Chapter11revised(3)

Chapter11revised(3) - I was also wondering if in chapter 9...

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Unformatted text preview: I was also wondering if in chapter 9 we were going to have equilibrium problems Organic Chemistry CHE 275 Chapter 11 Arenes and Aromaticity Structure of Benzene Structural studies of benzene do not support the Structural 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. All C—C Bond All Distances = 140 pm Distances 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 single bond 140 distance and the double bond distance in 1,3distance butadiene. Kekulé Formulation of Kekulé Benzene Instead of Kekulé's suggestion of a rapid Instead equilibrium between two structures: H H H H H H H H H H H H Resonance Formulation of Benzene of Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures) structures) 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 Thermochemical Measures Thermochemical of Stability heat of hydrogenation: compare experimental compare value with "expected" value for hypothetical "cyclohexatriene" + 3H2 Pt Pt ∆ H°= – 208 kJ Cyclic Conjugation vs. Acyclic Conjugation 3H2 3H Pt heat of hydrogenation = 208 kJ/mol heat 3H2 3H Pt heat of hydrogenation = 337 kJ/mol heat 3 x cyclohexene cyclohexene "expected" heat "expected" of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene cyclohexene 120 kJ/mol 120 360 kJ/mol 3 x cyclohexene 360 kJ/mol 360 231 kJ/mol 231 120 kJ/mol 120 208 kJ/mol 3 x cyclohexene cyclohexene observed heat of observed hydrogenation is 152 kJ/mol less than "expected" "expected" benzene is 152 benzene kJ/mol more stable than than expected 152 kJ/mol is the 152 resonance energy of benzene benzene 360 kJ/mol 208 kJ/mol hydrogenation of 1,31,3cyclohexadiene cyclohexadiene (2H2) gives off (2H gives more heat than hydrogenation of benzene (3H2)! benzene 231 kJ/mol 231 208 kJ/mol The π Molecular Orbitals of Benzene Benzene MOs Antibonding orbitals Energy Energy Bonding orbitals 6 p AOs combine to give 6 π MOs 3 MOs are bonding; 3 are MOs Benzene MOs Antibonding orbitals Energy Energy Bonding orbitals All bonding MOs are filled No electrons in antibonding orbitals Benzene MOs Substituted Derivatives of Benzene and Their Nomenclature Ortho, Meta, and Para Para alternative locants for disubstituted alternative derivatives of benzene 1,2 = ortho (abbreviated o-) (abbreviated 1,3 = meta 1,3 (abbreviated m-) (abbreviated 1,4 = para 1,4 (abbreviated p-) (abbreviated General Points 1) Benzene is considered as the parent and 1) comes last in the name. Br Bromobenzene Bromobenzene C(CH3)3 tert-Butylbenzene NO2 Nitrobenzene General Points 1) Benzene is considered as the parent and 1) 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 Example Example Cl Br F 2-bromo-1-chloro-4-fluorobenzene 2-bromo-1-chloro-4-fluorobenzene Physical Physical Properties Properties Arenes (aromatic hydrocarbons) resemble Arenes other hydrocarbons. They are: nonpolar insoluble in water less dense than water 1. Reactions of Reactions Arenes Reactions involving the ring a) Reduction a) 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) Reduction of Benzene Reduction Rings catalytic hydrogenation hydrogenation Birch reduction 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 H Birch Reduction of Benzene H H H H H H H H Na, NH3 H H CH3OH H H H H (80%) (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. Mechanism of the Mechanism Birch Reduction Birch Step 1: Electron transfer from sodium H H H H + • Na H H H H H • H •• – H H + Na+ Mechanism of the Mechanism Birch Reduction Birch Step 2: Proton transfer from methanol Step H H H H H H H H • H H – •• • OCH3 • •• • H •• – H H H • OCH3 • •• Mechanism of the