Chapter 11 - Humason

Chapter 11 - Humason - Ch Chapter 11 11 Arenes and...

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Unformatted text preview: Ch Chapter 11 11 Arenes and Aromaticity 11.2 The Structure of Benzene 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 Kekulé Formulation of Benzene Kekulé Formulation of Benzene Later, Kekulé revised his proposal by suggesting 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 Kekulé Formulation of Benzene Kekulé Formulation of Benzene However, this proposal suggested isomers of the 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 Structure of Benzene Structure of Benzene Structural studies of benzene do not support the studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the double bonds, all of the bonds are the same length. Benzene has the shape of a regular hexagon. All C— All C—C bond distances = 140 pm 140 pm pm 140 pm 140 pm 140 pm pm 146 pm pm 140 pm pm 140 pm 134 pm 140 pm is the average between the C—C C— single bond distance and the double bond distance in 1,3-butadiene. 1,3- Kekulé Kekulé Formulation of Benzene H H H H H H H H H H H H Resonance Resonance Formulation of Benzene Instead of Kekulé's suggestion of a rapid equilibrium between two structures, express the structure of benzene as a resonance resonance hybrid of the two Lewis structures. H H H H H H H H H H H H Resonance Resonance Formulation of Benzene Circle-inCircle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé description of benzene (hybrid of two Kekulé structures) structures) 11.3 The Stability of Benzene Th St 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 molecule than would be expected on greater for a molecule than would be expected on the the basis of any of the Lewis structures written for it Thermochemical Measures of Stability Thermochemical Measures of Stability heat of hydrogenation: compare experimental heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene" Pt + 3H2 ΔH°= – 208 kJ Figure 11.2 (p 452) 3 x cyclohexene cyclohexene 360 kJ/mol 231 231 kJ/mol 120 kJ/mol 208 kJ/mol Cyclic Cyclic conjugation versus noncyclic conjugation 3H 3H2 Pt Pt heat of hydrogenation = 208 kJ/mol 3H2 Pt heat of hydrogenation = 337 kJ/mol Resonance Resonance Energy of Benzene compared to localized 1,3,5-cyclohexatriene 1,3,5152 kJ/mol compared to 1,3,5-hexatriene to 129 kJ/mol exact value of resonance energy of benzene depends on what it is compared to, but depends on what it is compared to, but regardless regardless of model, benzene is more stable than expected by a substantial amount 11.4 An Orbital Hybridization View of Bonding in Benzene Orbital Orbital Hybridization Model of Bonding in Benzene Planar ring of 6 sp2 hybridized carbons sp Figure 11.3 Orbital Orbital Hybridization Model of Bonding in Benzene Each carbon contributes Each carbon contributes a p orbital Six p orbitals overlap to give cyclic π system; six electrons delocalized throughout six π electrons delocalized throughout π system Figure 11.3 Orbital Orbital Hybridization Model of Bonding in Benzene High electron density above and below plane of ring Figure 11.3 11.5 The π Molecular Orbitals of Benzene Benzene Benzene MOs Antibonding orbitals Energy Bonding orbitals 6 p AOs combine to give 6 π MOs AOs combine to give 3 MOs are bonding; 3 are antibonding Benzene Benzene MOs Antibonding orbitals Energy Bonding orbitals All bonding MOs are filled bonding MOs are filled No electrons in antibonding orbitals Benzene MOs Figure 11.4 p 454 11.6 11.6 Substituted Derivatives of Benzene and Their Nomenclature General Points General Points 1) Benzene is considered as the parent and Benzene is considered as the parent and comes last in the name. Br Bromobenzene C(CH3)3 tert-Butylbenzene NO2 Nitrobenzene General Points General Points 1) Benzene is considered as the parent and 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 Number ring in direction that gives lowest locant locant at first point of difference Example Example Cl Br F 2-bromo-1-chloro-4-fluorobenzene bromo- chloro- fluorobenzene Ortho, Ortho, Meta, and Para alternative locants for disubstituted derivatives of benzene derivatives of benzene 1,2 = ortho (abbreviated o-) 1,3 = meta (abbreviated m-) 1,4 = para (abbreviated p-) Examples Examples NO NO2 Cl Cl CH2CH3 Cl o-ethylnitrobenzene m-dichlorobenzene (1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene) Table 11.1 (p 436) Table 11.1 (p 436) Certain