1. Chapter 10 Conjugation in Alkadienes and Allylic Systems
conjugare is a Latin verb meaning "to link or yoke together"
Dr. Wolf's CHM 201 & 202 1

2. The Double Bond as a SubstituentC +
allylic carbocation
Dr. Wolf's CHM 201 & 202 2

3. The Double Bond as a SubstituentC + • C
allylic carbocation
allylic radical
Dr. Wolf's CHM 201 & 202 2

4. The Double Bond as a SubstituentC + • C
allylic carbocation
allylic radical C
conjugated diene
Dr. Wolf's CHM 201 & 202 2

5. The Allyl Group C H
Dr. Wolf's CHM 201 & 202 3

6. Vinylic versus Allylic
carbon
vinylic carbons
Dr. Wolf's CHM 201 & 202 4

7. Vinylic versus AllylicH C C H C H
vinylic hydrogens are attached to vinylic carbons
Dr. Wolf's CHM 201 & 202 4

8. Vinylic versus AllylicH
allylic hydrogens are attached to allylic carbons
Dr. Wolf's CHM 201 & 202 4

9. Vinylic versus AllylicX C C X C X
vinylic substituents are attached to vinylic carbons
Dr. Wolf's CHM 201 & 202 4

10. Vinylic versus AllylicX X C C X C
allylic substituents are attached to allylic carbons
Dr. Wolf's CHM 201 & 202 4

11. Allylic Carbocations C +
Dr. Wolf's CHM 201 & 202 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)
Dr. Wolf's CHM 201 & 202 6

13. Allylic Carbocations
provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations
CH3
CH3
H2C CH C + +
CH3 C
CH3
CH3
formed faster
Dr. Wolf's CHM 201 & 202 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 C +
CH3
CH3
H2C=CH— stabilizes C+ better than CH3—
Dr. Wolf's CHM 201 & 202 6

15. Stabilization of Allylic Carbocations
Delocalization of electrons in the double bond stabilizes the carbocation
resonance model orbital overlap model
Dr. Wolf's CHM 201 & 202 9

16. Resonance Model CH3 H2C CH + C CH3 CH3 H2C CH + C 10
Dr. Wolf's CHM 201 & 202 10

17. Resonance Model CH3 H2C CH + C CH3 CH3 H2C CH + C CH3 d+ d+ H2C CH C
Dr. Wolf's CHM 201 & 202 10

18. Orbital Overlap Model + +
Dr. Wolf's CHM 201 & 202 12

19. Orbital Overlap Model
Dr. Wolf's CHM 201 & 202 12

20. Orbital Overlap Model
Dr. Wolf's CHM 201 & 202 12

21. Orbital Overlap Model
Dr. Wolf's CHM 201 & 202 12

22. SN1 Reactions of Allylic Halides
Dr. Wolf's CHM 201 & 202 5

23. Hydrolysis of an Allylic HalideCl
CH3 C
H2C CH
H2O
Na2CO3
CH3 OH
CH3 C
H2C CH +
HOCH2 CH C
CH3
(15%)
(85%)
Dr. Wolf's CHM 201 & 202 16

24. Corollary Experiment CH3 ClCH2 CH C H2O Na2CO3 CH3 HOCH2 CH C OH CH3 C+
(15%)
(85%)
Dr. Wolf's CHM 201 & 202 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
Dr. Wolf's CHM 201 & 202 16

26. 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 + CH C
H2C CH C
CH3
CH3
Dr. Wolf's CHM 201 & 202 16

27. more positive charge on tertiary carbon; therefore more tertiary alcohol in product+
H2C CH C +
H2C CH C
CH3
CH3
Dr. Wolf's CHM 201 & 202 16

28. (85%) (15%) CH3 CH3 C + H2C CH OH HOCH2 CH C CH3 CH3
more positive charge on tertiary carbon; therefore more tertiary alcohol in product
CH3
CH3 +
H2C + CH C
H2C CH C
CH3
CH3
Dr. Wolf's CHM 201 & 202 16

29. SN2 Reactions of Allylic Halides
Dr. Wolf's CHM 201 & 202 5

30. relative rates: (I-, acetone)
Allylic SN2 Reactions
Allylic halides also undergo SN2 reactions
faster than simple primary alkyl halides. Cl
CH2
H2C CH
H3C
CH2
CH2 Cl 80 1
relative rates: (I-, acetone)
Dr. Wolf's CHM 201 & 202 6