Mechanism Birch Reduction Birch Step 3: Electron transfer from sodium Step H H H H + • Na H • H H H H – H H + •• H H H Na+ Mechanism of the Birch Reduction Birch Step 4: Proton transfer from methanol Step H H •• • OCH3 • – •• • • OCH3 •• H H H H H H – H H H H H •• H H H H H Birch Reduction Birch of an Alkylbenzene of H H Na, NH3 CH3OH C(CH3)3 H 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. The Benzene Ring as a Substituent C C C C • allylic radical • benzylic radical benzylic carbon is analogous to allylic carbon Recall: Bond-dissociation energy for C—H bond is equal to ∆ H° for: R—H R• + and is about 400 kJ/mol for alkanes. The more stable the free radical R•, the weaker the bond, and the smaller the bond-dissociation energy. •H Bond-Dissociation Energies of H Propene and Toluene H2C CH C H 368 kJ/mol 368 -H• H H2C CH C• H H C H H 356 kJ/mol -H• H H C• H Low BDEs indicate allyl and benzyl radical are more Low stable than simple alkyl radicals. stable Resonance in Benzyl Radical Radical H • C H H H H H H unpaired electron is delocalized between benzylic unpaired carbon and the ring carbons that are ortho and para to it it Resonance in Benzyl Resonance Radical 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 it Resonance in Benzyl Resonance Radical 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 it Resonance in Benzyl Resonance Radical Radical H H C H • H H H H unpaired electron is delocalized between benzylic unpaired carbon and the ring carbons that are ortho and para to it it Spin Density in Benzyl Radical unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para carbon Free-Radical Chlorination of Toluene CH3 CH Toluene Cl2 CH2Cl CH light or heat Benzyl chloride industrial process industrial highly regioselective for benzylic position Free-Radical Chlorination of Toluene Similarly, dichlorination and trichlorination are Similarly, selective for the benzylic carbon. Further chlorination gives: CHCl2 CHCl (Dichloromethyl)benzene CCl3 CCl (Trichloromethyl)benzene Benzylic Benzylic Bromination is used in the laboratory to introduce a halogen at the benzylic position halogen CH2Br CH CH3 CH + Br2 NO2 p-Nitrotoluene CCl4, 80°C + HBr light NO2 p-Nitrobenzyl bromide (71%) N-Bromosuccinimide is a convenient reagent(NBS) bromination for benzylic Br O NBr + NBr O CH2CH3 CCl4 benzoyl peroxide, heat O CHCH3 NH + NH O (87%) Oxidation of Alkyl Benzenes CH3 CH or CH2R CH or CHR2 CHR Na2Cr2O7 H2SO4 H2O heat O COH COH Example Example O CH3 CH Na2Cr2O7 H2SO4 COH H2O heat NO2 p-Nitrotoluene NO2 NO p-Nitrobenzoic acid (82-86%) Example CH(CH3)2 CH(CH O Na2Cr2O7 Na H2SO4 COH H2O heat CH3 COH COH O (45%) O2N Nucleophilic Substitution of Benzyl Halides of CH2Cl O Mechanism is SN2 NaOCCH3 acetic acid O O2N CH2OCCH3 (78-82%) What About What SN1? aqueous acetone Relative solvolysis rates in CH3 C CH3 600 CH3 Cl CH3 C CH3 1 tertiary benzylic carbocation is formed tertiary more rapidly than tertiary carbocation; therefore, more stable Cl What About What SN1? SN1? Relative rates of formation: CH3 C+ CH3 more stable CH3 CH3 C+ CH3 less stable Compare: C C C + allylic carbocation allylic C + benzylic carbocation benzylic benzylic carbon is analogous to allylic carbon Resonance in Benzyl Cation Cation H + C H H H H H H positive charge is delocalized between benzylic carbon positive and the ring carbons that are ortho and para to it and Resonance in Benzyl Resonance Cation Cation H H C H H + H H H positive charge is delocalized between benzylic carbon positive and the ring carbons that are ortho and para to it and Resonance in Benzyl Resonance Cation Cation H C H H H H + H H positive charge is delocalized between benzylic carbon positive and the ring carbons that are ortho and para