monosubstituted derivatives of benzene have unique names Table 11.1 (p 436) Table 11.1 (p 436) O O CH COH Benzaldehyde OH Phenol Benzoic acid OCH3 Anisole Table 11.1 (p 436) Table 11.1 (p 436) O NH2 CCH3 Aniline Acetophenone CH Styrene CH3 CH2 Toluene Names in Table 11 can be used as parent Names in Table 11.1 can be used as parent OCH OCH3 OCH3 NO2 Anisole p-Nitroanisole or 4-Nitroanisole Easily confused names Easily confused names phenyl phenol benzyl OH a group group a compound compound CH2— a group group 11 1 1 .7 Polycyclic Aromatic Hydrocarbons Naphthalene Naphthalene resonance energy 255 kJ/mol resonance energy = 255 kJ/mol most stable Lewis structure; stable Lewis structure; both rings correspond to Kekulé benzene Kekulé benzene Anthracene and Phenanthrene Anthracene and Phenanthrene Anthracene Anthracene Phenanthrene resonance energy: energy: 347 kJ/mol 381 kJ/mol 11 11.8 - Physical Properties Properties Arenes (aromatic hydrocarbons) resemble (aromatic hydrocarbons) resemble other hydrocarbons. They are: nonpolar insoluble in water less dense than water 11.9 11.9 Reactions of Arenes: renes: A Preview 1. Some reactions involve the ring. 2. In other reactions the ring is a substituent. 11.4 11.4 Catalytic Reduction of Benzene H H H H H H H H H Pt acetic acid H H H H H H 30 ̊ C H H H 11.10 The Birch Reduction Birch Birch Reduction of Benzene H H H H H H H H H H Na, NH3 CH3OH H H H H (80%) Product is non-conjugated diene. nonReaction stops here. There is no further reduction. Reaction is not hydrogenation. H2 is not involved in any way. Mechanism Mechanism of the Birch Reduction (Mechanism 11.1) Step 1: Electron transfer from sodium H H H H + • Na H H H H H • – H •• H H + Na+ Mechanism Mechanism of the Birch Reduction (Mechanism 11.1) 11. Step 2: Proton transfer from methanol H H H H H H H H • H H • – H •• H H – •• • OCH3 • •• H • OCH3 • •• Mechanism Mechanism of the Birch Reduction (Mechanism 11.1) Step 3: Electron transfer from sodium H H H H + • Na H • H H H H – H H + •• H H H Na+ Mechanism Mechanism of the Birch Reduction (Mechanism 11.1) Step 4: Proton transfer from methanol H H – •• • • OCH3 •• •• • • OCH3 H H H H H H – H H H H H •• H H H Birch Birch Reduction of an Alkylbenzene H H H H Na, NH3 CH3OH H C(CH3)3 H H H H H C(CH3)3 H H (86%) 86%) If an alkyl group is present on the ring, it ends up as a substituent on the double bond. substituent on the double bond 11.11 11.11 FreeFree-Radical Halogenation of Alkylbenzenes of Alkylbenzenes The The Benzene Ring as a Substituent C C C• allylic radical C• benzylic radical benzylic carbon is analogous to allylic carbon Recall: Recall: Bond-dissociation energy for C—H bond is is equal to ΔH° for: R—H R• + •H and is about 400 kJ/mol for alkanes. The more stable the free radical R• the weaker The more stable the free radical R•, the weaker the the bond, and the smaller the bond-dissociation bondenergy energy. BondBond-dissociation energies of propene and toluene H H2C CH C H 368 kJ/mol kJ/mol H H2C CH C• -H• H H H H 356 kJ/mol C H H -H• C• H Low BDEs indicate allyl and benzyl radical are Low BDEs indicate allyl and benzyl radical are more more stable than simple alkyl radicals. Resonance Resonance in Benzyl Radical H H • C H H H H H H H C H H H • C H H H H H C H H H H H • HH • H H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho ortho and para to it Spin Spin Density in Benzyl Radical (Figure 11.9, p 444) 11 444) unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho ortho and para to it Free Free-radical chlorination of toluene chlorination of toluene CH CH3 Toluene Cl2 CH2Cl light or heat Benzyl chloride industrial process highly regioselective for benzylic position Free Free-radical chlorination of toluene chlorination of toluene Similarly, dichlorination and trichlorination are dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives: CHCl CHCl2 (Dichloromethyl)benzene CCl3 (Trichloromethyl)benzene Benzylic Benzylic Bromination is used in the laboratory to introduce a halogen at the benzylic position halogen at the benzylic position CH2Br CH3 + Br2 Br NO2 p-Nitrotoluene CCl4, 80°C 80° + HBr light NO2 p-Nitrobenzyl bromide (71%) N-Bromosuccinimide (NBS) is a convenient reagent for benzylic bromination Br CH CH2CH3 O NBr + O CCl4 benzoyl peroxide, heat CHCH3 O NH + O (87%) 11.12 11.12 Oxidation of Alkylbenzenes Site of Oxidation is Benzylic Carbon Site of Oxidation is Benzylic Carbon CH CH3 or Na2C 2O7 Cr H2SO4 CH2R or CHR2 