8Chapter 11

8Chapter 11 - Chapter 11 Arenes and Aromaticity 11-1...

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Unformatted text preview: Chapter 11 Arenes and Aromaticity 11-1 Examples of Aromatic Hydrocarbons Examples of Aromatic Hydrocarbons CH3 H H H H H H H H H H Benzene H H H H H H Toluene H H H Naphthalene 11-2 11.1 Benzene 11-3 Some history Some history 1834 Eilhardt Mitscherlich isolates a new hydrocarbon and determines its empirical formula to be CnHn. Compound comes formula Compound to be called benzene. benzene 1845 August W. von Hofmann isolates benzene from coal tar. from 1866 August Kekulé proposes structure of benzene. benzene. 11-4 11.2 Kekulé and the Structure of Benzene 11-5 Kekulé Formulation of Benzene Kekulé Formulation of Benzene Kekulé proposed a cyclic structure for C6H6 with alternating single and double bonds. H H H H H H 11-6 Kekulé Formulation of Benzene Kekulé Formulation of Benzene Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. H H H H H H H H H H H H 11-7 Kekulé Formulation of Benzene Kekulé Formulation of Benzene However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. X X H X H X H H H H H H 11-8 Structure of Benzene Structure of Benzene 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. Benzene has the shape of a regular hexagon. 11-9 All C—C bond distances = 140 pm All C—C bond distances = 140 pm 140 pm 140 pm 140 pm 140 pm 140 pm 140 pm 11-10 All C—C bond distances = 140 pm All C—C bond distances = 140 pm 140 pm 140 pm 140 pm 140 pm 146 pm 146 140 pm 140 pm 134 pm 140 pm is the average between the C—C 140 single bond distance and the double bond distance in 1,3-butadiene. distance 11-11 A Resonance Picture of Bonding in Benzene 11-12 Kekulé Formulation of Benzene Kekulé Formulation of Benzene Instead of Kekulé's suggestion of a rapid equilibrium between two structures: H H H H H H H H H H H H 11-13 Resonance Formulation of Benzene Resonance Formulation of Benzene express the structure of benzene as a resonance express resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double not bonds, bonds, but are H delocalized over all six ring carbons. H H H H H H H H H H H 11-14 Resonance Formulation of Benzene Resonance Formulation of Benzene Circle-in-a-ring notation stands for resonance Circle-in-a-ring description of benzene (hybrid of two Kekulé structures) structures) 11-15 11.3 The Stability of Benzene benzene is the best and most familiar example benzene of a substance that possesses "special stability" or "aromaticity" or aromaticity is a level of stability that is substantially aromaticity greater for a molecule than would be expected on the basis of any of the Lewis structures written for it 11-16 Thermochemical Measures of Stability Thermochemical Measures of Stability heat of hydrogenation: compare experimental heat compare value with "expected" value for hypothetical "cyclohexatriene" + 3H2 Pt Pt ∆ H°= – 208 kJ 11-17 Figure 11.2 (p 404) Figure 11.2 (p 404) 3 x cyclohexene cyclohexene 360 kJ/mol 360 kJ/mol 231 kJ/mol 231 kJ/mol 120 kJ/mol 120 kJ/mol 120 208 kJ/mol 208 kJ/mol 11-18 Figure 11.2 (p 404) Figure 11.2 (p 404) "expected" "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene of 3 x cyclohexene cyclohexene 360 kJ/mol 360 kJ/mol 120 kJ/mol 120 kJ/mol 120 11-19 Figure 11.2 (p 404) Figure 11.2 (p 404) observed heat of observed hydrogenation is 152 kJ/mol less than "expected" than benzene is 152 benzene kJ/mol more stable than stable expected 152 kJ/mol is the 152 resonance energy of benzene of 3 x cyclohexene cyclohexene 360 kJ/mol 360 kJ/mol 208 kJ/mol 