1. Dienes 1

2. isolated dienes: double bonds react independently of one another
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 6

3. Addition of Hydrogen Halides to Conjugated Dienes7

4. Electrophilic Addition to Conjugated Dienes+ H X H
Proton adds to end of diene system
Carbocation formed is allylic 8

5. Example: H
HCl Cl H H Cl ? ? 9

6. Example: H
HCl Cl H 9

7. via: H + H H X H + 10

8. and: H + Cl H
Cl–
3-Chlorocyclopentene H + H H Cl H H H H H 10

9. 1,2-Addition versus 1,4-Addition
1,2-addition of XY X Y 12

10. 1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY X Y X Y 12

11. 1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY X Y X Y
via X + 12

12. HBr Addition to 1,3-Butadiene
H2C
CHCH
CH2
HBr Br
CH2
CH3CHCH +
CHCH2Br
CH3CH
electrophilic addition
1,2 and 1,4-addition both observed
product ratio depends on temperature 13

13. Rationale
3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. Br
CH2
CH3CHCH +
CHCH2Br
CH3CH
via: + +
CH2
CH3CHCH
CHCH2
CH3CH 13

14. Rationale
3-Bromo-1-butene is formed faster than 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. Br
CH2
CH3CHCH +
CHCH2Br
CH3CH
formed faster 13

15. Rationale
1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond. Br
CH2
CH3CHCH +
CHCH2Br
CH3CH
more stable 13

16. Rationale
The two products equilibrate at 25°C. Once equilibrium is established, the more stable isomer predominates. Br
CH2
CH3CHCH
CHCH2Br
CH3CH
major product at -80°C
major product at 25°C
(formed faster)
(more stable) 13

17. Kinetic Control versus Thermodynamic 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

18. CH2
CH3CHCH
CHCH2
CH3CH +
HBr
H2C
CHCH
CH2 17

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

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 ? 19

23. Problem + HCl Think mechanistically.
Protonation occurs: at end of diene system in 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 19

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

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

26. Think mechanistically
Problem
Think mechanistically H Cl
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 20

27. Think mechanistically
Problem
Think mechanistically H Cl
Cl– Cl + +
one resonance form is tertiary carbocation; other is primary
major product 20

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 23

31. Example H2C CHCH CH2 Br2 Br CH2 BrCH2CHCH + CHCH2Br BrCH2CH (37%)
(63%) 13