1. Chapter 10 Conjugation in Alkadienes and Allylic Systems
Conjugare is a Latin verb meaning "to link or yoke together"
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2. 10.1 The Allyl Group C H R 3

3. Vinylic versus Allylic
carbon
vinylic carbons 4

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

5. Vinylic versus AllylicX X X C C C C X C X C X
Vinylic substituents are
attached to vinylic carbons.
Allylic substituents are
attached to allylic carbons. 4

6. The Double Bond as a Substituent
allylic carbocation C +
allylic radical • C
allylic carbanion C -
a conjugated diene C 2

7. Allylic Molecular Orbitals
A linear combination of three p orbitals results in the formation of three new molecular orbitals. C H + 3

8. Molecular Orbitals of the Allylic Group2

9. Electrons in the Allylic MOs
Highest energy
Lowest energy 2

10. 10.2 Allylic Carbocations C + 5

11. relative rates: (ethanolysis, 45°C)
Allylic Carbocations
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

12. Provides good evidence that allylic carbocations
are more stable than alkyl carbocations.
CH3
H2C CH +
CH3 C
CH3 C +
CH3
H2C=CH— stabilizes C+ better than does CH3— 6

13. Stabilization of Allylic Carbocations
Extra stabilization comes from delocalization of electrons in the double bond. This stabilizes the carbocation.
Two models: resonance model orbital overlap model 9

14. Resonance stabilization of C+.
Resonance Model
Resonance stabilization of C+.
CH3
H2C CH + C
CH3
H2C CH + C C
CH3
H2C CH + 10

15. Orbital Overlap Model
Empty orbital of C+. 12

16. Filled pi bond orbitals.
Orbital Overlap Model
Filled pi bond orbitals. 12

17. Orbital overlap stabilization of C+.
Orbital Overlap Model
Orbital overlap stabilization of C+. 12

18. 10.3 SN1 Reactions of Allylic Halides5

19. Hydrolysis of an Allylic HalideCl
CH3 C
H2C CH
Carbonate binds the proton from water.
H2O
Na2CO3
(15%) +
CH3
HOCH2 CH C
(85%) OH
CH3 C
H2C CH 16

20. Corollary Experiment C CH3 ClCH2 CH Same products. H2O Na2CO3 (15%) +
HOCH2 CH C
(85%) OH
CH3 C
H2C CH 16

21. Give the same products because they form the same carbocation.Cl
CH3 C
H2C CH
CH3
ClCH2 CH C
and
Give the same products because they form the same carbocation.
H2C
CH3 CH + C 16

22. (15%) + CH3 HOCH2 CH C (85%) OH CH3 C H2C CH
More positive charge on tertiary carbon; therefore more tertiary alcohol in product.
H2C
CH3 CH + C 16

23. 10.4 SN2 Reactions of Allylic Halides5

24. Allylic halides also undergo SN2 reactions
Allylic SN2 Reactions
Allylic halides also undergo SN2 reactions
faster than simple primary alkyl halides. Cl
CH2
H2C CH
H3C
CH2
CH2 Cl 1 80
relative rates
In reaction with I-, a good nucleophile,
and run in acetone, an SN2 solvent. 6

25. relative rates: (I-, acetone)
Allylic SN2 reactions
Two factors:
Steric:
Trigonal carbon smaller than tetrahedral carbon. Cl
CH2
H2C CH
H3C
CH2
CH2 Cl 80 1
relative rates: (I-, acetone) 6

26. relative rates: (I-, acetone)
Allylic SN2 reactions
Two factors:
Electronic:
Electron delocalization lowers LUMO energy
which means lower activation energy.
See pg. 329 of the text. Cl
CH2
H2C CH
H3C
CH2
CH2 Cl 80 1
relative rates: (I-, acetone) 6

27. 10.5 Allylic Free Radicals C • 5

28. Allylic Free Radicals are Stabilized by Electron Delocalization• C • 22

29. 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 the radical (propyl) from propane. 23

30. 10.6 Allylic Halogenation 5

31. Chlorination of Propene
Normal addition
ClCH2CHCH3 Cl
CHCH3
H2C +
Cl2
CHCH2Cl
H2C
500 °C
+ HCl
Substitution,
Allylic chlorination 25

32. It is selective for replacement of an allylic hydrogen.
Allylic Halogenation
It is selective for replacement of an allylic hydrogen.
Proceeds by a free radical mechanism.
The allylic radical is an intermediate in this process. 26

