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

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

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

2 10-2 Dr. Wolf's CHM 201 & 202 allylic carbocation The Double Bond as a Substituent C + C C

3 10-3 Dr. Wolf's CHM 201 & 202 allylic carbocation allylic radical The Double Bond as a Substituent C + C C C C C

4 10-4 Dr. Wolf's CHM 201 & 202 allylic carbocation allylic radical conjugated diene The Double Bond as a Substituent C + C C C C C C C C C

5 10-5 Dr. Wolf's CHM 201 & 202 C C C HHH H H The Allyl Group

6 10-6 Dr. Wolf's CHM 201 & 202 vinylic carbons allyliccarbon Vinylic versus Allylic C C C

7 10-7 Dr. Wolf's CHM 201 & 202 vinylic hydrogens are attached to vinylic carbons Vinylic versus Allylic C C C H H H

8 10-8 Dr. Wolf's CHM 201 & 202 allylic hydrogens are attached to allylic carbons Vinylic versus Allylic C C CHH H

9 10-9 Dr. Wolf's CHM 201 & 202 Vinylic versus Allylic C C C X X X vinylic substituents are attached to vinylic carbons

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

11 10-11 Dr. Wolf's CHM 201 & 202 Allylic Carbocations C C C +

12 10-12 Dr. Wolf's CHM 201 & 202 the fact that a tertiary allylic halide undergoes solvolysis (S N 1) faster than a simple tertiary alkyl halide Cl CH 3 relative rates: (ethanolysis, 45°C) 1231 Allylic Carbocations C Cl CH 3 C H2CH2CH2CH2C CH

13 10-13 Dr. Wolf's CHM 201 & 202 provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations CH 3 formed faster Allylic Carbocations CC H2CH2CH2CH2C CH ++ CH 3

14 10-14 Dr. Wolf's CHM 201 & 202 provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations CH 3 H 2 C=CH— stabilizes C+ better than CH 3 — Allylic Carbocations CC CH 3 H2CH2CH2CH2C CH + +

15 10-15 Dr. Wolf's CHM 201 & 202 Delocalization of electrons in the double bond stabilizes the carbocation resonance model orbital overlap model Stabilization of Allylic Carbocations

16 10-16 Dr. Wolf's CHM 201 & 202 Resonance Model CH 3 H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH + C

17 10-17 Dr. Wolf's CHM 201 & 202 Resonance Model CH 3 H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH ++++ C ++++

18 10-18 Dr. Wolf's CHM 201 & 202 Orbital Overlap Model ++++ ++++

19 10-19 Dr. Wolf's CHM 201 & 202 Orbital Overlap Model

20 10-20 Dr. Wolf's CHM 201 & 202 Orbital Overlap Model

21 10-21 Dr. Wolf's CHM 201 & 202 Orbital Overlap Model

22 10-22 Dr. Wolf's CHM 201 & 202 S N 1 Reactions of Allylic Halides

23 10-23 Dr. Wolf's CHM 201 & 202 H2OH2OH2OH2O Na 2 CO 3 (85%) (15%) Hydrolysis of an Allylic Halide Cl CH 3 C H2CH2CH2CH2C CH OH C H2CH2CH2CH2C CH + HOCH 2 CH C CH 3

24 10-24 Dr. Wolf's CHM 201 & 202 H2OH2OH2OH2O Na 2 CO 3 (85%) (15%) Corollary Experiment OH CH 3 C H2CH2CH2CH2C CH + HOCH 2 CH C CH 3 ClCH 2 CH C CH 3

25 10-25 Dr. Wolf's CHM 201 & 202 CH 3 ClCH 2 CH C CH 3 Cl C H2CH2CH2CH2C CH and give the same products because they form the same carbocation

26 10-26 Dr. Wolf's CHM 201 & 202 CH 3 ClCH 2 CH C CH 3 Cl C H2CH2CH2CH2C CH and give the same products because they form the same carbocation CH 3 H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH + C