31. 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)
Dr. Wolf's CHM 201 & 202 6

32. relative rates: (I-, acetone)
Allylic SN2 reactions
Two factors:
Electronic
Electron delocalization lowers LUMO energy
which means lower activation energy. Cl
CH2
H2C CH
H3C
CH2
CH2 Cl 80 1
relative rates: (I-, acetone)
Dr. Wolf's CHM 201 & 202 6

33. Allylic Free Radicals C •
Dr. Wolf's CHM 201 & 202 5

34. Allylic free radicals are stabilized by electron delocalization• C •
Dr. Wolf's CHM 201 & 202 22

35. 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
Dr. Wolf's CHM 201 & 202 23

36. Allylic Halogenation
Dr. Wolf's CHM 201 & 202 5

37. Chlorination of Propene
addition
ClCH2CHCH3 Cl
CHCH3
H2C +
Cl2
CHCH2Cl
H2C
500 °C
+ HCl
substitution
Dr. Wolf's CHM 201 & 202 25

38. selective for replacement of allylic hydrogen free radical mechanism
Allylic Halogenation
selective for replacement of allylic hydrogen
free radical mechanism
allylic radical is intermediate
Dr. Wolf's CHM 201 & 202 26

39. 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
Dr. Wolf's CHM 201 & 202 27

40. Hydrogen-atom abstraction stepCl : .. • C C H H C
410 kJ/mol
368 kJ/mol H
Dr. Wolf's CHM 201 & 202 27

41. reagent used (instead of Br2) for allylic bromination
N-Bromosuccinimide
reagent used (instead of Br2) for allylic bromination O
NBr Br O NH
heat + +
CCl4
(82-87%)
Dr. Wolf's CHM 201 & 202 29

42. 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
Dr. Wolf's CHM 201 & 202 30

43. Cyclohexene satisfies both requirements
Example H
Cyclohexene satisfies both requirements
All allylic hydrogens are equivalent
Dr. Wolf's CHM 201 & 202 31

44. Cyclohexene satisfies both requirements
Example H
Cyclohexene satisfies both requirements
All allylic hydrogens are equivalent H • H •
Both resonance forms are equivalent
Dr. Wolf's CHM 201 & 202 31

45. 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.
Dr. Wolf's CHM 201 & 202 31

46. Allylic Anions
Dr. Wolf's CHM 201 & 202 1

47. Allylic anions are stabilized by electron delocalization
CH3
H2C CH - C
CH3
CH3
H2C CH - C
Dr. Wolf's CHM 201 & 202

48. Acidity of Propene H3C CH CH2 H3C CH2 CH3 pKa ~ 43 pKa ~ 62 H2C CH -
Propene is significantly more acidic than propane.
Dr. Wolf's CHM 201 & 202 10

49. Resonance Model - H2C CH - CH2 H2C CH CH2 CH2 H2C CH -
Charge is delocalized to both terminal carbons,
stabilizing the conjugate base.
Dr. Wolf's CHM 201 & 202 10

50. Classes of Dienes
Dr. Wolf's CHM 201 & 202 1

51. Classification of Dienes
isolated diene
conjugated diene
cumulated diene C
Dr. Wolf's CHM 201 & 202 2

52. Nomenclature (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene
Dr. Wolf's CHM 201 & 202 2

53. Relative Stabilities of Dienes
Dr. Wolf's CHM 201 & 202 3

54. Heats of Hydrogenation
1,3-pentadiene is 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
Dr. Wolf's CHM 201 & 202 4

55. Heats of Hydrogenation
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
Dr. Wolf's CHM 201 & 202 5

56. Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
Dr. Wolf's CHM 201 & 202 5

57. Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated
Dr. Wolf's CHM 201 & 202 5

58. Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
this extra 15 kJ/mol is known by several terms stabilization energy delocalization energy resonance energy
Dr. Wolf's CHM 201 & 202 5

59. Heats of Hydrogenation
Cumulated double bonds have relatively high heats of hydrogenation
H2C C +
CH2
2H2
CH3CH2CH3
DH° = -295 kJ +
H2C
CH2CH3 H2
CH3CH2CH3
DH° = -125 kJ
Dr. Wolf's CHM 201 & 202 7

