Understanding Dienes Reactivity: Addition Patterns and Thermodynamic Control

1. Dienes

2. Reactions of Dienes • isolated dienes: double bonds react independently of one another • conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own • cumulated dienes: specialized topic

3. Addition of Hydrogen HalidestoConjugated Dienes

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

5. H H H H H H H H H H Cl H H Cl H H H H H H H H Example: HCl ? ?

6. H H H H H H H H Cl H H H H H Example: HCl

7. H H H H + H H H H H H H H H H H + H H H H H via: H X

8. 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

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

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

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

12. 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 +

13. 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: + +

14. 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

15. 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 +

16. 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)

17. 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

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

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

21. Consider the reaction coordinate diagram …

22. Problem • 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 ?

23. Problem • 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

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

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

26. H Cl Problem 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

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

28. The 1,4-addition product is NOT always the thermodynamic product … Always compare the intermediates and products to assign the kinetic (PATHWAY) and the thermodynamic product ( Lower ENERGY STATE)

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

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