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10.9 Preparation of Dienes

10.9 Preparation of Dienes. CH 2. H 2 C. CHCH. 1,3-Butadiene. 590-675°C. CH 3 CH 2 CH 2 CH 3. 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. +. 2H 2. OH.

JasminFlorian
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10.9 Preparation of Dienes

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  1. 10.9Preparation of Dienes

  2. 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

  3. OH Dehydration of Alcohols KHSO4 heat

  4. OH Dehydration of Alcohols • major product; 88% yield KHSO4 heat

  5. Br Dehydrohalogenation of Alkyl Halides KOH heat

  6. Br Dehydrohalogenation of Alkyl Halides • major product; 78% yield KOH heat

  7. 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

  8. 10.10Addition of Hydrogen HalidestoConjugated Dienes

  9. Electrophilic Addition to Conjugated Dienes • Proton adds to end of diene system • Carbocation formed is allylic + H X H

  10. H H H H H H H H H H Cl H H Cl H H H H H H H H Example: HCl ? ?

  11. H H H H H H H H Cl H H H H H Example: HCl

  12. H H H H + H H H H H H H H H H H + H H H H H via: H X

  13. 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

  14. Y X 1,2-Addition versus 1,4-Addition 1,2-addition of XY

  15. Y Y X X 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY

  16. Y Y X X + X 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY via

  17. 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 +

  18. 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: + +

  19. 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

  20. CH3CHCH CH3CH CH2 CHCH2Br Br Rationale 1-Bromo-2-butene is more stable than3-bromo-1-butene because it has amore highly substituted double bond. • more stable +

  21. CH3CHCH CH3CH CH2 CHCH2Br Br Rationale The two products equilibrate at 25°C.Once equilibrium is established, the morestable isomer predominates. major product at -80°C major product at 25°C (formed faster) (more stable)

  22. Kinetic ControlversusThermodynamic Control • Kinetic control: major product is the one formed at the fastest rate • Thermodynamic control: major product is the one that is the most stable

  23. + + CH3CHCH CH3CH CH2 CHCH2 CH2 H2C CHCH HBr

  24. CH3CHCH CH2 Br CH3CH CHCH2Br + higher activation energy CH3CHCH CH2 + CH3CH CHCH2 formed more slowly

  25. Problem 10.10(page 382) • Addition of hydrogen chloride to 2-methyl-1,3-butadiene is a kinetically controlled reaction and gives one product in much greater amounts than any isomers. What is this product? + HCl ?

  26. Problem 10.10(page 382) • Think mechanistically. • Protonation occurs:at end of diene systemin direction that gives most stable carbocation • Kinetically controlled product corresponds to attack by chloride ion at carbon that has the greatest share of positive charge in the carbocation + HCl

  27. H Cl Problem 10.10(page 382) Think mechanistically • one resonance form is tertiary carbocation; other is primary + +

  28. H Cl + + Problem 10.10(page 382) Think mechanistically • one resonance form is secondary carbocation; other is primary Cl H + + one resonance form is tertiary carbocation; other is primary

  29. H Cl Problem 10.10(page 382) Think mechanistically • More stable carbocation • Is attacked by chloride ion at carbon that bears greater share of positive charge + + one resonance form is tertiary carbocation; other is primary

  30. H Cl Problem 10.10(page 382) Think mechanistically Cl– Cl + + one resonance form is tertiary carbocation; other is primary majorproduct

  31. 10.11Halogen Addition to Dienes • gives mixtures of 1,2 and 1,4-addition products

  32. CH2 H2C CHCH BrCH2CHCH BrCH2CH CH2 CHCH2Br Br Example Br2 + (37%) (63%)

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