60. Bonding in Conjugated Dienes
Dr. Wolf's CHM 201 & 202 8

61. Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene 10
Dr. Wolf's CHM 201 & 202 10

62. p bonds are independent of each other
Isolated diene
p bonds are independent of each other
1,3-pentadiene
Conjugated diene
Dr. Wolf's CHM 201 & 202 10

63. p bonds are independent of each other
Isolated diene
p bonds are independent of each other
p orbitals overlap to give extended p bond encompassing four carbons
Conjugated diene
Dr. Wolf's CHM 201 & 202 10

64. less electron delocalization; less stable
Isolated diene
less electron delocalization; less stable
more electron delocalization; more stable
Conjugated diene
Dr. Wolf's CHM 201 & 202 10

65. Conformations of DienesH 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)
Dr. Wolf's CHM 201 & 202 11

66. Conformations of DienesH 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)
Dr. Wolf's CHM 201 & 202 11

67. Conformations of Dienes
s-trans
s-cis
Both conformations allow electron delocalization via overlap of p orbitals to give extended p system
Dr. Wolf's CHM 201 & 202 11

68. s-trans is more stable than s-cis
Interconversion of conformations requires two p bonds to be at right angles to each other and prevents conjugation
12 kJ/mol
Dr. Wolf's CHM 201 & 202 12

69. Dr. Wolf's CHM 201 & 202 12

70. 16 kJ/mol
12 kJ/mol
Dr. Wolf's CHM 201 & 202 15

71. Bonding in Allenes
Dr. Wolf's CHM 201 & 202 16

72. cumulated dienes are less stable than isolated and conjugated dienes
(see Problem 10.7 on p 375)
Dr. Wolf's CHM 201 & 202 17

73. linear arrangement of carbons
Structure of Allene
118.4°
131 pm
linear arrangement of carbons
nonplanar geometry
Dr. Wolf's CHM 201 & 202 19

74. linear arrangement of carbons
Structure of Allene
118.4°
131 pm
linear arrangement of carbons
nonplanar geometry
Dr. Wolf's CHM 201 & 202 19

75. Bonding in Allene
sp 2 sp
sp 2
Dr. Wolf's CHM 201 & 202 19

76. Bonding in Allene
Dr. Wolf's CHM 201 & 202 19

77. Bonding in Allene
Dr. Wolf's CHM 201 & 202 19

78. Bonding in Allene
Dr. Wolf's CHM 201 & 202 19

79. Allenes of the type shown are chiral
Chiral Allenes
Allenes of the type shown are chiral A X C B Y
A ¹ B; X ¹ Y
Have a stereogenic axis
Dr. Wolf's CHM 201 & 202 25

80. analogous to difference between:
Stereogenic 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
Dr. Wolf's CHM 201 & 202 26

81. Preparation of Dienes
Dr. Wolf's CHM 201 & 202 1

82. used to prepare synthetic rubber (See "Diene Polymers" box)
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)
Dr. Wolf's CHM 201 & 202 2

83. Dehydration of Alcohols
KHSO4 OH
heat
Dr. Wolf's CHM 201 & 202 3

84. Dehydration of Alcohols
KHSO4 OH
heat
major product; 88% yield
Dr. Wolf's CHM 201 & 202 3

85. Dehydrohalogenation of Alkyl Halides
KOH Br
heat
Dr. Wolf's CHM 201 & 202 3

86. Dehydrohalogenation of Alkyl Halides
KOH Br
heat
major product; 78% yield
Dr. Wolf's CHM 201 & 202 3

87. isolated dienes: double bonds react independently of one another
Reactions of Dienes
isolated dienes: double bonds react independently of one another
cumulated dienes: specialized topic
conjugated dienes: reactivity pattern requires us to think of conjugated diene system as a functional group of its own
Dr. Wolf's CHM 201 & 202 6

88. Addition of Hydrogen Halides to Conjugated Dienes
Dr. Wolf's CHM 201 & 202 7

89. Electrophilic Addition to Conjugated Dienes+ H X H
Proton adds to end of diene system
Carbocation formed is allylic
Dr. Wolf's CHM 201 & 202 8