27 10-27 Dr. Wolf's CHM 201 & 202 more positive charge on tertiary carbon; therefore more tertiary alcohol in product CH 3 H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH + C

28 10-28 Dr. Wolf's CHM 201 & 202 CH 3 HOCH 2 CH C CH 3 OH C H2CH2CH2CH2CCH more positive charge on tertiary carbon; therefore more tertiary alcohol in product CH 3 H2CH2CH2CH2C CH + C H2CH2CH2CH2C CH + C + (85%)(15%)

29 10-29 Dr. Wolf's CHM 201 & 202 S N 2 Reactions of Allylic Halides

30 10-30 Dr. Wolf's CHM 201 & 202 Allylic halides also undergo S N 2 reactions faster than simple primary alkyl halides. relative rates: (I -, acetone) 80 1 Allylic S N 2 ReactionsCl CH 2 H2CH2CH2CH2C CH Cl H3CH3CH3CH3C

31 10-31 Dr. Wolf's CHM 201 & 202 Two factors: Steric Trigonal carbon smaller than tetrahedral carbon. relative rates: (I -, acetone) 80 1 Allylic S N 2 ReactionsCl CH 2 H2CH2CH2CH2C CH Cl H3CH3CH3CH3C

32 10-32 Dr. Wolf's CHM 201 & 202 Two factors: Electronic Electron delocalization lowers LUMO energy which means lower activation energy. relative rates: (I -, acetone) 80 1 Allylic S N 2 reactions Cl CH 2 H3CH3CH3CH3C Cl H2CH2CH2CH2C CH

33 10-33 Dr. Wolf's CHM 201 & 202 Allylic Free Radicals C C C

34 10-34 Dr. Wolf's CHM 201 & 202 Allylic free radicals are stabilized by electron delocalization C C C C C C

35 10-35 Dr. Wolf's CHM 201 & 202 Free-radical stabilities are related to bond-dissociation energies CH 3 CH 2 CH 2 —H 410 kJ/mol CH 3 CH 2 CH 2 + H 368 kJ/mol + H CHCH 2 —H H2CH2CH2CH2C CHCH 2 H2CH2CH2CH2C C—H bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane

36 10-36 Dr. Wolf's CHM 201 & 202 Allylic Halogenation

37 10-37 Dr. Wolf's CHM 201 & 202 ClCH 2 CHCH 3 Cl addition 500 °C substitution CHCH 3 H2CH2CH2CH2C + Cl 2 CHCH 2 Cl H2CH2CH2CH2C + HCl Chlorination of Propene

38 10-38 Dr. Wolf's CHM 201 & 202 selective for replacement of allylic hydrogen free radical mechanism allylic radical is intermediate Allylic Halogenation

39 10-39 Dr. Wolf's CHM 201 & kJ/mol Hydrogen-atom abstraction step C C CHH H H H H 368 kJ/mol Cl :..... allylic C—H bond weaker than vinylic chlorine atom abstracts allylic H in propagation step

40 10-40 Dr. Wolf's CHM 201 & kJ/mol Hydrogen-atom abstraction step C C C H H H H H 368 kJ/mol H Cl : :....

41 10-41 Dr. Wolf's CHM 201 & 202 Br reagent used (instead of Br 2 ) for allylic bromination + heat CCl 4 (82-87%) +OO NBr OO NH N-BromosuccinimideN-Bromosuccinimide

42 10-42 Dr. Wolf's CHM 201 & 202 all of the allylic hydrogens are equivalent and the resonance forms of allylic radical are equivalent Limited Scope Allylic halogenation is only used when:

43 10-43 Dr. Wolf's CHM 201 & 202 ExampleExample Cyclohexene satisfies both requirements HHHH All allylic hydrogens are equivalent

44 10-44 Dr. Wolf's CHM 201 & 202 ExampleExample Cyclohexene satisfies both requirements HHHH All allylic hydrogens are equivalent HH H HH H Both resonance forms are equivalent

45 10-45 Dr. Wolf's CHM 201 & 202 ExampleExample 2-Butene All allylic hydrogens are equivalent Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides. CH 3 CH CHCH 3 But CH 3 CH CH CH 2 CH 3 CH CH CH 2