208 kJ/mol 11-20 Figure 11.2 (p 404) Figure 11.2 (p 404) hydrogenation hydrogenation of 1,3of cyclohexadiene cyclohexadiene (2H2) gives off (2H gives more heat than hydrogenation of benzene (3H2)! (3H 231 kJ/mol 231 kJ/mol 231 208 kJ/mol 208 kJ/mol 11-21 Cyclic conjugation versus noncyclic conjugation Cyclic conjugation versus noncyclic conjugation Cyclic 3H2 3H Pt heat of hydrogenation = 208 kJ/mol 3H2 3H Pt heat of hydrogenation = 337 kJ/mol heat 11-22 Resonance Energy of Benzene Resonance Energy of Benzene Resonance 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 exact depends on what it is compared to, but regardless of model, benzene is more stable than expected by a substantial amount 11-23 11.4 An Orbital Hybridization View of Bonding in Benzene 11-24 Orbital Hybridization Model of Orbital Hybridization Model of Orbital Orbital Bonding in Benzene Bonding in Benzene Bonding Bonding Planar ring of 6 sp2 hybridized carbons Planar sp Figure 11.3 11-25 Orbital Hybridization Model of Orbital Hybridization Model of Orbital Orbital Bonding in Benzene Bonding in Benzene Bonding Bonding Each carbon contributes a p orbital Each Six p orbitals overlap to give cyclic π system; Six six π electrons delocalized throughout π system six Figure 11.3 11-26 Orbital Hybridization Model of Orbital Hybridization Model of Orbital Orbital Bonding in Benzene Bonding in Benzene Bonding Bonding High electron density above and below plane High of ring of Figure 11.3 11-27 11.5 The π Molecular Orbitals of Benzene 11-28 Benzene MOs Benzene MOs Antibonding orbitals Energy Bonding orbitals 6 p AOs combine to give 6 π MOs 3 MOs are bonding; 3 are antibonding 11-29 Benzene MOs Benzene MOs Antibonding orbitals Energy Bonding orbitals All bonding MOs are filled No electrons in antibonding orbitals 11-30 The Three Bonding π MOs of Benzene The Three Bonding π MOs of Benzene The The 11-31 11.6 Substituted Derivatives of Benzene Substituted and Their Nomenclature 11-32 General Points General Points 1) Benzene is considered as the parent and comes last in the name. 11-33 Examples Examples Br Bromobenzene Bromobenzene C(CH3)3 tert-Butylbenzene NO2 Nitrobenzene 11-34 General Points General Points 1) Benzene is considered as the parent and comes last in the name. 2) List substituents in alphabetical order 2) 3) Number ring in direction that gives lowest locant at first point of difference locant 11-35 Example Example Example Cl Br F 2-bromo-1-chloro-4-fluorobenzene 11-36 Ortho, Meta, and Para Ortho, Meta, and Para alternative locants for disubstituted alternative derivatives of benzene 1,2 = ortho (abbreviated o-) (abbreviated 1,3 = meta (abbreviated m-) (abbreviated 1,4 = para (abbreviated p-) (abbreviated 11-37 Examples Examples NO2 Cll C CH2CH3 Cl o-ethylnitrobenzene m-dichlorobenzene (1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene) 11-38 Benzene Derivatives Benzene Derivatives Certain monosubstituted derivatives of benzene Certain have unique names have 11-39 Benzene Derivatives Benzene Derivatives O CH Benzaldehyde 11-40 Benzene Derivatives Benzene Derivatives O COH Benzoic acid 11-41 Benzene Derivatives Benzene Derivatives CH CH2 CH Styrene 11-42 Benzene Derivatives Benzene Derivatives CH3 CH Toluene 11-43 Benzene Derivatives Benzene Derivatives O CCH3 Acetophenone 11-44 Benzene Derivatives Benzene Derivatives OH OH Phenol 11-45 Benzene Derivatives Benzene Derivatives OCH3 OCH Anisole 11-46 Benzene Derivatives Benzene Derivatives NH2 NH Aniline 11-47 Benzene derivative names can be used as parent Benzene derivative names can be used as parent