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11-1 Dr. Wolf's CHM 201 & 202 Chapter 11 Arenes and Aromaticity.

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Presentation on theme: "11-1 Dr. Wolf's CHM 201 & 202 Chapter 11 Arenes and Aromaticity."— Presentation transcript:

1 11-1 Dr. Wolf's CHM 201 & 202 Chapter 11 Arenes and Aromaticity

2 11-2 Dr. Wolf's CHM 201 & 202 BenzeneToluene Naphthalene Examples of Aromatic Hydrocarbons HHH HH H CH 3 H H HH H HH H HH HHH

3 11-3 Dr. Wolf's CHM 201 & 202 Benzene

4 11-4 Dr. Wolf's CHM 201 & 202 Some history 1834 Eilhardt Mitscherlich isolates a new hydrocarbon and determines its empirical formula to be C n H n. Compound comes to be called benzene August W. von Hofmann isolates benzene from coal tar August Kekulé proposes structure of benzene.

5 11-5 Dr. Wolf's CHM 201 & 202 Kekulé and the Structure of Benzene

6 11-6 Dr. Wolf's CHM 201 & 202 Kekulé proposed a cyclic structure for C 6 H 6 with alternating single and double bonds. Kekulé Formulation of Benzene HHH HH H

7 11-7 Dr. Wolf's CHM 201 & 202 Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. Kekulé Formulation of Benzene HHH HH H HHH HH H

8 11-8 Dr. Wolf's CHM 201 & 202 However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. Kekulé Formulation of Benzene HXX HH H HXX HH H

9 11-9 Dr. Wolf's CHM 201 & 202 Structural studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same length. Structure of Benzene Benzene has the shape of a regular hexagon.

10 11-10 Dr. Wolf's CHM 201 & pm All C—C bond distances = 140 pm

11 11-11 Dr. Wolf's CHM 201 & pm 146 pm 134 pm All C—C bond distances = 140 pm 140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene.

12 11-12 Dr. Wolf's CHM 201 & 202 A Resonance Picture of Bonding in Benzene

13 11-13 Dr. Wolf's CHM 201 & 202 Instead of Kekulé's suggestion of a rapid equilibrium between two structures: HHH HH H HHH HH H Kekulé Formulation of Benzene

14 11-14 Dr. Wolf's CHM 201 & 202 express the structure of benzene as a resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double bonds, but are delocalized over all six ring carbons. Resonance Formulation of Benzene HHH HH H HHH HH H

15 11-15 Dr. Wolf's CHM 201 & 202 Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures) Resonance Formulation of Benzene

16 11-16 Dr. Wolf's CHM 201 & 202 The Stability of Benzene benzene is the best and most familiar example of a substance that possesses "special stability" or "aromaticity" aromaticity is a level of stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it

17 11-17 Dr. Wolf's CHM 201 & 202 heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene"  H°= – 208 kJ Thermochemical Measures of Stability + 3H 2 Pt

18 11-18 Dr. Wolf's CHM 201 & kJ/mol 231 kJ/mol 208 kJ/mol 360 kJ/mol 3 x cyclohexene Figure 11.2 (p 404)

19 11-19 Dr. Wolf's CHM 201 & kJ/mol 360 kJ/mol 3 x cyclohexene Figure 11.2 (p 404) "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene

20 11-20 Dr. Wolf's CHM 201 & kJ/mol 360 kJ/mol 3 x cyclohexene Figure 11.2 (p 404) observed heat of hydrogenation is 152 kJ/mol less than "expected" benzene is 152 kJ/mol more stable than expected 152 kJ/mol is the resonance energy of benzene

21 11-21 Dr. Wolf's CHM 201 & 202 hydrogenation of 1,3- cyclohexadiene (2H 2 ) gives off more heat than hydrogenation of benzene (3H 2 )! 231 kJ/mol 208 kJ/mol Figure 11.2 (p 404)

22 11-22 Dr. Wolf's CHM 201 & 202 heat of hydrogenation = 208 kJ/mol heat of hydrogenation = 337 kJ/mol 3H 2 Pt Pt Cyclic conjugation versus noncyclic conjugation