33. Hydrogen-atom Abstraction StepCl : . .. H
410 kJ/mol
368 kJ/mol H
Allylic C—H bond is weaker than vinylic.
Chlorine atom abstracts allylic H in propagation step. 27

34. Hydrogen-atom Abstraction StepCl : .. • C C H H C
410 kJ/mol
368 kJ/mol H 27

35. Allylic bromination uses N-Bromosuccinimide
NBS is the reagent used (instead of Br2 directly) for allylic bromination.
CCl4 Br +
heat
(82-87%) O NH O
NBr 29

36. Allylic halogenation is best used when:
Limited Scope
Allylic halogenation is best used when:
1. All of the allylic hydrogens are equivalent
and
2. The resonance forms of allylic radical are equivalent. 30

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

38. All allylic hydrogens are equivalent.
Example
All allylic hydrogens are equivalent. •
CH3CH CH
CH2
2-Butene
CH3CH
CHCH3
But
Two resonance forms are not equivalent; so a mixture of isomeric allylic bromides forms. Br Br
CH3CH CH
CH2
and
CH3CH CH
CH2 31

39. 10.7 Allylic Anions C - 5

40. Acidity of Propene H3C CH CH2 H3C CH2 CH3 pKa ~ 43 pKa ~ 62 H2C CH -
Allylic hydrogens of propene are significantly more acidic than hydrogens of propane and are easier to remove. 10

41. Charge is delocalized to both terminal carbons,
Resonance Model -
H2C CH -
CH2
H2C CH
CH2
CH2
H2C CH -
Charge is delocalized to both terminal carbons,
stabilizing the conjugate base. 10

42. 10.8 Classes of Dienes 5

43. Classification of Dienes
Isolated diene
Conjugated diene
Cumulated diene C 2

44. Nomenclature (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene

45. 10.9 Relative Stabilities of Dienes5

46. Heats of Hydrogenation, conjugated vs isolated
1,3-pentadiene is 26 kJ/mol more stable than 1,4-pentadiene, but some of this stabilization is because it also contains a more highly substituted double bond.
252 kJ/mol
226 kJ/mol 4

47. Heats of Hydrogenation, conjugated vs isolated
Δ = 126 kJ/mol
Δ = 111 kJ/mol
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol 5

48. Heats of Hydrogenation, conjugated vs isolated
Δ = 126 kJ/mol
Δ = 111 kJ/mol
When a terminal double bond is conjugated with another double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated. 5

49. Heats of Hydrogenation, conjugated vs isolated
Δ = 126 kJ/mol
Δ = 111 kJ/mol
This extra 15 kJ/mol is known by several terms: conjugation energy or delocalization energy or resonance energy. 5

50. Heats of Hydrogenation
Cumulated double bonds have relatively high heats of hydrogenation.
H2C C +
CH2
2H2
CH3CH2CH3
H° = -295 kJ
H° = -125 kJ +
H2C
CH2CH3 H2
CH3CH2CH3 7

51. 10.10 Bonding in Conjugated Dienes5

52. Isolated diene
1,4-pentadiene
1,3-pentadiene
Conjugated diene 10

53.  bonds are independent of each other.
Isolated diene
 bonds are independent of each other.
p orbitals overlap to extend  bond to encompass four carbons.
Conjugated diene 10

54. less electron delocalization; less stable
Isolated diene
less electron delocalization; less stable
more electron delocalization; more stable
Conjugated diene 10

55. Conformations of Dienes, s-trans vs s-cisH H
s-trans
s-cis
s prefix designates conformation around single bond.
s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case). 11

56. Conformations of Dienes
s-trans
s-cis
Both conformations allow electron delocalization via overlap of p orbitals to give extended  system. 11

57. s-trans is More Stable Than s-cis
Interconversion of conformations involves the two  bonds being at right angles to each other and prevents conjugation during the process.
12 kJ/mol 12

58. No conjugation
Conjugated
s-trans
Conjugated
s-cis 12

59. No conjugation Conjugated s-trans Conjugated s-cis 16 kJ/mol 12 kJ/mol15

60. 10.11 Bonding in Allenes 5

61. Cumulated dienes are less stable than isolated and conjugated dienes.
(see Problem 10.10) 17

62. Structure of Allene, CH2=C=CH2
118.4°
131 pm
Linear arrangement of carbons
Nonplanar geometry 19

63. p-orbitals on the sp2 carbons are orthogonal.
Bonding in Allene
p-orbitals on the sp2 carbons are orthogonal.
sp 2 sp
sp 2 19

64. p-orbitals on the sp2 carbons are orthogonal.
Bonding in Allene
p-orbitals on the sp2 carbons are orthogonal. 19

