjugation, and the more stable the alkene.” Chapter 6 - alkene

  24. Bond Strength p 206. “ More highly substituted alkenes always have a higher ratio of sp3 - sp2 to sp3 - sp3 bonds than less highly substituted alkenes and are therefore more stable.” Chapter 6 - alkene

  25. p 204, Table 6.2 Heats of Hydrogenation - cis Chapter 6 - alkene

  26. p 204, Table 6.2 Heats of Hydrogenation - trans Chapter 6 - alkene

  27. Heats of Hydrogenation - cis & trans Chapter 6 - alkene

  28. E -120 -115 RC Heats of Hydrogenation - cis & trans reaction profile Chapter 6 - alkene

  29. Chapter 6 • Alkenes - CnH2n - Naming. • Degree of unsaturation. • Stereoisomers - cis, trans, E, & Z. • Alkene stability. • Electrophilic Addition. • The Hammond Postulate. Chapter 6 - alkene

  30. or Br- Electrophilic Addition - symmetric 2o carbocation nucleophile electrophile 2o carbocation (Lewis base) (Lewis acid) nucleophile (Lewis base) electrophile only one product !!!! (Lewis acid) Chapter 6 - alkene

  31. or Br- Electrophilic Addition - asymmetric 3o carbocation nucleophile electrophile (Lewis acid) 2o carbocation (Lewis base) electrophile (Lewis acid) Markonikov’s Rule (Major) nucleophile (Lewis base) regiospecific (Minor) electrophile Chapter 6 - alkene (Lewis acid)

  32. Markonikov’s Rule regiospecific Electrophilic Addition - examples Predict major product: ether Chapter 6 - alkene

  33. Markonikov’s Rule regiospecific Electrophilic Addition - examples Predict major product: ether Chapter 6 - alkene

  34. Markonikov’s Rule regiospecific Electrophilic Addition - examples Predict major product: Chapter 6 - alkene

  35. Markonikov’s Rule regiospecific ether Electrophilic Addition - examples Predict major product: ether Chapter 6 - alkene

  36. Markonikov’s Rule regiospecific Electrophilic Addition - examples Predict major product: ether Chapter 6 - alkene

  37. Markonikov’s Rule regiospecific Electrophilic Addition - examples Predict major product: ether Chapter 6 - alkene

  38. 1o E 2o 3o Stability of Carbocation Chapter 6 - alkene

  39. Evidence of Carbocation minor major H-shift Chapter 6 - alkene

  40. Markonikov’s Rule regiospecific Electrophilic Addition - examples Predict all products: Chapter 6 - alkene

  41. H-shift Chapter 6 - alkene

  42. Stability of Carbocation • Inductive effect. • Hyperconjugation Chapter 6 - alkene

  43. H R C+ H Direction electron density shifts. R” R R’ Inductive Effect Ris an alkyl group, larger and polarizable. C+ Chapter 6 - alkene

  44. H CH3 H sp2 sp2 sp3 Hyperconjugation Chapter 6 - alkene

  45. Chapter 6 • Alkenes - CnH2n - Naming. • Degree of unsaturation. • Stereoisomers - cis, trans, E, & Z. • Alkene stability. • Electrophilic Addition. • The Hammond Postulate. Chapter 6 - alkene

  46. Experimental Data heat CH3CH2CH3 + 2Cl2 CH3CHClCH3 + CH3CH2CH2Cl +2HCl ? % ? % heat CH3CH2CH3 + 2Br2 CH3CHBrCH3 + CH3CH2CH2Br +2HBr ? % ? % Chapter 6 - alkene

  47. Experimental Data heat CH3CH2CH3 + 2Cl2 CH3CHClCH3 + CH3CH2CH2Cl +2HCl 43% 57% heat CH3CH2CH3 + 2Br2 CH3CHBrCH3 + CH3CH2CH2Br +2HBr 8% 92% Regiospecificity stronger in Bromine and Chlorine. Why? Chapter 6 - alkene

  48. Hammond Postulate CH3CH3 + •Cl  CH3CH2 • + HCl H = -21 kJ/mol CH3CH3 + •Br  CH3CH2 • + HBr H = +42 kJ/mol Chapter 6 - alkene

  49. Hammond Postulate [CH3CH2------- H -- Br]‡ [CH3CH2 -- H ------- Cl]‡ CH3CH2 • + HBr E E CH3CH3 + •Cl -21kJ/mol +42 kJ/mol CH3CH2 • + HCl CH3CH3 + •Br RC RC Chapter 6 - alkene

  50. Table 5.3 p 172 Stability Reactivity Carbocation Ease of breaking: 3o > 2o > 1o Chapter 6 - alkene

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