23 11-23 Dr. Wolf's CHM 201 & 202 compared to localized 1,3,5-cyclohexatriene 152 kJ/mol compared to 1,3,5-hexatriene 129 kJ/mol exact value of resonance energy of benzene depends on what it is compared to, but regardless of model, benzene is more stable than expected by a substantial amount Resonance Energy of Benzene

24 11-24 Dr. Wolf's CHM 201 & 202 An Orbital Hybridization View of Bonding in Benzene

25 11-25 Dr. Wolf's CHM 201 & 202 Orbital Hybridization Model of Bonding in Benzene Figure 11.3 Planar ring of 6 sp 2 hybridized carbons

26 11-26 Dr. Wolf's CHM 201 & 202 Orbital Hybridization Model of Bonding in Benzene Figure 11.3 Each carbon contributes a p orbital Six p orbitals overlap to give cyclic  system; six  electrons delocalized throughout  system

27 11-27 Dr. Wolf's CHM 201 & 202 Orbital Hybridization Model of Bonding in Benzene Figure 11.3 High electron density above and below plane of ring

28 11-28 Dr. Wolf's CHM 201 & 202 The  Molecular Orbitals of Benzene

29 11-29 Dr. Wolf's CHM 201 & 202 Energy Bonding orbitals Antibonding orbitals Benzene MOs 6 p AOs combine to give 6  MOs 3 MOs are bonding; 3 are antibonding

30 11-30 Dr. Wolf's CHM 201 & 202 Energy Bonding orbitals Antibonding orbitals Benzene MOs All bonding MOs are filled No electrons in antibonding orbitals

31 11-31 Dr. Wolf's CHM 201 & 202 The Three Bonding  MOs of Benzene

32 11-32 Dr. Wolf's CHM 201 & 202 Substituted Derivatives of Benzene and Their Nomenclature

33 11-33 Dr. Wolf's CHM 201 & 202 1) Benzene is considered as the parent and comes last in the name. General Points

34 11-34 Dr. Wolf's CHM 201 & 202 ExamplesExamples Bromobenzene tert-Butylbenzene Nitrobenzene NO 2 C(CH 3 ) 3 Br

35 11-35 Dr. Wolf's CHM 201 & 202 1) Benzene is considered as the parent and comes last in the name. 2) List substituents in alphabetical order 3) Number ring in direction that gives lowest locant at first point of difference General Points

36 11-36 Dr. Wolf's CHM 201 & bromo-1-chloro-4-fluorobenzene ExampleExample Br Cl F

37 11-37 Dr. Wolf's CHM 201 & 202 Ortho, Meta, and Para alternative locants for disubstituted derivatives of benzene 1,2 = ortho (abbreviated o-) 1,3 = meta (abbreviated m-) 1,4 = para (abbreviated p-)

38 11-38 Dr. Wolf's CHM 201 & 202 ExamplesExamples o-ethylnitrobenzene NO 2 CH 2 CH 3 ClCl m-dichlorobenzene (1-ethyl-2-nitrobenzene)(1,3-dichlorobenzene)

39 11-39 Dr. Wolf's CHM 201 & 202 Certain monosubstituted derivatives of benzene have unique names Benzene Derivatives

40 11-40 Dr. Wolf's CHM 201 & 202 Benzaldehyde Benzene Derivatives CHO

41 11-41 Dr. Wolf's CHM 201 & 202 Benzoic acid Benzene Derivatives COHO

42 11-42 Dr. Wolf's CHM 201 & 202 Styrene Benzene Derivatives CH 2 CH

43 11-43 Dr. Wolf's CHM 201 & 202 Toluene Benzene Derivatives CH 3

44 11-44 Dr. Wolf's CHM 201 & 202 Acetophenone Benzene Derivatives CCH 3 O

45 11-45 Dr. Wolf's CHM 201 & 202 Phenol Benzene Derivatives OH

46 11-46 Dr. Wolf's CHM 201 & 202 Anisole Benzene Derivatives OCH 3

47 11-47 Dr. Wolf's CHM 201 & 202 Aniline Benzene Derivatives NH 2

48 11-48 Dr. Wolf's CHM 201 & 202 Benzene derivative names can be used as parent OCH 3 NO 2 OCH 3 Anisole p-Nitroanisole or 4-Nitroanisole

