1 / 61

10.8 Classes of Dienes

10.8 Classes of Dienes. C. Classification of Dienes. isolated diene conjugated diene cumulated diene. C. Nomenclature. (2 E ,5 E )-2,5-heptadiene (2 E ,4 E )-2,4-heptadiene 3,4-heptadiene. 10.9 Relative Stabilities of Dienes. Heats of Hydrogenation.

genna
Télécharger la présentation

10.8 Classes of Dienes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 10.8 Classes of Dienes Dr. Wolf's CHM 201 & 202

  2. C Classification of Dienes isolated diene conjugated diene cumulated diene Dr. Wolf's CHM 201 & 202

  3. C Nomenclature (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene 3,4-heptadiene Dr. Wolf's CHM 201 & 202

  4. 10.9Relative Stabilitiesof Dienes Dr. Wolf's CHM 201 & 202

  5. Heats of Hydrogenation 1,3-pentadiene is 26 kJ/mol more stable than 1,4-pentadiene, but some of this stabilization is because it also contains a more highly substituted double bond 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202

  6. Heats of Hydrogenation 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202

  7. Heats of Hydrogenation 126 kJ/mol 111 kJ/mol 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol Dr. Wolf's CHM 201 & 202

  8. Heats of Hydrogenation when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated 126 kJ/mol 111 kJ/mol Dr. Wolf's CHM 201 & 202

  9. Heats of Hydrogenation this extra 15 kJ/mol is known by several termsstabilization energydelocalization energyresonance energy 126 kJ/mol 111 kJ/mol Dr. Wolf's CHM 201 & 202

  10. H2C CH2CH3 Heats of Hydrogenation Cumulated double bonds have relatively high heats of hydrogenation + CH3CH2CH3 C 2H2 H2C CH2 DH° = -295 kJ + CH3CH2CH3 H2 DH° = -125 kJ Dr. Wolf's CHM 201 & 202

  11. 10.10Bondingin Conjugated Dienes Dr. Wolf's CHM 201 & 202

  12. Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene Dr. Wolf's CHM 201 & 202

  13. Isolated diene p bonds are independent of each other 1,3-pentadiene Conjugated diene Dr. Wolf's CHM 201 & 202

  14. Isolated diene p bonds are independent of each other p orbitals overlap to give extended p bond encompassing four carbons Conjugated diene Dr. Wolf's CHM 201 & 202

  15. Isolated diene less electron delocalization; less stable more electron delocalization; more stable Conjugated diene Dr. Wolf's CHM 201 & 202

  16. H H H H H H H H H H H H Conformations of Dienes s prefix designates conformation around single bond s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) s-trans s-cis Dr. Wolf's CHM 201 & 202

  17. H H H H H H H H H H H H Conformations of Dienes s prefix designates conformation around single bond s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) s-trans s-cis Dr. Wolf's CHM 201 & 202

  18. Conformations of Dienes Both conformations allow electron delocalization via overlap of p orbitals to give extended p system s-trans s-cis Dr. Wolf's CHM 201 & 202

  19. s-trans is more stable than s-cis Interconversion of conformations requires two p bonds to be at right angles to each other and prevents conjugation 12 kJ/mol Dr. Wolf's CHM 201 & 202

  20. Dr. Wolf's CHM 201 & 202

  21. 16 kJ/mol 12 kJ/mol Dr. Wolf's CHM 201 & 202

  22. 10.11Bonding in Allenes Dr. Wolf's CHM 201 & 202

  23. C C C Cumulated Dienes cumulated dienes are less stable thanisolated and conjugated dienes (see Problem 10.7 on p 375) Dr. Wolf's CHM 201 & 202

  24. Structure of Allene linear arrangement of carbons nonplanar geometry 118.4° 131 pm Dr. Wolf's CHM 201 & 202

  25. Structure of Allene linear arrangement of carbons nonplanar geometry 118.4° 131 pm Dr. Wolf's CHM 201 & 202

  26. Bonding in Allene sp sp 2 sp 2 Dr. Wolf's CHM 201 & 202

  27. Bonding in Allene Dr. Wolf's CHM 201 & 202

  28. Bonding in Allene Dr. Wolf's CHM 201 & 202

  29. Bonding in Allene Dr. Wolf's CHM 201 & 202

  30. C C C Chiral Allenes Allenes of the type shown are chiral X A Y B A ¹B; X ¹Y Have a stereogenic axis Dr. Wolf's CHM 201 & 202

  31. Stereogenic Axis analogous to difference between: a screw with a right-hand thread and one with a left-hand thread a right-handed helix and a left-handed helix Dr. Wolf's CHM 201 & 202

  32. 10.12Preparation of Dienes Dr. Wolf's CHM 201 & 202

  33. CH2 H2C CHCH 1,3-Butadiene 590-675°C CH3CH2CH2CH3 More than 4 billion pounds of 1,3-butadiene prepared by this method in U.S. each year used to prepare synthetic rubber (See "Diene Polymers" box) chromia- alumina + 2H2 Dr. Wolf's CHM 201 & 202

  34. OH Dehydration of Alcohols KHSO4 heat Dr. Wolf's CHM 201 & 202

  35. OH Dehydration of Alcohols major product; 88% yield KHSO4 heat Dr. Wolf's CHM 201 & 202

  36. Br Dehydrohalogenation of Alkyl Halides KOH heat Dr. Wolf's CHM 201 & 202

  37. Br Dehydrohalogenation of Alkyl Halides major product; 78% yield KOH heat Dr. Wolf's CHM 201 & 202

  38. Reactions of Dienes isolated dienes: double bonds react independently of one another cumulated dienes: specialized topic conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own Dr. Wolf's CHM 201 & 202

  39. 10.13Addition of Hydrogen HalidestoConjugated Dienes Dr. Wolf's CHM 201 & 202

  40. Electrophilic Addition to Conjugated Dienes Proton adds to end of diene system Carbocation formed is allylic + H X H Dr. Wolf's CHM 201 & 202

  41. H H H H H H H H H H Cl H H Cl H H H H H H H H Example: HCl ? ? Dr. Wolf's CHM 201 & 202

  42. H H H H H H H H Cl H H H H H Example: HCl Dr. Wolf's CHM 201 & 202

  43. H H H H + H H H H H H H H H H H + H H H H H via: H X Dr. Wolf's CHM 201 & 202

  44. H H H H Cl + H H H H H H H H H H H H + H H H H H and: Cl– 3-Chlorocyclopentene H H Cl H H H H H Dr. Wolf's CHM 201 & 202

  45. Y X 1,2-Addition versus 1,4-Addition 1,2-addition of XY Dr. Wolf's CHM 201 & 202

  46. Y Y X X 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY Dr. Wolf's CHM 201 & 202

  47. Y Y X X + X 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY via Dr. Wolf's CHM 201 & 202

  48. CH2 H2C CHCH CH3CHCH CH3CH CH2 CHCH2Br Br HBr Addition to 1,3-Butadiene electrophilic addition 1,2 and 1,4-addition both observed product ratio depends on temperature HBr + Dr. Wolf's CHM 201 & 202

  49. CH3CHCH CH3CH CH2 CHCH2Br Br CH3CHCH CH3CH CH2 CHCH2 Rationale 3-Bromo-1-butene is formed faster than1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. + via: + + Dr. Wolf's CHM 201 & 202

  50. CH3CHCH CH3CH CH2 CHCH2Br Br Rationale 3-Bromo-1-butene is formed faster than1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. + formed faster Dr. Wolf's CHM 201 & 202

More Related