46 10-46 Dr. Wolf's CHM 201 & 202 Allylic Anions

47 10-47 Dr. Wolf's CHM 201 & 202 CH 3 H2CH2CH2CH2C CH - C H2CH2CH2CH2C CH - C Allylic anions are stabilized by electron delocalization

48 10-48 Dr. Wolf's CHM 201 & 202 Acidity of Propene H2CH2CH2CH2C CH- CH 2 Propene is significantly more acidic than propane. H3CH3CH3CH3C CH CH 2 H3CH3CH3CH3C CH 3 H2CH2CH2CH2C CH 2 - CH 3 pKa ~ 43 pKa ~ 62

49 10-49 Dr. Wolf's CHM 201 & 202 Resonance Model H2CH2CH2CH2C CH- CH 2 H2CH2CH2CH2C CH - H2CH2CH2CH2C CH ---- ---- Charge is delocalized to both terminal carbons, stabilizing the conjugate base.

50 10-50 Dr. Wolf's CHM 201 & 202 Classes of Dienes

51 10-51 Dr. Wolf's CHM 201 & 202 isolated diene conjugated diene cumulated diene C Classification of Dienes

52 10-52 Dr. Wolf's CHM 201 & 202 (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene 3,4-heptadiene C NomenclatureNomenclature

53 10-53 Dr. Wolf's CHM 201 & 202 Relative Stabilities of Dienes

54 10-54 Dr. Wolf's CHM 201 & kJ/mol 226 kJ/mol 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 Heats of Hydrogenation

55 10-55 Dr. Wolf's CHM 201 & kJ/mol 226 kJ/mol 126 kJ/mol 115 kJ/mol Heats of Hydrogenation

56 10-56 Dr. Wolf's CHM 201 & kJ/mol 226 kJ/mol 126 kJ/mol 115 kJ/mol 126 kJ/mol 111 kJ/mol Heats of Hydrogenation

57 10-57 Dr. Wolf's CHM 201 & kJ/mol 111 kJ/mol Heats of Hydrogenation when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated

58 10-58 Dr. Wolf's CHM 201 & kJ/mol 111 kJ/mol Heats of Hydrogenation this extra 15 kJ/mol is known by several terms stabilization energy delocalization energy resonance energy

59 10-59 Dr. Wolf's CHM 201 & 202 Cumulated double bonds have relatively high heats of hydrogenation  H° = -295 kJ Heats of Hydrogenation H2CH2CH2CH2C CH 2 C + 2H 2 CH 3 CH 2 CH 3  H° = -125 kJ H2CH2CH2CH2C CH 2 CH 3 + H2H2H2H2 CH 3 CH 2 CH 3

60 10-60 Dr. Wolf's CHM 201 & 202 Bonding in Conjugated Dienes

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

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

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

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

65 10-65 Dr. Wolf's CHM 201 & 202 s-trans s-cis Conformations of Dienes 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) H HH HHHHH HHHH

66 10-66 Dr. Wolf's CHM 201 & 202 s-trans s-cis Conformations of Dienes 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) H HH HHHHH HHHH

67 10-67 Dr. Wolf's CHM 201 & 202 s-trans s-cis Conformations of Dienes Both conformations allow electron delocalization via overlap of p orbitals to give extended  system

68 10-68 Dr. Wolf's CHM 201 & 202 s-trans is more stable than s-cis 12 kJ/mol Interconversion of conformations requires two  bonds to be at right angles to each other and prevents conjugation

69 10-69 Dr. Wolf's CHM 201 & 202

70 10-70 Dr. Wolf's CHM 201 & kJ/mol 12 kJ/mol

71 10-71 Dr. Wolf's CHM 201 & 202 Bonding in Allenes

72 10-72 Dr. Wolf's CHM 201 & 202 cumulated dienes are less stable than isolated and conjugated dienes (see Problem 10.7 on p 375) Cumulated Dienes CC C