49 11-49 Dr. Wolf's CHM 201 & 202 Easily confused names phenylphenolbenzyl OH CH 2 —

50 11-50 Dr. Wolf's CHM 201 & 202 Polycyclic Aromatic Hydrocarbons

51 11-51 Dr. Wolf's CHM 201 & 202 resonance energy = 255 kJ/mol most stable Lewis structure; both rings correspond to Kekulé benzene NaphthaleneNaphthalene

52 11-52 Dr. Wolf's CHM 201 & 202 Anthracene Phenanthrene resonance energy: 347 kJ/mol 381 kJ/mol Anthracene and Phenanthrene

53 11-53 Dr. Wolf's CHM 201 & 202 Physical Properties of Arenes

54 11-54 Dr. Wolf's CHM 201 & 202 Resemble other hydrocarbons nonpolar insoluble in water less dense than water Physical Properties

55 11-55 Dr. Wolf's CHM 201 & 202 Reactions of Arenes: A Preview 1. Some reactions involve the ring. 2. In other reactions the ring is a substituent.

56 11-56 Dr. Wolf's CHM 201 & 202 a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 12b) 1. Reactions involving the ring 2. The ring as a substituent (Sections )

57 11-57 Dr. Wolf's CHM 201 & 202 catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) HH HH HH HHH HH H HH Reduction of Benzene Rings HH H HHHH HHH H H

58 11-58 Dr. Wolf's CHM 201 & 202 The Birch Reduction

59 11-59 Dr. Wolf's CHM 201 & 202 (80%)HH HH HH HHH HH H HH Na, NH 3 CH 3 OH Birch Reduction of Benzene Product is non-conjugated diene. Reaction stops here. There is no further reduction. Reaction is not hydrogenation. H 2 is not involved in any way.

60 11-60 Dr. Wolf's CHM 201 & 202HH HH HH Step 1: Electron transfer from sodium + Na + Na + Mechanism of the Birch Reduction (Figure 11.8) H H HHH H –

61 11-61 Dr. Wolf's CHM 201 & 202 Step 2: Proton transfer from methanol Mechanism of the Birch Reduction (Figure 11.8) H H H H H H – OCH 3 H

62 11-62 Dr. Wolf's CHM 201 & 202 Step 2: Proton transfer from methanol Mechanism of the Birch Reduction (Figure 11.8) H H H H H H – OCH 3 H H H H H H H H –

63 11-63 Dr. Wolf's CHM 201 & 202 Step 3: Electron transfer from sodium Mechanism of the Birch Reduction (Figure 11.8) H H HHH H H + Na

64 11-64 Dr. Wolf's CHM 201 & 202 Step 3: Electron transfer from sodium Mechanism of the Birch Reduction (Figure 11.8) H H HHH H H + Na H H H H H H H + Na + –

65 11-65 Dr. Wolf's CHM 201 & 202 Step 4: Proton transfer from methanol Mechanism of the Birch Reduction (Figure 11.8) H H H H H H H – OCH 3 H

66 11-66 Dr. Wolf's CHM 201 & 202 Step 4: Proton transfer from methanol Mechanism of the Birch Reduction (Figure 11.8) H H H H H H H – OCH 3 H HHH HH H HH –

67 11-67 Dr. Wolf's CHM 201 & 202 (86%)HH H C(CH 3 ) 3 HH HHH H H HH Na, NH 3 CH 3 OH Birch Reduction of an Alkylbenzene If an alkyl group is present on the ring, it ends up as a substituent on the double bond.

68 11-68 Dr. Wolf's CHM 201 & 202 a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 23) 1. Reactions involving the ring 2. The ring as a substituent (Sections )

69 11-69 Dr. Wolf's CHM 201 & 202 Free-Radical Halogenation of Alkylbenzenes

70 11-70 Dr. Wolf's CHM 201 & 202 allylic radical The Benzene Ring as a Substituent C C C C benzylic radical benzylic carbon is analogous to allylic carbon

71 11-71 Dr. Wolf's CHM 201 & 202 The more stable the free radical R, the weaker the bond, and the smaller the bond-dissociation energy. Recall:Recall: R—HRH + Bond-dissociation energy for C—H bond is equal to  H° for: and is about 400 kJ/mol for alkanes.