90. Example: H
HCl Cl H H Cl ? ?
Dr. Wolf's CHM 201 & 202 9

91. Example: H
HCl Cl H
Dr. Wolf's CHM 201 & 202 9

92. via: H + H H X H +
Dr. Wolf's CHM 201 & 202 10

93. and: H + Cl H Cl– 3-Chlorocyclopentene H + H H Cl H H H H H 10
Dr. Wolf's CHM 201 & 202 10

94. 1,2-Addition versus 1,4-Addition
1,2-addition of XY X Y
Dr. Wolf's CHM 201 & 202 12

95. 1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY X Y X Y
Dr. Wolf's CHM 201 & 202 12

96. 1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY X Y X Y
via X +
Dr. Wolf's CHM 201 & 202 12

97. 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
Dr. Wolf's CHM 201 & 202 13

98. 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
Dr. Wolf's CHM 201 & 202 13

99. 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
Dr. Wolf's CHM 201 & 202 13

100. 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
Dr. Wolf's CHM 201 & 202 13

101. 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)
Dr. Wolf's CHM 201 & 202 13

102. 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
Dr. Wolf's CHM 201 & 202 16

103. CH2
CH3CHCH
CHCH2
CH3CH +
HBr
H2C
CHCH
CH2
Dr. Wolf's CHM 201 & 202 17

104. higher activation energy CH3CHCH CH2+
higher activation energy
CH3CHCH
CH2 +
CH3CH
CHCH2
formed more slowly Br
CH2
CH3CHCH
CHCH2Br
CH3CH
Dr. Wolf's CHM 201 & 202 17

105. 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 ?
Dr. Wolf's CHM 201 & 202 19

106. + 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
Dr. Wolf's CHM 201 & 202 19

107. Think mechanisticallyCl + +
one resonance form is tertiary carbocation; other is primary
Dr. Wolf's CHM 201 & 202 20

108. Think mechanisticallyCl Cl H + + + +
one resonance form is tertiary carbocation; other is primary
one resonance form is secondary carbocation; other is primary
Dr. Wolf's CHM 201 & 202 20

109. Think mechanisticallyCl
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
Dr. Wolf's CHM 201 & 202 20

110. Think mechanisticallyCl
Cl– Cl + +
one resonance form is tertiary carbocation; other is primary
major product
Dr. Wolf's CHM 201 & 202 20

111. Halogen Addition to Dienes
gives mixtures of 1,2 and 1,4-addition products
Dr. Wolf's CHM 201 & 202 23

112. Example H2C CHCH CH2 Br2 Br CH2 BrCH2CHCH + CHCH2Br BrCH2CH (37%)
(63%)
Dr. Wolf's CHM 201 & 202 13

113. The Diels-Alder Reaction
Synthetic method for preparing compounds containing a cyclohexene ring
Dr. Wolf's CHM 201 & 202 1

114. In general... + conjugated diene alkene (dienophile) cyclohexene 2
Dr. Wolf's CHM 201 & 202 2

115. via
transition state
Dr. Wolf's CHM 201 & 202 3

116. Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202

117. a concerted reaction that proceeds through a cyclic transition state
Mechanistic features
concerted mechanism
cycloaddition
pericyclic reaction
a concerted reaction that proceeds through a cyclic transition state
Dr. Wolf's CHM 201 & 202 4

118. Recall the general reaction...+
conjugated diene
alkene (dienophile)
cyclohexene
The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile.
Dr. Wolf's CHM 201 & 202 2

119. What makes a reactive dienophile?
The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond.
Typical EWGs C
EWG C O C N
Dr. Wolf's CHM 201 & 202 5

120. Example CH O H2C CHCH CH2 + H2C CH benzene 100°C CH O (100%) 6
Dr. Wolf's CHM 201 & 202 6

121. Example CH O H2C CHCH CH2 + H2C CH benzene 100°C CH O via: CH O (100%)
Dr. Wolf's CHM 201 & 202 6

122. Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202

123. Example O H2C CHC CH2 CH3 + benzene 100°C H3C O (100%) 6
Dr. Wolf's CHM 201 & 202 6

124. Example O H2C CHC CH2 CH3 + benzene 100°C via: H3C O H3C O (100%) 6
Dr. Wolf's CHM 201 & 202 6

125. Acetylenic Dienophile
CCOCH2CH3
CH3CH2OCC
H2C
CHCH
CH2 +
benzene
100°C
COCH2CH3 O
(98%)
Dr. Wolf's CHM 201 & 202 6

126. Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202

127. Diels-Alder Reaction is Stereospecific*
syn addition to alkene
cis-trans relationship of substituents on alkene retained in cyclohexene product
*A stereospecific reaction is one in which stereoisomeric starting materials give stereoisomeric products; characterized by terms like syn addition, anti elimination, inversion of configuration, etc.
Dr. Wolf's CHM 201 & 202 9