73 10-73 Dr. Wolf's CHM 201 & pm Structure of Allene 118.4° linear arrangement of carbons nonplanar geometry

74 10-74 Dr. Wolf's CHM 201 & 202 Structure of Allene 131 pm 118.4° linear arrangement of carbons nonplanar geometry

75 10-75 Dr. Wolf's CHM 201 & 202 sp 2 sp Bonding in Allene sp 2

76 10-76 Dr. Wolf's CHM 201 & 202 Bonding in Allene

77 10-77 Dr. Wolf's CHM 201 & 202 Bonding in Allene

78 10-78 Dr. Wolf's CHM 201 & 202 Bonding in Allene

79 10-79 Dr. Wolf's CHM 201 & 202 Allenes of the type shown are chiral A B X Y A  B; X  Y Have a stereogenic axis Chiral Allenes CC C

80 10-80 Dr. Wolf's CHM 201 & 202 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 Stereogenic Axis

81 10-81 Dr. Wolf's CHM 201 & 202 Preparation of Dienes

82 10-82 Dr. Wolf's CHM 201 & 202 CH 3 CH 2 CH 2 CH °C chromia-alumina 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) 1,3-Butadiene1,3-Butadiene H2CH2CH2CH2C CHCH CH 2 + 2H 2

83 10-83 Dr. Wolf's CHM 201 & 202 KHSO 4 heat Dehydration of Alcohols OH

84 10-84 Dr. Wolf's CHM 201 & 202 KHSO 4 heat Dehydration of Alcohols OH major product; 88% yield

85 10-85 Dr. Wolf's CHM 201 & 202 KOH heat Dehydrohalogenation of Alkyl Halides Br

86 10-86 Dr. Wolf's CHM 201 & 202 KOH heat Br major product; 78% yield Dehydrohalogenation of Alkyl Halides

87 10-87 Dr. Wolf's CHM 201 & 202 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 Reactions of Dienes

88 10-88 Dr. Wolf's CHM 201 & 202 Addition of Hydrogen Halides to Conjugated Dienes

89 10-89 Dr. Wolf's CHM 201 & 202 Proton adds to end of diene system Carbocation formed is allylic Electrophilic Addition to Conjugated Dienes H X H +

90 10-90 Dr. Wolf's CHM 201 & 202 HCl Example:Example:HH HHH H ClHH HHH H H H HHHHCl H H ??

91 10-91 Dr. Wolf's CHM 201 & 202 HCl Example:Example:HH HHH H ClHH HHH H H

92 10-92 Dr. Wolf's CHM 201 & 202 via:HH HHH H H + HH HHH H H XHH HHH H H +

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

94 10-94 Dr. Wolf's CHM 201 & 202 1,2-Addition versus 1,4-Addition 1,2-addition of XY XY

95 10-95 Dr. Wolf's CHM 201 & 202 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY XY XY

96 10-96 Dr. Wolf's CHM 201 & 202 via 1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY XY XY X +

97 10-97 Dr. Wolf's CHM 201 & 202 electrophilic addition 1,2 and 1,4-addition both observed product ratio depends on temperature HBr Addition to 1,3-Butadiene H2CH2CH2CH2C CHCH CH 2 HBr Br CH 3 CHCH CHCH 2 Br CH 3 CH +

98 10-98 Dr. Wolf's CHM 201 & 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. RationaleRationale Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH + CH 2 CH 3 CHCH CHCH 2 CH 3 CH via: ++

99 10-99 Dr. Wolf's CHM 201 & 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. RationaleRationale Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH + formed faster

100 Dr. Wolf's CHM 201 & 202 more stable RationaleRationale Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH + 1-Bromo-2-butene is more stable than 3-bromo-1-butene because it has a more highly substituted double bond.