H2O heat O COH 11.13 11 Nucleophilic Substitution Reactions of Benzylic Halides SN1 SN1 Reactions Relative solvolysis rates in aqueous acetone CH CH3 C CH3 600 CH3 Cl CH3 C Cl CH3 1 tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; rapidly than tertiary carbocation; therefore, more stable SN1 Reactions Relative rates of formation: CH CH3 C+ CH3 more stable CH3 CH3 C+ CH3 less stable Solvolysis Solvolysis - SN1 CH CH3 Cl C CH3 CH3CH2OH CH CH3 C CH3 OCH2CH3 (87%) 11.14 11 Nucleophilic Substitution Reactions of Benzylic Halides SN2 Primary Primary Benzylic Halides – SN2 O2N CH2Cl O Mechanism is SN2 NaOCCH3 acetic acid O O2N CH2OCCH3 (78-82%) Compare. Compare. C C H C+ H allylic carbocation H C+ H benzylic carbocation benzylic carbon is analogous to allylic carbon Resonance Resonance in Benzyl Cation H H + C H H C H H H C + H H H H H C H H H H + H H H + HH + H H H H positive charge is delocalized between benzylic carbon and the ring carbons that are ortho and carbon and the ring carbons that are ortho and para para to it 11.15 Preparation of Alkenylbenzenes •dehydrogenation •dehydration •dehydrohalogenation Dehydrogenation •industrial preparation of styrene 12 billion lbs produced annually •Almost 12 billion lbs. produced annually in the United States CH2CH3 630° 630°C ZnO CH + H2 CH2 Acid-Catalyzed Dehydration of Benzylic Alcohols Cl Cl Cl KHSO4 CH CHCH3 heat CH2 (80-82%) OH Cl CHCH3 + Dehydrohalogenation CH2CHCH3 H3C Br NaO NaOCH2CH3 ethanol, 50°C 0° CH H3C (99%) 99%) CHCH3 11.16 Addition Reactions of Alkenylbenzenes Hydrogenation Hydrogenation CH3 C CH3 CHCH3 CHCH2CH3 H2 Pt Br Br (92%) Halogenation Halogenation Br2 CH CH2 CH CH2 Br Br (82%) Addition of Hydrogen Halides Cl Cl HCl (75(75-84%) + via benzylic carbocation Free Free-Radical Addition of HBr Addition of HBr HBr CH CH2CH2Br CH2 peroxides CH • CH2Br via benzylic radical 11 11.17 Polymerization of Styrene Polymerization of Styrene Polymerization of Styrene H2C CH2 CH C6H5 CH2 CHC6H5 CH C6H5 polystyrene CH2 CH C6H5 11.18 11.18 Cyclobutadiene and Cyclooctatetraene Heats Heats of Hydrogenation to give cyclohexane (kJ/mol) 120 231 208 heat of hydrogenation of benzene is 152 kJ/mol less than 3 times heat of hydrogenation of cyclohexene Heats Heats of Hydrogenation to give cyclooctane (kJ/mol) 97 97 205 303 410 heat of hydrogenation of cyclooctatetraene is more than times heat of hydrogenation of more than 4 times heat of hydrogenation of cyclooctene cyclooctene Structure Structure of Cyclooctatetraene cyclooctatetraene is not planar has alternating long (146 pm) and short (133 pm) bonds Structure of Cyclobutadiene Structure of Cyclobutadiene MO calculations give alternating short and long calculations give alternating short and long bonds for cyclobutadiene. H H 135 pm 156 pm H H Structure of Cyclobutadiene Structure of Cyclobutadiene structure of a stabilized derivative is characterized of stabilized derivative is characterized by alternating short bonds and long bonds C(CH3)3 (CH3)3C 138 pm 151 pm (CH3)3C CO2CH3 Stability of Cyclobutadiene Stability of Cyclobutadiene Cyclobutadiene is observed to be highly reactive Cyclobutadiene is observed to be highly reactive, and too unstable to be isolated and stored in the customary way. way Not only is cyclobutadiene not aromatic, it is antiaromatic antiaromatic. An antiaromatic substance is one that is destabilized An substa destab by cyclic conjugation. Requirements for Aromaticity Requirements for Aromaticity cyclic conjugation is necessary but not sufficient cyclic conjugation is necessary, but not sufficient not not aromatic Antiaromatic An when square aromatic not aromatic Antiaromatic when planar 11.19 11.19 Hückel's Rule: the additional factor that influences the additional factor that influences aromaticity aromaticity is the number of π electrons Hückel's Hückel's Rule among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 π electrons possess special stability (are aromatic) n 4n+2 0 2 1 6 2 10 3 14 4 18 Benzene! Hückel's Hückel's Rule Frost's circle is a mnemonic that allows us to draw a diagram showing the relative energies of the orbitals of cyclic conjugated system the π orbitals of a cyclic conjugated system. 