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Addition of Hydrogen Halides to Conjugated Dienes

Addition of Hydrogen Halides to Conjugated Dienes. Electrophilic Addition to Conjugated Dienes. Proton adds to end of diene system Carbocation formed is allylic. +. H. X. H. H. H. H. H. H. H. H. H. H. H. Cl. H. H. Cl. H. H. H. H. H. H. H. H. Example:. H Cl. ?. ?.

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Addition of Hydrogen Halides to Conjugated Dienes

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  1. Addition of Hydrogen HalidestoConjugated Dienes

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

  3. H H H H H H H H H H Cl H H Cl H H H H H H H H Example: HCl ? ?

  4. H H H H H H H H Cl H H H H H Example: HCl

  5. H H H H + H H H H H H H H H H H + H H H H H via: H X

  6. 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

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

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

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

  10. 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 +

  11. 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: + +

  12. 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

  13. 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 +

  14. 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)

  15. 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

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

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

  18. 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 ?

  19. 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

  20. H Cl Think mechanistically • one resonance form is tertiary carbocation; other is primary + +

  21. H Cl + + Think mechanistically • one resonance form is secondary carbocation; other is primary Cl H + + one resonance form is tertiary carbocation; other is primary

  22. H Cl 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

  23. H Cl Think mechanistically Cl– Cl + + one resonance form is tertiary carbocation; other is primary majorproduct

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

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

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