72 11-72 Dr. Wolf's CHM 201 & 202 Bond-dissociation energies of propene and toluene 368 kJ/mol 356 kJ/mol H H2CH2CH2CH2C CH C HH H C HH H2CH2CH2CH2C CH -H -H H CH H CH Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals.

73 11-73 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Radical C H H HH H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

74 11-74 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Radical CHH HH H HH unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

75 11-75 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Radical CHH HH H HH unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

76 11-76 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Radical C H H HH H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

77 11-77 Dr. Wolf's CHM 201 & 202 industrial process highly regioselective for benzylic position CH 3 Free-radical chlorination of toluene Cl 2 light or heat CH 2 Cl Toluene Benzyl chloride

78 11-78 Dr. Wolf's CHM 201 & 202 Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives: Free-radical chlorination of toluene CCl 3 (Dichloromethyl)benzene CHCl 2 (Trichloromethyl)benzene

79 11-79 Dr. Wolf's CHM 201 & 202 is used in the laboratory to introduce a halogen at the benzylic position Benzylic Bromination CH 3 NO 2 + Br 2 CCl 4, 80°C light + HBr NO 2 CH 2 Br p-Nitrotoluene p-Nitrobenzyl bromide (71%)

80 11-80 Dr. Wolf's CHM 201 & 202 is a convenient reagent for benzylic bromination N-Bromosuccinimide (NBS) CCl 4 benzoylperoxide,heat CH 2 CH 3 + NBr OO CHCH 3 NHOO + Br (87%)

81 11-81 Dr. Wolf's CHM 201 & 202 Oxidation of Alkylbenzenes

82 11-82 Dr. Wolf's CHM 201 & 202 Site of Oxidation is Benzylic Carbon CH 3 CH 2 R CHR 2 or or COHO Na 2 Cr 2 O 7 H 2 SO 4 H2OH2OH2OH2Oheat

83 11-83 Dr. Wolf's CHM 201 & 202 ExampleExample Na 2 Cr 2 O 7 H 2 SO 4 H2OH2OH2OH2Oheat COHO CH 3 NO 2 p-Nitrotoluene NO 2 p-Nitrobenzoic acid (82-86%)

84 11-84 Dr. Wolf's CHM 201 & 202 ExampleExample Na 2 Cr 2 O 7 H 2 SO 4 H2OH2OH2OH2Oheat CH(CH 3 ) 2 CH 3 (45%) COHOCOH O

85 11-85 Dr. Wolf's CHM 201 & 202 S N 1 Reactions of Benzylic Halides S N 1 Reactions of Benzylic Halides

86 11-86 Dr. Wolf's CHM 201 & 202 tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; therefore, more stable What about S N 1? C CH 3 Cl 6001 C Cl Relative solvolysis rates in aqueous acetone

87 11-87 Dr. Wolf's CHM 201 & 202 What about S N 1? C more stable less stable C CH 3 Relative rates of formation: CH 3 + +

88 11-88 Dr. Wolf's CHM 201 & 202 allylic carbocation Compare.Compare. + C C C + C benzylic carbocation benzylic carbon is analogous to allylic carbon

89 11-89 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Cation C H H HH H H H + unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

90 11-90 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Cation C H H HH H H H + unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

91 11-91 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Cation C H H HH H H H + unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

92 11-92 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Cation C H H HH H H H + unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

93 11-93 Dr. Wolf's CHM 201 & 202 SolvolysisSolvolysis C CH 3 Cl CH 3 CH 2 OH C CH 3 OCH 2 CH 3 (87%)

94 11-94 Dr. Wolf's CHM 201 & 202 S N 2 Reactions of Benzylic Halides

95 11-95 Dr. Wolf's CHM 201 & 202 Primary Benzylic Halides acetic acid CH 2 Cl O2NO2NO2NO2N NaOCCH 3 O CH 2 OCCH 3 O2NO2NO2NO2NO Mechanism is S N 2 (78-82%)

96 11-96 Dr. Wolf's CHM 201 & 202 Preparation of Alkenylbenzenes dehydrogenation dehydration dehydrohalogenation