1) draw a circle 2) inscribe a regular polygon inside the circle so that one of its corners is at the bottom 3) every point where a corner of the polygon touches the circle corresponds to a π electron energy level 4) the middle of the circle separates bonding and antibonding orbitals Frost's Frost's Circle Antibonding Bonding π MOs of Benzene MOs of Benzene π-MOs of Benzene Antibonding Benzene Benzene 6 p orbitals give 6 π orbitals orbitals give 3 orbitals are bonding; 3 are antibonding Bonding π-MOs of Benzene Antibonding Benzene Benzene 6 π electrons fill all of the bonding orbitals fill all of the bonding orbitals all π antibonding orbitals are empty Bonding π-MOs of Cyclobutadiene (square planar) planar) Antibonding CycloCyclobutadiene Bonding 4 p orbitals give 4π orbitals 1 orbital is bonding, one is antibonding, and 2 are nonbonding π-MOs of Cyclobutadiene (square planar) planar) Antibonding CycloCyclobutadiene Bonding 4 π electrons; bonding orbital is filled; other 2 π electrons singly occupy two nonbonding orbitals π-MOs of Cyclooctatetraene (square planar) planar) Antibonding CycloCyclooctatetraene Bondin Bonding 8 p orbitals give 8 π orbitals 3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding π-MOs of Cyclooctatetraene (square planar) planar) Antibonding CycloCyclooctatetraene Bondin Bonding 8 π electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled π-Electron Requirement for Aromaticity Requirement for Aromaticity 4 π electrons not aromatic 6 π electrons 8 π electrons aromatic not aromatic Completely Conjugated Polyenes Completely Conjugated Polyenes 6 π electrons; completely conjugated 6 π electrons; not completely conjugated H aromatic H not aromatic 11.20 Annulenes 11.20 Annulenes Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind kind of hydrocarbons treated by Hückel's rule. [10]Annulene [10]Annulene 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° regular polygon has angles of 144 [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 [14]Annulene [14]Annulene HH HH 14 π electrons satisfies Hückel's rule van der Waals strain between hydrogens inside the ring ring [16]Annulene 16]Annulene 16 π electrons does not satisfy Hückel's rule alternating short (134 pm) and long (146 pm) bonds is an antiaromatic 4n π-electron system [18]Annulene [18]Annulene H HH H H H 18 π electrons satisfies Hückel's rule resonance energy = 418 kJ/mol bond distances range between 137-143 pm 137- 11.21 11.21 Aromatic Ions Cycloheptatrienyl 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 Cycloheptatrienyl Cycloheptatrienyl Cation H H H H H H H + H + H H H H H H Cycloheptatrienyl 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 mp 203 °C; soluble in water; insoluble in diethyl ether Cyclopentadienide Cyclopentadienide Anion H H H •• H – H 6 π electrons delocalized over 5 carbons negative charge dispersed over 5 carbons stabilized anion Cyclopentadienide Cyclopentadienide Anion H H H H – H •• H – H H H H Acidity Acidity of Cyclopentadiene H H H H H+ + H H H H pKa = 16 Ka = 10-16 10 H •• – H Cyclopentadiene is unusually acidic for a hydrocarbon. Increased acidity is due to stability of cyclopentadienide anion. H Electron 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 Compare Compare Acidities of Cyclopentadiene and Cycloheptatriene and Cycloheptatriene H H H H H H H H H H H H H H pKa = 16 pKa = 36 Ka = 10-16 10 Ka = 10-36 10 Compare Compare Acidities of Cyclopentadiene and Cycloheptatriene and Cycloheptatriene H H H H H H •• – H H H Aromatic anion 6 π electrons H •• – H H Antiaromatic anion 8 π electrons Cyclopropenyl Cyclopropenyl Cation H H + H also written as H H + H n=0 4n +2 = 2 π electrons +2 Cyclooctatetraene Dianion Cyclooctatetraene Dianion H H H –H H •• •• – H H H H H H also written as 2– H H H H n=2 4n +2 = 10 π electrons H 11.22 11.22 Heterocyclic Aromatic Compounds Examples Examples N •• •• N •• O •• •• S •• Furan Thiophene H Pyridine Pyrrole Examples Examples N • N• •• Quinoline Isoquinoline 11. 11.23 Heterocyclic Aromatic Compounds Aromatic Compounds and Hückel's Rule Pyridine Pyridine 6 π electrons in ring lone pair on nitrogen is in an pair on nitrogen is in an N •• sp2 hybridized orbital; hybridized not part of π system of ring not system Pyrrole Pyrrole lone pair on nitrogen must be part of ring of ring π system if ring is to have if ring is to have •• N H 6 π electrons lone pair must be in a p orbital in order to overlap with ring π system Furan Furan two lone pairs on oxygen one pair is in one pair is in a p orbital and is part and is part •• of ring π system; other is in an •• sp2 hybridized orbital and is not O part of ring π system ...
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