OCH3 OCH OCH3 OCH NO2 Anisole p-Nitroanisole or 4-Nitroanisole 11-48 Easily confused names Easily confused names Easily OH phenyl phenyl phenol CH2— benzyl 11-49 11.7 Polycyclic Aromatic Hydrocarbons 11-50 Naphthalene Naphthalene resonance energy = 255 kJ/mol most stable Lewis structure; both rings correspond to both Kekulé benzene Kekulé 11-51 Anthracene and Phenanthrene Anthracene and Phenanthrene Anthracene Phenanthrene resonance energy: 347 kJ/mol 381 kJ/mol 11-52 11.8 Physical Properties of Arenes 11-53 Physical Properties Physical Properties Resemble other hydrocarbons nonpolar insoluble in water less dense than water 11-54 11.9 Reactions of Arenes: A Preview 1. Some reactions involve the ring. 2. In other reactions the ring is a substituent. 11-55 1. Reactions involving the ring a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 12b) 2. The ring as a substituent (Sections 11.12-11.17) 11-56 Reduction of Benzene Rings Reduction of Benzene Rings catalytic catalytic hydrogenation hydrogenation (Section 11.4) H H H H H H H H H H H H H H H H H H Birch reduction (Section 11.11) H H H H H H H H 11-57 11.10 The Birch Reduction 11-58 Birch Reduction of Benzene Birch Reduction of Benzene H H H H H H H H Na, NH3 H H CH3OH H H H H (80%) Product is non-conjugated diene. Reaction stops here. There is no further reduction. Reaction is not hydrogenation. H2 is not involved in any way. 11-59 Mechanism of the Birch Reduction (Figure 11.8) Mechanism of the Birch Reduction (Figure 11.8) Step 1: Electron transfer from sodium H H H H + • Na H H H H H • H •• – H + Na+ H 11-60 Mechanism of the Birch Reduction (Figure 11.8) Mechanism of the Birch Reduction (Figure 11.8) Step 2: Proton transfer from methanol H H H • H •• – H H H • OCH3 • •• 11-61 Mechanism of the Birch Reduction (Figure 11.8) Mechanism of the Birch Reduction (Figure 11.8) Step 2: Proton transfer from methanol H H H H H H H H • H H – •• • OCH3 • •• • H •• – H H H • OCH3 • •• 11-62 Mechanism of the Birch Reduction (Figure 11.8) Mechanism of the Birch Reduction (Figure 11.8) Step 3: Electron transfer from sodium H H H + • Na • H H H H 11-63 Mechanism of the Birch Reduction (Figure 11.8) Mechanism of the Birch Reduction (Figure 11.8) Step 3: Electron transfer from sodium H H H H + • Na H • H H H H – H H + •• Na+ H H H 11-64 Mechanism of the Birch Reduction (Figure 11.8) Mechanism of the Birch Reduction (Figure 11.8) Step 4: Proton transfer from methanol •• • OCH3 • H H – H H •• H H H H 11-65 Mechanism of the Birch Reduction (Figure 11.8) Mechanism of the Birch Reduction (Figure 11.8) Step 4: Proton transfer from methanol H H – •• • • OCH3 •• •• • OCH3 • H H H H H H – H H H H H •• H H H 11-66 Birch Reduction of an Alkylbenzene Birch Reduction of an Alkylbenzene H H H H Na, NH3 CH3OH C(CH3)3 H H H H H H C(CH3)3 H H (86%) If an alkyl group is present on the ring, it ends up as a substituent on the double bond. 11-67 1. Reactions involving the ring a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 23) 2. The ring as a substituent (Sections 11.12-11.17) 11-68 11.11 Free-Radical Halogenation of Alkylbenzenes 11-69 The Benzene Ring as a Substituent The Benzene Ring as a Substituent C C C • allylic radical C • benzylic radical benzylic benzylic carbon is analogous to allylic carbon 11-70 Recall: Recall: Bond-dissociation energy for C—H bond Bond-dissociation is equal to ∆ H° for: R—H R• + •H and is about 400 kJ/mol for alkanes. The more stable the free radical R•, the weaker The the bond, and the smaller the bond-dissociation energy. energy. 