97 11-97 Dr. Wolf's CHM 201 & 202 industrial preparation of styrene Dehydrogenation CH 2 CH 3 630°C ZnO CH 2 CH + H 2

98 11-98 Dr. Wolf's CHM 201 & 202 Acid-Catalyzed Dehydration of Benzylic Alcohols KHSO 4 heat (80-82%) CH 2 CH CHCH 3 OHClCl+ H2OH2OH2OH2O

99 11-99 Dr. Wolf's CHM 201 & 202 Acid-Catalyzed Dehydration of Benzylic Alcohols KHSO 4 heat (80-82%) CH 2 CH CHCH 3 OHClCl Cl+

100 Dr. Wolf's CHM 201 & 202 Dehydrohalogenation NaOCH 2 CH 3 ethanol, 50°C (99%) H3CH3CH3CH3C CH 2 CHCH 3 Br CH H3CH3CH3CH3C CHCH 3

101 Dr. Wolf's CHM 201 & 202 hydrogenation halogenation addition of hydrogen halides Addition Reactions of Alkenylbenzenes

102 Dr. Wolf's CHM 201 & 202 Hydrogenation H2H2H2H2Pt (92%) Br C CH 3 CHCH 3 Br CHCH 2 CH 3 CH 3

103 Dr. Wolf's CHM 201 & 202 Halogenation CH 2 CH Br 2 CH 2 CH BrBr (82%)

104 Dr. Wolf's CHM 201 & 202 Addition of Hydrogen Halides HCl(75-84%)Cl

105 Dr. Wolf's CHM 201 & 202 Addition of Hydrogen Halides HCl via benzylic carbocation Cl+

106 Dr. Wolf's CHM 201 & 202 Free-Radical Addition of HBr CH 2 CH CH 2 CH 2 Br HBr peroxides

107 Dr. Wolf's CHM 201 & 202 Free-Radical Addition of HBr CH 2 CH CH 2 CH 2 Br HBr peroxides via benzylic radical CH 2 Br CH

108 Dr. Wolf's CHM 201 & 202 Polymerization of Styrene

109 Dr. Wolf's CHM 201 & 202 Polymerization of Styrene H2CH2CH2CH2C CHC 6 H 5 CH 2 CH C6H5C6H5C6H5C6H5 CH C6H5C6H5C6H5C6H5 CH C6H5C6H5C6H5C6H5 polystyrene

110 Dr. Wolf's CHM 201 & 202 Mechanism RO H2CH2CH2CH2C CHC 6 H 5

111 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 RO

112 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 RO H2CH2CH2CH2C

113 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 RO H2CH2CH2CH2C

114 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 H2CH2CH2CH2C RO

115 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 H2CH2CH2CH2C RO H2CH2CH2CH2C

116 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 H2CH2CH2CH2C RO H2CH2CH2CH2C

117 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 H2CH2CH2CH2C H2CH2CH2CH2C RO

118 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 H2CH2CH2CH2C H2CH2CH2CH2C H2CH2CH2CH2C RO

119 Dr. Wolf's CHM 201 & 202 Mechanism H2CH2CH2CH2C CHC 6 H 5 H2CH2CH2CH2C H2CH2CH2CH2C H2CH2CH2CH2C RO

120 Dr. Wolf's CHM 201 & 202 Cyclobutadiene and Cyclooctatetraene

121 heat of hydrogenation of benzene is 152 kJ/mol less than 3 times heat of hydrogenation of cyclohexene to give cyclohexane (kJ/mol) Heats of Hydrogenation

122 Dr. Wolf's CHM 201 & 202 Heats of Hydrogenation to give cyclooctane (kJ/mol) heat of hydrogenation of cyclooctatetraene is more than 4 times heat of hydrogenation of cyclooctene

123 Dr. Wolf's CHM 201 & 202 Structure of Cyclooctatetraene cyclooctatetraene is not planar has alternating long (146 pm) and short (133 pm) bonds

124 Dr. Wolf's CHM 201 & 202 structure of a stabilized derivative is characterized by alternating short bonds and long bonds Structure of Cyclobutadiene C(CH 3 ) 3 (CH 3 ) 3 C CO 2 CH 3 (CH 3 ) 3 C 138 pm 151 pm