65. Bonding in Allene 19

66. Bonding in Allene 19

67. Allenes of the type shown are chiral
Chiral Allenes
Allenes of the type shown are chiral A X C B Y
A  B; X  Y
Have a chirality axis (Section 7.9) 25

68. Analogous to difference between:
Chirality Axis
Analogous to difference between:
A screw with a right-hand thread and one with a left-hand thread A right-handed helix and a left-handed helix. 26

69. 10.12 Preparation of Dienes 5

70. Used to prepare synthetic rubber (See "Diene Polymers" box, p. 406).
1,3-Butadiene °C
CH3CH2CH2CH3
H2C
CHCH
CH2
chromia-
alumina +
2H2
More than 4 billion pounds of 1,3-butadiene prepared by this method in U.S. each year.
Used to prepare synthetic rubber (See "Diene Polymers" box, p. 406). 2

71. Dehydration of Alcohols
KHSO4 OH
heat
major product;
88% yield
Conjugated, more stable 3

72. Dehydrohalogenation of Alkyl Halides
KOH Br
heat
major product;
78% yield
Conjugated, more stable 3

73. Isolated dienes: double bonds react independently of one another.
Reactions of Dienes
Isolated dienes: double bonds react independently of one another.
Cumulated dienes: a specialized topic.
Conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own. 6

74. 10.13 Addition of Hydrogen Halides to Conjugated Dienes5

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

76. H
Example
HCl Cl H ? H Cl ?
Which 9

77. The favored reaction proceeds via:+ H H X H +
Protonation of the end of the diene unit (above) gives an allylic carbocation. The opposing protonation would give an isolated secondary carbocation. 10

78. The Allylic C+ gives Two Products:Cl H
1,2-addition of HCl
Cl– H +
3-Chlorocyclopentene
1,4-addition of HCl 10

79. 1,2-Addition vs 1,4-Addition
1,2-addition of XY
1,4-addition of XY X Y X Y
via X + 12

80. Addition of HBr to 1,3-Butadiene
H2C
CHCH
CH2
HBr Br
CH2
CH3CHCH +
CHCH2Br
CH3CH
3-Bromo-1-butene
1-bromo-2-butene
This is electrophilic addition.
1,2 and 1,4-addition both observed.
Product ratio depends on temperature. 13

81. Rationale:
3-Bromo-1-butene (left) is formed faster than 1-bromo-2-butene (right) 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)
CH2
CH3CHCH
CHCH2
CH3CH
via: + 13

82. Rationale:
However, 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
(formed faster)
(more stable) 13

83. Rationale:
The two products equilibrate at 25°C. At equilibrium, the more stable isomer predominates. Br
CH2
CH3CHCH
CHCH2Br
CH3CH
major product at -80°C
major product at 25°C
(more stable)
(formed faster)
1,2 addition product
1,4 addition product 13

84. Kinetic Control vs Thermodynamic Control
Kinetic control: the major product is the one formed at the fastest rate.
Thermodynamic control: the major product is the one that is the most stable. 16

85. Energy Diagram for Addition of HBr to 1,3-Butadiene
H2C
CHCH
CH2
HBr Br
CH2
CH3CHCH +
CHCH2Br
CH3CH 17

86. higher activation energy +
formed more slowly Br
CH2
CH3CHCH
CHCH2Br
CH3CH
Kinetic product
Thermodynamic product 17

87. Example 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

88. Example Problem + HCl Think mechanistically.
Protonation occurs: at end of diene system such that the most stable carbocation forms.
Kinetically controlled product corresponds to attack by chloride ion at carbon that has the greatest share of positive charge in the carbocation. +
HCl 19

89. Think mechanistically:
Example Problem
Think mechanistically: H Cl Cl H + +
One resonance form is a tertiary carbocation; other is primary.
One resonance form is a secondary carbocation; other is primary. 20

90. Think mechanistically:
Example Problem
Think mechanistically: H Cl
The more stable tertiary carbocation is attacked by chloride ion since this carbon bears a greater share of positive charge. + +
One resonance form is a favored tertiary carbocation. 20

91. Think mechanistically:
Example Problem
Think mechanistically: H Cl
Cl– Cl + +
One resonance form is tertiary carbocation; other is primary.
major product 20

92. 10.14 Halogen Addition to Conjugated Dienes
Gives mixtures of 1,2 and 1,4-addition products 5

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

94. 10.15 The Diels-Alder Reaction
Synthetic method for preparing compounds containing a cyclohexene ring 5