11-71 Bond-dissociation energies of propene and toluene Bond-dissociation energies of propene and toluene H H2C CH C H 368 kJ/mol -H• H H H2C CH C• H H C H H 356 kJ/mol -H• H C• H Low BDEs indicate allyl and benzyl radical are Low more stable than simple alkyl radicals. more 11-72 Resonance in Benzyl Radical Resonance in Benzyl Radical H • C H H H H H H unpaired electron is delocalized between unpaired benzylic carbon and the ring carbons that are ortho and para to it ortho 11-73 Resonance in Benzyl Radical Resonance in Benzyl Radical H H C H H • H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it ortho 11-74 Resonance in Benzyl Radical Resonance in Benzyl Radical H C H H H H • H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it ortho 11-75 Resonance in Benzyl Radical Resonance in Benzyl Radical Resonance H H C H • H H H H unpaired electron is delocalized between unpaired benzylic carbon and the ring carbons that are ortho and para to it ortho 11-76 Free-radical chlorination of toluene Free-radical chlorination of toluene industrial process highly regioselective for benzylic position CH3 CH Toluene Cl2 light or heat CH2Cl CH Benzyl chloride 11-77 Free-radical chlorination of toluene Free-radical chlorination of toluene Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives: CHCl2 CHCl (Dichloromethyl)benzene CCl3 CCl (Trichloromethyl)benzene 11-78 Benzylic Bromination Benzylic Bromination iis used in the laboratory to introduce a s halogen at the benzylic position halogen CH2Br CH CH3 CH + Br2 NO2 p-Nitrotoluene CCl4, 80°C + HBr light NO2 p-Nitrobenzyl bromide (71%) 11-79 N-Bromosuccinimide (NBS) N-Bromosuccinimide (NBS) N-Bromosuccinimide iis a convenient reagent for benzylic bromination s Br O NBr + O CH2CH3 CCl4 benzoyl peroxide, heat O CHCH3 NH + NH O (87%) 11-80 11.12 Oxidation of Alkylbenzenes 11-81 Site of Oxidation is Benzylic Carbon Site of Oxidation is Benzylic Carbon CH3 CH or CH2R CH or Na2Cr2O7 H2SO4 H2O heat O COH COH CHR2 CHR 11-82 Example Example O CH3 CH Na2Cr2O7 H2SO4 COH H2O heat NO2 p-Nitrotoluene NO2 NO p-Nitrobenzoic acid (82-86%) 11-83 Example Example O CH(CH3)2 CH(CH Na2Cr2O7 H2SO4 COH H2O heat CH3 COH COH O (45%) 11-84 11.13 11.13 SN1 Reactions of Benzylic Halides 11-85 What about SN1? What about SN1? Relative solvolysis rates in aqueous acetone CH3 C CH3 600 CH3 Cl CH3 C Cl CH3 1 tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; therefore, more stable 11-86 What about SN1? What about SN1? Relative rates of formation: CH3 C+ CH3 more stable CH3 CH3 C+ CH3 less stable 11-87 Compare. Compare. C C C + allylic carbocation C + benzylic carbocation benzylic benzylic carbon is analogous to allylic carbon 11-88 Resonance in Benzyl Cation Resonance in Benzyl Cation H + C H H H H H H unpaired electron is delocalized between unpaired benzylic carbon and the ring carbons that are ortho and para to it ortho 11-89 Resonance in Benzyl Cation Resonance in Benzyl Cation H H C H H + H H H unpaired electron is delocalized between unpaired benzylic carbon and the ring carbons that are ortho and para to it ortho 11-90 Resonance in Benzyl Cation Resonance in Benzyl Cation Resonance H C H H H H + H H unpaired electron is delocalized between unpaired benzylic carbon and the ring carbons that are ortho and para to it ortho 11-91 Resonance in Benzyl Cation Resonance in Benzyl Cation Resonance H H C H + H H H H unpaired electron is delocalized between unpaired benzylic carbon and the ring carbons that are ortho and para to it ortho 11-92 Solvolysis Solvolysis CH3 C Cl CH3 CH3CH2OH CH3 CH C CH3 OCH2CH3 (87%) 11-93 11.14 SN2 Reactions of Benzylic Halides 11-94 