128. Example O C6H5 COH C + H2C CHCH CH2 H H H C6H5 COH O only product 10
Dr. Wolf's CHM 201 & 202 10

129. Example C C6H5 COH H O + H2C CHCH CH2 H C6H5 COH O only product 10
Dr. Wolf's CHM 201 & 202 10

130. Cyclic dienes yield bridged bicyclic Diels-Alder adducts.
Dr. Wolf's CHM 201 & 202 12

131. Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202
Dr. Wolf's CHM 201 & 202

132. Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202

133. C
COCH3 H O
CH3OC + H
COCH3 O
Dr. Wolf's CHM 201 & 202 13

134. H
COCH3 O H
COCH3 O
is the same as
Dr. Wolf's CHM 201 & 202 14

135. The p Molecular Orbitals of Ethylene and 1,3-Butadiene
Dr. Wolf's CHM 201 & 202 1

136. Orbitals and Chemical Reactions
A deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals 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.
Dr. Wolf's CHM 201 & 202 6

137. Orbitals and Chemical Reactions
We can illustrate HOMO-LUMO interactions by way of the Diels-Alder reaction between ethylene and 1,3-butadiene.
We need only consider only the p electrons of ethylene and 1,3-butadiene. We can ignore the framework of s bonds in each molecule.
Dr. Wolf's CHM 201 & 202 6

138. red and blue colors distinguish sign of wave function
The p MOs of Ethylene
red and blue colors distinguish sign of wave function
bonding p MO is antisymmetric with respect to plane of molecule
Bonding p orbital of ethylene; two electrons in this orbital
Dr. Wolf's CHM 201 & 202 6

139. LUMO HOMO The p MOs of Ethylene
Antibonding p orbital of ethylene; no electrons in this orbital
LUMO HOMO
Bonding p orbital of ethylene; two electrons in this orbital
Dr. Wolf's CHM 201 & 202 6

140. Two of these orbitals are bonding; two are antibonding.
The p MOs of 1,3-Butadiene
Four p orbitals contribute to the p system of 1,3-butadiene; therefore, there are four p molecular orbitals.
Two of these orbitals are bonding; two are antibonding.
Dr. Wolf's CHM 201 & 202 6

141. The Two Bonding p MOs of 1,3-Butadiene
HOMO
4 p electrons; 2 in each orbital
Lowest energy orbital
Dr. Wolf's CHM 201 & 202 6

142. The Two Antibonding p MOs of 1,3-Butadiene
Highest energy orbital
LUMO
Both antibonding orbitals are vacant
Dr. Wolf's CHM 201 & 202 6

143. A p Molecular Orbital Analysis of the Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202 1

144. MO Analysis of Diels-Alder Reaction
Inasmuch as electron-withdrawing groups increase the reactivity of a dienophile, we assume electrons flow from the HOMO of the diene to the LUMO of the dienophile.
Dr. Wolf's CHM 201 & 202 6

145. MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
HOMO of 1,3-butadiene and LUMO of ethylene are in phase with one another
allows s bond formation between the alkene and the diene
LUMO of ethylene (dienophile)
Dr. Wolf's CHM 201 & 202 6

146. MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
LUMO of ethylene (dienophile)
Dr. Wolf's CHM 201 & 202 6

147. A "forbidden" reaction H2C CH2 + H2C CH2
The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not?
Dr. Wolf's CHM 201 & 202 6

148. HOMO of one ethylene molecule
A "forbidden" reaction
H2C
CH2 +
HOMO of one ethylene molecule
HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new s bonds
LUMO of other ethylene molecule
Dr. Wolf's CHM 201 & 202 6

149. End of Chapter 10
Dr. Wolf's CHM 201 & 202 6