95. an alkene (dienophile)
Diels-Alder reaction +
a conjugated diene
an alkene (dienophile)
cyclohexene
Electron flow:
transition state 2

96. Mechanistic Features of Diels-Alder
It has a concerted mechanism, the addition is syn and an endo product is formed (called the endo rule or the Alder rule).
It is a (4 + 2) cycloaddition reaction (forms a cyclic structure).
Pericyclic reaction: A class of reactions that use pi electrons, have concerted mechanisms and proceed through a cyclic transition state (includes cycloadditions). 4

97. What Makes a Reactive Dienophile?
Ethylene itself is not an effective dienophile. The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond.
Typical EWGs C O N C
EWG 5

98. Example CH O H2C CHCH CH2 + H2C CH benzene 100°C (100%) CH O CH O via:6

99. Example O
CHC
CH2
H2C
CH3 +
benzene
100°C
H3C
via: O
(100%)
H3C O 6

100. Acetylenic Dienophile
CCOCH2CH3
CH3CH2OCC
H2C
CHCH
CH2 +
benzene
100°C
(98%)
COCH2CH3 O 6

101. Diels-Alder Reaction is Stereospecific*
*A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other; characterized by terms like syn addition, anti elimination, inversion of configuration, etc.
Diels-Alder: occurs by syn addition to alkene.
The cis-trans relationship of substituents on alkene retained in cyclohexene product. 9

102. Example O C6H5 COH C + H2C CHCH CH2 H H cis diene heat H C6H5 COH O
+ enantiomer 10

103. Example C C6H5 COH H O + H2C CHCH CH2 heat trans diene H C6H5 COH O
+ enantiomer 10

104. Cyclic Dienes Yield Bridged Bicyclic Diels-Alder Adducts
COCH3 H O
CH3OC + H
COCH3 O
Cyclic Dienes Yield Bridged Bicyclic Diels-Alder Adducts
+ enantiomer 13

105. H
COCH3 O H
COCH3 O
is the same as 14

106. Endo product formation10

107. 10.16 The  Molecular Orbitals of Ethylene and 1,3-Butadiene5

108. Orbitals and Chemical Reactions
A deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals (HOMO & LUMO) of the reactants.
Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital (LUMO) of the other. 6

109. Orbitals and Chemical Reactions
HOMO-LUMO interactions can be illustrated by way of the Diels-Alder reaction between ethylene and 1,3-butadiene.
Consider only the  electrons of ethylene and 1,3-butadiene and ignore the framework of  bonds in each molecule. 6

110. Red and blue colors distinguish sign of wave function.
The  MOs of Ethylene
Red and blue colors distinguish sign of wave function.
Bonding  MO is antisymmetric with respect to plane of molecule.
Bonding  orbital of ethylene; two electrons in this orbital. 6

111. Antibonding  orbital of ethylene; no electrons in this orbital.
The  MOs of Ethylene
Antibonding  orbital of ethylene; no electrons in this orbital.
LUMO HOMO
Bonding  orbital of ethylene; two electrons in this orbital. 6

112. Two of these orbitals are bonding; two are antibonding. __
The  MOs of 1,3-Butadiene __
Four p orbitals contribute to the  system of 1,3-butadiene; therefore, there are four  molecular orbitals.
Two of these orbitals are bonding; two are antibonding. __
LUMO
__ HOMO __ 6

113. The Two Bonding  MOs of 1,3-Butadiene
HOMO
4  electrons; 2 in each orbital.
Lowest energy orbital 6

114. The Two Antibonding  MOs of 1,3-Butadiene
Highest energy orbital
LUMO
Both antibonding orbitals are vacant. 6

115. 10.17 A  Molecular Orbital Analysis of the Diels-Alder Reaction5

116. MO Analysis of Diels-Alder Reaction
Since electron-withdrawing groups increase the reactivity of a dienophile, it is proposed that electrons flow from the HOMO of the diene to the LUMO of the dienophile. 6

117. MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene and LUMO of ethylene are in phase with one another.
HOMO of 1,3-butadiene
This allows  bond formation between the alkene and the diene.
LUMO of ethylene (dienophile) 6

118. A “Forbidden" Reaction H2C CH2 heat + H2C CH2
The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? 6

119. HOMO of one ethylene molecule.
A “Forbidden" Reaction
H2C
CH2
heat +
H2C
CH2
HOMO of one ethylene molecule.
HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new bonds.
LUMO of other ethylene molecule. 6

120. End of Chapter 10 Conjugation in Alkadienes and Allylic Systems5