1 / 25

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. ?. ?.

duy
Download Presentation

Addition of Hydrogen Halides to Conjugated 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. 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%)

More Related