125 Cyclobutadiene is observed to be highly reactive, and too unstable to be isolated and stored in the customary way. Not only is cyclobutadiene not aromatic, it is antiaromatic. An antiaromatic substance is one that is destabilized by cyclic conjugation. Stability of Cyclobutadiene

126 cyclic conjugation is necessary, but not sufficient Requirements for Aromaticity not aromatic aromatic Antiaromatic when square Antiaromatic when planar

127 Dr. Wolf's CHM 201 & 202 ConclusionConclusion there must be some factor in addition to cyclic conjugation that determines whether a molecule is aromatic or not

128 Dr. Wolf's CHM 201 & 202 Hückel's Rule: Annulenes the additional factor that influences aromaticity is the number of  electrons

129 Dr. Wolf's CHM 201 & 202 among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2  electrons possess special stability (are aromatic) n 4n Hückel's Rule

130 Dr. Wolf's CHM 201 & 202 among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2  electrons possess special stability (are aromatic) n 4n benzene! Hückel's Rule

131 Dr. Wolf's CHM 201 & 202 Hückel restricted his analysis to planar, completely conjugated, monocyclic polyenes he found that the  molecular orbitals of these compounds had a distinctive pattern one  orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highest and the lowest Hückel's Rule

132 Dr. Wolf's CHM 201 & 202  -MOs of Benzene Benzene Antibonding Bonding 6 p orbitals give 6  orbitals 3 orbitals are bonding; 3 are antibonding

133 Dr. Wolf's CHM 201 & 202 Benzene Antibonding Bonding 6  electrons fill all of the bonding orbitals all  antibonding orbitals are empty  -MOs of Benzene

134 Dr. Wolf's CHM 201 & 202 Cyclo- butadiene Antibonding Bonding 4 p orbitals give 4  orbitals 1 orbital is bonding, one is antibonding, and 2 are nonbonding  -MOs of Cyclobutadiene (square planar)

135 Dr. Wolf's CHM 201 & 202 Cyclo- butadiene Antibonding Bonding 4  electrons; bonding orbital is filled; other 2  electrons singly occupy two nonbonding orbitals  -MOs of Cyclobutadiene (square planar)

136 Dr. Wolf's CHM 201 & 202 Antibonding Bonding 8 p orbitals give 8  orbitals 3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding  -MOs of Cyclooctatetraene (square planar) Cyclo- octatetraene

137 Dr. Wolf's CHM 201 & 202 Antibonding Bonding 8  electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled  -MOs of Cyclooctatetraene (square planar) Cyclo- octatetraene

138 Dr. Wolf's CHM 201 & 202  -Electron Requirement for Aromaticity not aromatic aromatic not aromatic 4  electrons 6  electrons 8  electrons

139 Dr. Wolf's CHM 201 & 202 Completely Conjugated Polyenes aromatic 6  electrons; completely conjugated not aromatic 6  electrons; not completely conjugated HH

140 Annulenes

141 Dr. Wolf's CHM 201 & 202 Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule. AnnulenesAnnulenes

142 Dr. Wolf's CHM 201 & 202 predicted to be aromatic by Hückel's rule, but too much angle strain when planar and all double bonds are cis 10-sided regular polygon has angles of 144° [10]Annulene[10]Annulene

143 Dr. Wolf's CHM 201 & 202 incorporating two trans double bonds into the ring relieves angle strain but introduces van der Waals strain into the structure and causes the ring to be distorted from planarity [10]Annulene[10]Annulene

144 Dr. Wolf's CHM 201 & 202 incorporating two trans double bonds into the ring relieves angle strain but introduces van der Waals strain into the structure and causes the ring to be distorted from planarity [10]Annulene[10]Annulene van der Waals strain between these two hydrogens

145 Dr. Wolf's CHM 201 &  electrons satisfies Hückel's rule van der Waals strain between hydrogens inside the ring [14]Annulene[14]Annulene HH HH

146 Dr. Wolf's CHM 201 &  electrons does not satisfy Hückel's rule alternating short (134 pm) and long (146 pm) bonds not aromatic [16]Annulene[16]Annulene

