1. Chapter 10 Conjugation in Alkadienes and Allylic Systems
conjugare is a Latin verb meaning "to link or yoke together" 1

2. The Double Bond as a SubstituentC +
allylic carbocation 2

3. The Double Bond as a SubstituentC + • C
allylic carbocation
allylic radical 2

4. The Double Bond as a SubstituentC + • C
allylic carbocation
allylic radical C
conjugated diene 2

5. 10.1 The Allyl Group C H 3

6. Vinylic versus Allylic
carbon
vinylic carbons 4

7. Vinylic versus AllylicH C C H C H
vinylic hydrogens are attached to vinylic carbons 4

8. Vinylic versus AllylicH
allylic hydrogens are attached to allylic carbons 4

9. Vinylic versus AllylicX C C X C X
vinylic substituents are attached to vinylic carbons 4

10. Vinylic versus AllylicX X C C X C
allylic substituents are attached to allylic carbons 4

11. 10.2 Allylic Carbocations C + 5

12. relative rates: (ethanolysis, 45°C)
Allylic Carbocations
the fact that a tertiary allylic halide undergoes solvolysis (SN1) faster than a simple tertiary alkyl halide Cl
CH3 C
H2C CH
CH3 C
CH3 Cl
CH3
123 1
relative rates: (ethanolysis, 45°C) 6

13. Allylic Carbocations
provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations
CH3
CH3
H2C C + CH
CH3 C +
CH3
CH3
formed faster 6

14. H2C=CH— stabilizes C+ better than CH3—
Allylic Carbocations
provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations
CH3
H2C CH +
CH3 C
CH3 C +
CH3
H2C=CH— stabilizes C+ better than CH3— 6

15. Stabilization of Allylic Carbocations
Delocalization of electrons in the double bond stabilizes the carbocation
resonance model orbital overlap model 9

16. Resonance Model
CH3
H2C CH + C
CH3
CH3
H2C CH + C 10

17. Resonance Model CH3 H2C CH + C CH3 CH3 H2C CH + C CH3 d+ d+ H2C CH C10

18. Orbital Overlap Model d+ d+ 12

19. Orbital Overlap Model 12

20. Orbital Overlap Model 12

21. Orbital Overlap Model 12

22. Hydrolysis of an Allylic HalideCl
CH3 C
H2C CH
H2O
Na2CO3
CH3 OH
CH3 C
H2C CH +
HOCH2 CH C
CH3
(15%)
(85%) 16

23. Corollary Experiment CH3 ClCH2 CH C H2O Na2CO3 CH3 HOCH2 CH C OH CH3 C+
(15%)
(85%) 16

24. give the same products because they form the same carbocationCl
CH3 C
H2C CH
CH3
ClCH2 CH C
and
give the same products because they form the same carbocation 16

25. give the same products because they form the same carbocationCl
CH3 C
H2C CH
CH3
ClCH2 CH C
and
give the same products because they form the same carbocation
CH3
CH3 +
H2C C + CH
H2C CH C
CH3
CH3 16

26. more positive charge on tertiary carbon; therefore more tertiary alcohol in product+
H2C CH C +
H2C CH C
CH3
CH3 16

27. (85%) (15%) CH3 CH3 C + C H2C CH OH HOCH2 CH CH3 CH3
more positive charge on tertiary carbon; therefore more tertiary alcohol in product
CH3
CH3 +
H2C + CH C
H2C CH C
CH3
CH3 16

28. 10.3 Allylic Free Radicals C • 5

29. Allylic free radicals are stabilized by electron delocalization• C • 22

30. Free-radical stabilities are related to bond-dissociation energies
410 kJ/mol •
CH3CH2CH2—H
CH3CH2CH2 + H•
368 kJ/mol •
CHCH2—H
H2C
CHCH2
H2C + H•
C—H bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane 23

31. 10.4 Allylic Halogenation 5

32. Chlorination of Propene
addition
ClCH2CHCH3 Cl
CHCH3
H2C +
Cl2
CHCH2Cl
H2C
500 °C
+ HCl
substitution 25

33. selective for replacement of allylic hydrogen free radical mechanism
Allylic Halogenation
selective for replacement of allylic hydrogen
free radical mechanism
allylic radical is intermediate 26

34. Hydrogen-atom abstraction stepCl : . .. H
410 kJ/mol
368 kJ/mol H
allylic C—H bond weaker than vinylic
chlorine atom abstracts allylic H in propagation step 27

35. Hydrogen-atom abstraction stepCl : .. • C C H H C
410 kJ/mol
368 kJ/mol H 27

36. reagent used (instead of Br2) for allylic bromination
N-Bromosuccinimide
reagent used (instead of Br2) for allylic bromination Br O
NBr O NH
heat + +
CCl4
(82-87%) 29

37. Allylic halogenation is only used when:
Limited Scope
Allylic halogenation is only used when:
all of the allylic hydrogens are equivalent
and the resonance forms of allylic radical are equivalent 30

38. Cyclohexene satisfies both requirements
Example H
Cyclohexene satisfies both requirements
All allylic hydrogens are equivalent 31

39. Cyclohexene satisfies both requirements
Example H
Cyclohexene satisfies both requirements
All allylic hydrogens are equivalent H • H •
Both resonance forms are equivalent 31

40. All allylic hydrogens are equivalent 2-Butene CH3CH CHCH3
Example
All allylic hydrogens are equivalent
2-Butene
CH3CH
CHCH3
But •
CH3CH CH
CH2 •
CH3CH CH
CH2
Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides. 31