Primary Benzylic Halides Primary Benzylic Halides O2N CH2Cl O Mechanism is SN2 NaOCCH3 acetic acid O O2N CH2OCCH3 (78-82%) 11-95 11.15 Preparation of Alkenylbenzenes •dehydrogenation •dehydration •dehydrohalogenation 11-96 Dehydrogenation Dehydrogenation •industrial preparation of styrene CH2CH3 CH 630°C ZnO CH CH2 CH + H2 11-97 Acid-Catalyzed Dehydration of Acid-Catalyzed Dehydration of Benzylic Alcohols Benzylic Alcohols Cl Cll C KHSO4 CHCH3 OH heat CH CH2 + H2O (80-82%) 11-98 Acid-Catalyzed Dehydration of Acid-Catalyzed Dehydration of Benzylic Alcohols Benzylic Alcohols Cl Cll C KHSO4 CHCH3 CH heat OH CH2 (80-82%) Cl CHCH3 CHCH + 11-99 Dehydrohalogenation Dehydrohalogenation CH2CHCH3 H3C Br NaOCH2CH3 ethanol, 50°C H3C CH CH CHCH3 CHCH (99%) 11-100 11.16 Addition Reactions of Alkenylbenzenes •hydrogenation •halogenation •addition of hydrogen halides 11-101 Hydrogenation Hydrogenation CH3 C CH3 CHCH2CH3 CHCH CHCH3 H2 Pt Br Br (92%) 11-102 Halogenation Halogenation Br2 CH CH2 CH CH CH2 CH Br Br (82%) 11-103 Addition of Hydrogen Halides Addition of Hydrogen Halides Cl HCl (75-84%) (75-84%) 11-104 Addition of Hydrogen Halides Addition of Hydrogen Halides Cl HCl + via benzylic carbocation via 11-105 Free-Radical Addition of HBr Free-Radical Addition of HBr CH CH2 CH HBr peroxides CH2CH2Br CH 11-106 Free-Radical Addition of HBr Free-Radical Addition of HBr CH CH2 CH HBr peroxides CH • CH2CH2Br CH CH2Br CH via benzylic radical 11-107 11.17 Polymerization of Styrene 11-108 Polymerization of Styrene Polymerization of Styrene H2C CH2 CH C6H5 CH2 CHC6H5 CH C6H5 CH2 CH C6H5 polystyrene 11-109 Mechanism •• • RO • • H2C CHC6H5 11-110 Mechanism •• • RO • H2C CHC6H5 • 11-111 Mechanism •• • RO • H2C CHC6H5 • H2C CHC6H5 11-112 Mechanism •• • RO • H2C CHC6H5 • H2C CHC6H5 11-113 Mechanism •• • RO • H2C CHC6H5 H2C CHC6H5 • 11-114 Mechanism •• • RO • H2C CHC6H5 H2C CHC6H5 • H2C CHC6H5 11-115 Mechanism •• • RO • H2C CHC6H5 H2C CHC6H5 • H2C CHC6H5 11-116 Mechanism •• • RO • H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 • 11-117 Mechanism •• • RO • H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 • H2C CHC6H5 11-118 Mechanism •• • RO • H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 • H2C CHC6H5 11-119 11.18 Cyclobutadiene and Cyclooctatetraene 11-120 Heats of Hydrogenation to give cyclohexane (kJ/mol) 120 120 231 208 heat of hydrogenation of benzene is 152 kJ/mol heat less than 3 times heat of hydrogenation of cyclohexene cyclohexene 11-121 Heats of Hydrogenation Heats of Hydrogenation to give cyclooctane (kJ/mol) 97 205 303 410 heat of hydrogenation of cyclooctatetraene is heat more than 4 times heat of hydrogenation of cyclooctene cyclooctene 11-122 Structure of Cyclooctatetraene Structure of Cyclooctatetraene cyclooctatetraene is not planar has alternating long (146 pm) and short (133 pm) bonds 11-123 Structure of Cyclobutadiene Structure of Cyclobutadiene structure of a stabilized derivative is characterized by alternating short bonds and long bonds C(CH3)3 (CH3)3C 138 pm (CH3)3C 151 pm CO2CH3 11-124 Stability of Cyclobutadiene Cyclobutadiene is observed to be highly reactive, Cyclobutadiene and too unstable to be isolated and stored in the and customary way. Not only is cyclobutadiene not aromatic, it is Not antiaromatic. antiaromatic An antiaromatic substance is one that is destabilized An destabilized by cyclic conjugation. 