147 Dr. Wolf's CHM 201 &  electrons satisfies Hückel's rule resonance energy = 418 kJ/mol bond distances range between pm [18]Annulene[18]Annulene H H H H H H

148 Dr. Wolf's CHM 201 & 202 Aromatic Ions

149 Dr. Wolf's CHM 201 &  electrons delocalized over 7 carbons positive charge dispersed over 7 carbons very stable carbocation also called tropylium cation Cycloheptatrienyl Cation HHHH HH H +

150 Dr. Wolf's CHM 201 & 202 Cycloheptatrienyl Cation HHHH HH H + +HHHH HH H

151 Dr. Wolf's CHM 201 & 202 Tropylium cation is so stable that tropylium bromide is ionic rather than covalent. mp 203 °C; soluble in water; insoluble in diethyl ether Cycloheptatrienyl Cation HBr + Br – IonicCovalent

152 Dr. Wolf's CHM 201 & 202 Cyclopentadienide Anion HHHH H – 6  electrons delocalized over 5 carbons negative charge dispersed over 5 carbons stabilized anion

153 Dr. Wolf's CHM 201 & 202 Cyclopentadienide Anion HHHH H –HHHH H –

154 Dr. Wolf's CHM 201 & 202 Acidity of Cyclopentadiene HHHH HHHHHH H – H+H+H+H+ + pK a = 16 K a = Cyclopentadiene is unusually acidic for a hydrocarbon. Increased acidity is due to stability of cyclopentadienide anion.

155 Dr. Wolf's CHM 201 & 202 Electron Delocalization in Cyclopentadienide Anion HHHH H –

156 Dr. Wolf's CHM 201 & 202 Electron Delocalization in Cyclopentadienide Anion HHHH H –HHHH H –

157 Dr. Wolf's CHM 201 & 202 Electron Delocalization in Cyclopentadienide Anion HHHH H –HHHH H – HHHH H –

158 Dr. Wolf's CHM 201 & 202 Electron Delocalization in Cyclopentadienide Anion HHHH H –HHHH H – HHHH H –HHHH H –

159 Dr. Wolf's CHM 201 & 202 Electron Delocalization in Cyclopentadienide Anion HHHH H –HHHH H – HH HH HHHHH H –HHHH H –

160 Dr. Wolf's CHM 201 & 202 Compare Acidities of Cyclopentadiene and Cycloheptatriene HHHH HH pK a = 16 K a = pK a = 36 K a = HHHHH HH HH

161 Dr. Wolf's CHM 201 & 202HHHH H – Compare Acidities of Cyclopentadiene and Cycloheptatriene Aromatic anion 6  electrons Anion not aromatic 8  electrons HHHH HH H –

162 Dr. Wolf's CHM 201 & 202 n = 0 4n +2 = 2  electrons Cyclopropenyl Cation +HHHHHH + also written as

163 Dr. Wolf's CHM 201 & 202 n = 2 4n +2 = 10  electrons Cyclooctatetraene Dianion HHHH H H H H H H H H HHHH 2– – – also written as

164 Dr. Wolf's CHM 201 & 202 Heterocyclic Aromatic Compounds

165 Dr. Wolf's CHM 201 & 202 Pyridine N ExamplesExamples O S N H PyrroleFuranThiophene

166 Dr. Wolf's CHM 201 & 202 Quinoline ExamplesExamples N N Isoquinoline

167 Dr. Wolf's CHM 201 & 202 Heterocyclic Aromatic Compounds and Hückel's Rule

168 Dr. Wolf's CHM 201 & 202 N PyridinePyridine 6  electrons in ring lone pair on nitrogen is in an sp 2 hybridized orbital; not part of  system of ring

169 Dr. Wolf's CHM 201 & 202 PyrrolePyrrole lone pair on nitrogen must be part of ring  system if ring is to have 6  electrons lone pair must be in a p orbital in order to overlap with ring  system N H

170 Dr. Wolf's CHM 201 & 202 FuranFuran two lone pairs on oxygen one pair is in a p orbital and is part of ring  system; other is in an sp 2 hybridized orbital and is not part of ring  system O

171 Dr. Wolf's CHM 201 & 202 End of Chapter 11


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