101 Dr. Wolf's CHM 201 & 202 major product at -80°C RationaleRationale major product at 25°C The two products equilibrate at 25°C. Once equilibrium is established, the more stable isomer predominates. Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH (formed faster) (more stable)

102 Dr. Wolf's CHM 201 & 202 Kinetic Control versus Thermodynamic Control Kinetic control: major product is the one formed at the fastest rateKinetic control: major product is the one formed at the fastest rate Thermodynamic control: major product is the one that is the most stableThermodynamic control: major product is the one that is the most stable

103 Dr. Wolf's CHM 201 & 202 H2CH2C CHCH CH 2 HBr CH 2 CH 3 CHCH CHCH 2 CH 3 CH ++

104 Dr. Wolf's CHM 201 & 202 CH 2 CH 3 CHCH + CHCH 2 CH 3 CH + Br CH 2 CH 3 CHCH CHCH 2 Br CH 3 CH higher activation energy formed more slowly

105 Dr. Wolf's CHM 201 & 202 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?

106 Dr. Wolf's CHM 201 & 202 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

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

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

109 Dr. Wolf's CHM 201 & 202 Think mechanistically H Cl + + one resonance form is tertiary carbocation; other is primary More stable carbocation Is attacked by chloride ion at carbon that bears greater share of positive charge

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

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

112 Dr. Wolf's CHM 201 & 202 ExampleExample H2CH2CH2CH2C CHCH CH 2 Br 2 Br CH 2 BrCH 2 CHCH CHCH 2 Br BrCH 2 CH + (37%) (63%)

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

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

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

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

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

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

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

120 Dr. Wolf's CHM 201 & benzene 100°C (100%) H2CH2CH2CH2C CHCH CH 2 H2CH2CH2CH2C CH CHO CHOExampleExample

121 Dr. Wolf's CHM 201 & benzene 100°C (100%) H2CH2CH2CH2C CHCH CH 2 H2CH2CH2CH2C CH CHO CHOExampleExample CHOvia:

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

123 Dr. Wolf's CHM 201 & benzene 100°C (100%)OO O ExampleExample H2CH2CH2CH2C CHC CH 2 CH 3 H3CH3CH3CH3COO O

124 Dr. Wolf's CHM 201 & benzene 100°C (100%)OO O ExampleExample H2CH2CH2CH2C CHC CH 2 CH 3 H3CH3CH3CH3COO O via: H3CH3CH3CH3C O O O

125 Dr. Wolf's CHM 201 & benzene 100°C (98%) H2CH2CH2CH2C CHCH CH 2 Acetylenic Dienophile O CCOCH 2 CH 3 CH 3 CH 2 OCC O COCH 2 CH 3 OO

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

127 Dr. Wolf's CHM 201 & 202 syn addition to alkene cis-trans relationship of substituents on alkene retained in cyclohexene product Diels-Alder Reaction is Stereospecific* *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.

128 Dr. Wolf's CHM 201 & 202 only product + H2CH2CH2CH2C CHCH CH 2 ExampleExample C C C6H5C6H5C6H5C6H5 COH HH OH C6H5C6H5C6H5C6H5 H COH O

129 Dr. Wolf's CHM 201 & 202 only product + H2CH2CH2CH2C CHCH CH 2 ExampleExample C C C6H5C6H5C6H5C6H5 COH H HO H C6H5C6H5C6H5C6H5H COH O

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

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

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

133 Dr. Wolf's CHM 201 & C C COCH 3 H HO CH 3 OC O H H COCH 3 O O

134 Dr. Wolf's CHM 201 & 202 is the same as H H COCH 3 O O H H O O

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

136 Dr. Wolf's CHM 201 & 202 Orbitals and Chemical Reactions A deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals of the reactants.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.Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital (LUMO) of the other.

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

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

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

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

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

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

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

144 Dr. Wolf's CHM 201 & 202 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.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.

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

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

147 Dr. Wolf's CHM 201 & 202 A "forbidden" reaction The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not?The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? H2CH2CH2CH2C CH 2 H2CH2CH2CH2C +

148 Dr. Wolf's CHM 201 & 202 A "forbidden" reaction H2CH2CH2CH2C CH 2 H2CH2CH2CH2C + HOMO of one ethylene molecule LUMO of other ethylene molecule HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new  bonds

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


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