to it and Resonance in Benzyl Resonance Cation Cation H H C H + H H H H positive charge is delocalized between benzylic carbon positive and the ring carbons that are ortho and para to it and Solvolysis CH3 C Cl CH3 CH3CH2OH CH3 CH C CH3 OCH2CH3 (87%) Preparation of Alkenylbenzenes •dehydration •dehydrohalogenation Acid-Catalyzed Dehydration of Benzylic Alcohols Cl Cll C KHSO4 CHCH3 CH heat OH CH2 (80-82%) Cl CHCH3 CHCH + Dehydrohalogenation CH2CHCH3 H3C Br NaOCH2CH3 NaOCH ethanol, 50°C H3C CH (99%) CHCH3 CHCH Addition Reactions of Alkenylbenzenes •hydrogenation •halogenation •addition of hydrogen halides Hydrogenation CH3 C CH3 CHCH2CH3 CHCH CHCH3 H2 Pt Br Br (92%) Halogenation Br2 CH CH2 CH CH CH2 CH Br Br (82%) 11.18 11.18 Cyclobutadiene and Cyclooctatetraene Heats of Hydrogenation Heats to give cyclohexane (kJ/mol) 120 120 231 208 heat of hydrogenation of benzene is 152 kJ/mol less than 3 times heat of hydrogenation of cyclohexene cyclohexene Heats of Hydrogenation Heats to give cyclooctane (kJ/mol) to 97 205 303 heat of hydrogenation of cyclooctatetraene is heat more than 4 times heat of hydrogenation of cyclooctene cyclooctene 410 Structure of Cyclooctatetraene cyclooctatetraene is not planar cyclooctatetraene has alternating long (146 pm) and short (133 pm) bonds Structure of Cyclobutadiene MO calculations give alternating short and long MO bonds for cyclobutadiene. H H 135 pm 156 pm H H Structure of Cyclobutadiene structure of a stabilized derivative is characterized structure by alternating short bonds and long bonds C(CH3)3 (CH3)3C 138 pm (CH3)3C 151 pm CO2CH3 Stability of Cyclobutadiene Cyclobutadiene is observed to be highly reactive, 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. Requirements for Aromaticity cyclic conjugation is necessary, but not sufficient not not aromatic Antiaromatic Antiaromatic when square when aromatic not aromatic Antiaromatic Antiaromatic when planar when Conclusion there must be some factor in addition there to cyclic conjugation that determines to whether a molecule is aromatic or not whether Hückel's Rule: the additional factor that influences aromaticity is the the number of π electrons Hückel's Rule among planar, monocyclic, completely 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! Hückel's Rule Hückel restricted his analysis to planar, Hückel 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 Hückel's Rule Frost's circle is a mnemonic that allows us to Frost's draw a diagram showing the relative energies of draw the π orbitals of a cyclic conjugated system. the 1) draw a circle 1) 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 touches energy level 4) the middle of the circle separates bonding and antibonding orbitals Frost's Circle Antibonding Bonding π MOs of Benzene MOs π-MOs of Benzene Antibonding Benzene 6 p orbitals give 6 π orbitals 3 orbitals are bonding; 3 are antibonding Bonding π-MOs of Benzene Antibonding Benzene Benzene 6 π electrons fill all of the bonding orbitals all π antibonding orbitals are empty all Bonding π-MOs of Cyclobutadiene (square planar) CycloCyclobutadiene 4 p orbitals give 4π orbitals 1 orbital is bonding, one is antibonding, and 2 orbital are nonbonding are Antibonding Bonding π-MOs of Cyclobutadiene (square planar) CycloCyclobutadiene Antibonding Bonding 4 π electrons; bonding orbital is filled; other 2 π electrons singly occupy two nonbonding orbitals π-MOs of Cyclobutadiene (square planar) Antibonding Antibonding Cyclooctatetraene 8 p orbitals give 8 π orbitals 3 orbitals are bonding, 3 are antibonding, and 2 orbitals are nonbonding are Bonding π-MOs of Cyclobutadiene (square planar) Antibonding Antibonding Cyclooctatetraene 8 π electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled Bonding π-Electron Requirement for Aromaticity 4 π electrons not not aromatic 6 π electrons 6 π electrons 8 π electrons aromatic aromatic not aromatic Completely Conjugated Polyenes Conjugated 6 π electrons; 6 π electrons; completely conjugated completely conjugated 6 π electrons; not completely conjugated H aromatic aromatic H not aromatic Annulenes Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule. rule. [10]Annulene predicted to be aromatic by Hückel's rule, predicted but too much angle strain when planar and but all double bonds are cis all 10-sided regular polygon has angles of 144° [10]Annulene van der Waals van strain between these two hydrogens incorporating two trans double bonds into incorporating 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 HH HH 14 π electrons satisfies Hückel's rule van der Waals strain between hydrogens inside the ring [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 H HH H H 18 π electrons satisfies Hückel's rule resonance energy = 418 kJ/mol bond distances range between 137-143 pm H Aromatic Ions: 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 Cation H H H H H H + H H H H + H H H H Cycloheptatrienyl Cycloheptatrienyl Cation Cation + Br– H Ionic Br Covalent Tropylium cation is so stable that tropylium Tropylium bromide is ionic rather than covalent. mp 203 °C; soluble in water; insoluble in diethyl ether 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 Anion H H H •• •• – H H H H H – H H Acidity of HCyclopentadiene H H H H H+ + H H H pKa = 16 Ka = 10-16 H •• •• – H Cyclopentadiene is unusually Cyclopentadiene acidic for a hydrocarbon. acidic Increased acidity is due to Increased stability of cyclopentadienide anion. anion. H 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 Acidities of 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 Acidities of Cyclopentadiene and Cycloheptatriene H H H H H H •• •• – H H Aromatic anion 6 π electrons H H •• – H H Antiaromatic anion 8 π electrons Cyclopropenyl Cyclopropenyl Cation H H + also written as H H H + H n=0 4n +2 = 2 π electrons Cyclooctatetraene Dianion Cyclooctatetraene H H H –H – •• H •• H – H H also written as H H H 2– H H H H n=2 4n +2 = 10 π electrons H 11.22 11.22 Heterocyclic Aromatic Compounds Heterocyclic Aromatic Compounds N •• •• N •• •• O S Furan Thiophene •• •• H Pyridine Pyridine Pyrrole Heterocyclic Aromatic Compounds N •• Quinoline Quinoline • N• Isoquinoline 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 Pyrolle lone pair on nitrogen must be part •• N H 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 Furan two lone pairs on oxygen two •• one pair is in a p orbital and is part one •• of ring π system; other is in an of O sp2 hybridized orbital and is not sp part of ring π system part Additional information. Examples Examples NO2 NO Cll C CH2CH3 Cl o-ethylnitrobenzene m-dichlorobenzene (1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene) Unique Names O CH Benzaldehyde Benzaldehyde Unique Names O COH Benzoic acid Benzoic Unique Names CH Styrene Styrene CH2 CH Unique Names OCH3 OCH Anisole Anisole Unique Names O CCH3 Acetophenone Acetophenone Unique Names OH OH Phenol Phenol Unique Names NH2 NH Aniline Aniline Unique Names OCH3 OCH OCH3 OCH NO2 Anisole p-Nitroanisole or 4-Nitroanisole Easily Confused Names Easily phenyl phenyl phenol benzyl OH OH a group a compound CH2— CH a group Polycyclic Aromatic Hydrocarbons Naphthalene resonance energy = 255 kJ/mol resonance most stable Lewis structure; both rings correspond to both Kekulé benzene Kekulé Anthracene and Phenanthrene Anthracene Anthracene Phenanthrene resonance energy: 347 kJ/mol 381 kJ/mol ...
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