11-125 Requirements for Aromaticity Requirements cyclic conjugation is necessary, but not sufficient cyclic not not aromatic Antiaromatic Antiaromatic when square when aromatic not aromatic Antiaromatic Antiaromatic when planar when 11-126 Conclusion Conclusion there must be some factor in addition to cyclic conjugation that determines to whether a molecule is aromatic or not whether 11-127 11.19 Hückel's Rule: Annulenes the additional factor that influences the aromaticity is the number of π electrons 11-128 Hückel's Rule Hückel's Rule among planar, monocyclic, completely among conjugated polyenes, only those with 4n + 2 conjugated π electrons possess special stability (are aromatic) aromatic) n 4n+2 0 2 1 6 2 10 3 14 4 18 11-129 Hückel's Rule Hückel's Rule among planar, monocyclic, completely among conjugated polyenes, only those with 4n + 2 conjugated π electrons possess special stability (are aromatic) aromatic) n 4n+2 0 2 1 6 2 10 3 14 4 18 benzene! 11-130 Hückel's Rule Hückel's Rule Hückel restricted his analysis to planar, completely conjugated, monocyclic polyenes he found that the π molecular orbitals of he these compounds had a distinctive pattern one π orbital was lowest in energy, another one was highest in energy, and the others were arranged in pairs between the highest were and the lowest 11-131 π-MOs of Benzene π-MOs of Benzene Antibonding Benzene 6 p orbitals give 6 π orbitals 3 orbitals are bonding; 3 are antibonding Bonding 11-132 π-MOs of Benzene π-MOs of Benzene Antibonding Benzene Benzene 6 π electrons fill all of the bonding orbitals all π antibonding orbitals are empty all Bonding 11-133 π-MOs of Cyclobutadiene π-MOs of Cyclobutadiene (square planar) (square planar) Antibonding Cyclobutadiene Bonding 4 p orbitals give 4π orbitals 1 orbital is bonding, one is antibonding, and 2 orbital are nonbonding are 11-134 π-MOs of Cyclobutadiene π-MOs of Cyclobutadiene (square planar) (square planar) Antibonding Cyclobutadiene Bonding 4 π electrons; bonding orbital is filled; other 2 π electrons singly occupy two nonbonding orbitals orbitals 11-135 π-MOs of Cyclooctatetraene π-MOs of Cyclooctatetraene (square planar) (square planar) Antibonding Cyclooctatetraene Bonding 8 p orbitals give 8 π orbitals 3 orbitals are bonding, 3 are antibonding, and 2 orbitals are nonbonding are 11-136 π-MOs of Cyclooctatetraene π-MOs of Cyclooctatetraene (square planar) (square planar) Antibonding Cyclooctatetraene Bonding 8 π electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled 11-137 π-Electron Requirement for Aromaticity π-Electron Requirement for Aromaticity 4 π electrons not not aromatic 6 π electrons 6 π electrons 8 π electrons aromatic aromatic not aromatic 11-138 Completely Conjugated Polyenes Completely Conjugated Polyenes 6 π electrons; 6 π electrons; completely conjugated completely conjugated 6 π electrons; not completely conjugated H aromatic aromatic H not aromatic 11-139 11.20 Annulenes 11-140 Annulenes Annulenes Annulenes are planar, monocyclic, completely Annulenes conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule. rule. 11-141 [10]Annulene [10]Annulene predicted to be aromatic by Hückel's rule, but too much angle strain when planar and but all double bonds are cis all 10-sided regular polygon has angles of 144° 11-142 [10]Annulene [10]Annulene 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 11-143 [10]Annulene [10]Annulene van der Waals strain between these two hydrogens 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 11-144 [14]Annulene [14]Annulene HH HH 14 π electrons satisfies Hückel's rule 14 van der Waals strain between hydrogens inside the ring 11-145 [16]Annulene [16]Annulene 16 π electrons does not satisfy Hückel's rule 16 alternating short (134 pm) and long (146 pm) alternating bonds bonds not aromatic 11-146 [18]Annulene [18]Annulene H HH H H H 18 π electrons satisfies Hückel's rule 18 resonance energy = 418 kJ/mol bond distances range between 137-143 pm 11-147 11.21 Aromatic Ions 11-148 Cycloheptatrienyl Cation Cycloheptatrienyl Cation H H H H H + H H 6 π electrons delocalized over 7 carbons positive charge dispersed over 7 carbons very stable carbocation also called tropylium cation 11-149 Cycloheptatrienyl Cation Cycloheptatrienyl Cation H H H H H H + H H H H + H H H H 11-150 Cycloheptatrienyl Cation Cycloheptatrienyl Cation + Br– H Ionic Br Covalent Tropylium cation is so stable that tropylium bromide is ionic rather than covalent. mp 203 °C; soluble in water; insoluble in diethyl ether 11-151 Cyclopentadienide Anion Cyclopentadienide Anion H H H •• •• – H H 6 π electrons delocalized over 5 carbons negative charge dispersed over 5 carbons stabilized anion 11-152 Cyclopentadienide Anion Cyclopentadienide Anion H H H •• •• – H H H H H – H H 11-153 Acidity of Cyclopentadiene Acidity of Cyclopentadiene H H H H H H H pKa = 16 Ka = 10-16 H+ + H H •• •• – H H Cyclopentadiene is Cyclopentadiene unusually acidic for a hydrocarbon. hydrocarbon. Increased acidity is due to Increased stability of cyclopentadienide anion. anion. 11-154 Electron Delocalization in Cyclopentadienide Anion Electron Delocalization in Cyclopentadienide Anion H H H •• – H H 11-155 Electron Delocalization in Cyclopentadienide Anion Electron Delocalization in Cyclopentadienide Anion H H H H – •• H •• – H H H H H 11-156 Electron Delocalization in Cyclopentadienide Anion Electron Delocalization in Cyclopentadienide Anion H H H H – •• H •• – H H H H H H H • •– H H H 11-157 Electron Delocalization in Cyclopentadienide Anion Electron Delocalization in Cyclopentadienide Anion H H H H – •• H •• – H H H H H H H H H –• • H H H • •– H H H 11-158 Electron Delocalization in Cyclopentadienide Anion Electron Delocalization in Cyclopentadienide Anion H H H H – •• H •• – H H H H H H H H •• •• H H H H H H –• • H H H • •– H H H 11-159 Compare Acidities of Compare Acidities of Cyclopentadiene and Cycloheptatriene Cyclopentadiene and Cycloheptatriene H H H H H H H H H H H H H H pKa = 16 pKa = 36 Ka = 10-16 Ka = 10-36 11-160 Compare Acidities of Compare Acidities of Cyclopentadiene and Cycloheptatriene Cyclopentadiene and Cycloheptatriene H H H H H H •• •• – H H Aromatic anion 6 π electrons H H •• – H H Anion not aromatic 8 π electrons 11-161 Cyclopropenyl Cation Cyclopropenyl Cation H H + H also written as H H + H n=0 4n +2 = 2 π electrons 11-162 Cyclooctatetraene Dianion Cyclooctatetraene Dianion H H H –H – •• H •• H – H H also written as H H H 2– H H H H H n=2 4n +2 = 10 π electrons 11-163 11.22 Heterocyclic Aromatic Compounds 11-164 Examples Examples N •• •• N •• •• O S Furan Thiophene •• •• H Pyridine Pyrrole 11-165 Examples Examples N •• Quinoline • N• Isoquinoline 11-166 11.23 Heterocyclic Aromatic Compounds and Hückel's Rule 11-167 Pyridine Pyridine 6 π electrons in ring lone pair on nitrogen is in an N •• sp2 hybridized orbital; hybridized not part of π system of ring not system 11-168 Pyrrole Pyrrole •• N H llone pair on nitrogen must be part one of ring π system if ring is to have 6 π electrons llone pair must be in a p orbital one iin order to overlap with ring π n system 11-169 Furan Furan two lone pairs on oxygen one pair is in a p orbital and is part one •• O •• of ring π system; other is in an of sp2 hybridized orbital and is not sp part of ring π system part 11-170 End of Chapter 11 11-171 ...
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