Chapter 11 Arenes and Aromaticity Dr. Wolf's CHM 201 & 202 3

Examples of Aromatic Hydrocarbons Benzene Toluene Naphthalene Dr. Wolf's CHM 201 & 202 2

Benzene Dr. Wolf's CHM 201 & 202 3

1845 August W. von Hofmann isolates benzene from coal tar. Some history 1834 Eilhardt Mitscherlich isolates a new hydrocarbon and determines its empirical formula to be CnHn. Compound comes to be called benzene. 1845 August W. von Hofmann isolates benzene from coal tar. 1866 August Kekulé proposes structure of benzene. Dr. Wolf's CHM 201 & 202 4

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

Kekulé Formulation of Benzene Kekulé proposed a cyclic structure for C6H6 with alternating single and double bonds. H Dr. Wolf's CHM 201 & 202 6

Kekulé Formulation of Benzene Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. H H Dr. Wolf's CHM 201 & 202 6

Kekulé Formulation of Benzene However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. H X H X Dr. Wolf's CHM 201 & 202 6

Benzene has the shape of a regular hexagon. 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. Dr. Wolf's CHM 201 & 202 6

All C—C bond distances = 140 pm Dr. Wolf's CHM 201 & 202 9

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. Dr. Wolf's CHM 201 & 202 9

A Resonance Picture of Bonding in Benzene Dr. Wolf's CHM 201 & 202 5

Kekulé Formulation of Benzene Instead of Kekulé's suggestion of a rapid equilibrium between two structures: H H Dr. Wolf's CHM 201 & 202 6

Resonance Formulation of Benzene 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. H H Dr. Wolf's CHM 201 & 202 6

Resonance Formulation of Benzene Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures) Dr. Wolf's CHM 201 & 202 14

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 Dr. Wolf's CHM 201 & 202 15

Thermochemical Measures of Stability heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene" Pt + 3H2 DH°= – 208 kJ Dr. Wolf's CHM 201 & 202 17

Figure 11.2 (p 404) 3 x cyclohexene 360 kJ/mol 231 kJ/mol 208 kJ/mol Dr. Wolf's CHM 201 & 202 18

Figure 11.2 (p 404) 3 x cyclohexene "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene 360 kJ/mol 120 kJ/mol Dr. Wolf's CHM 201 & 202 18

observed heat of hydrogenation is 152 kJ/mol less than "expected" Figure 11.2 (p 404) 3 x cyclohexene 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 360 kJ/mol 208 kJ/mol Dr. Wolf's CHM 201 & 202 18

Figure 11.2 (p 404) hydrogenation of 1,3-cyclohexadiene (2H2) gives off more heat than hydrogenation of benzene (3H2)! 231 kJ/mol 208 kJ/mol Dr. Wolf's CHM 201 & 202 18

Cyclic conjugation versus noncyclic conjugation 3H2 Pt heat of hydrogenation = 208 kJ/mol 3H2 Pt heat of hydrogenation = 337 kJ/mol Dr. Wolf's CHM 201 & 202 20

Resonance Energy of Benzene 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 Dr. Wolf's CHM 201 & 202 21

An Orbital Hybridization View of Bonding in Benzene Dr. Wolf's CHM 201 & 202 22

Orbital Hybridization Model of Bonding in Benzene Planar ring of 6 sp2 hybridized carbons Figure 11.3 Dr. Wolf's CHM 201 & 202 23

Orbital Hybridization Model of Bonding in Benzene Each carbon contributes a p orbital Six p orbitals overlap to give cyclic p system; six p electrons delocalized throughout p system Figure 11.3 Dr. Wolf's CHM 201 & 202 23

Orbital Hybridization Model of Bonding in Benzene High electron density above and below plane of ring Figure 11.3 Dr. Wolf's CHM 201 & 202 23

The p Molecular Orbitals of Benzene Dr. Wolf's CHM 201 & 202 26

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

All bonding MOs are filled No electrons in antibonding orbitals Benzene MOs Antibonding orbitals Energy Bonding orbitals All bonding MOs are filled No electrons in antibonding orbitals Dr. Wolf's CHM 201 & 202 6

The Three Bonding p MOs of Benzene Dr. Wolf's CHM 201 & 202 28

Substituted Derivatives of Benzene and Their Nomenclature Dr. Wolf's CHM 201 & 202 1

1) Benzene is considered as the parent and comes last in the name. General Points 1) Benzene is considered as the parent and comes last in the name. Dr. Wolf's CHM 201 & 202 2

Examples Br C(CH3)3 NO2 Bromobenzene tert-Butylbenzene Nitrobenzene 3 Dr. Wolf's CHM 201 & 202 3

1) Benzene is considered as the parent and comes last in the name. General Points 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 Dr. Wolf's CHM 201 & 202 2

2-bromo-1-chloro-4-fluorobenzene Example Cl Br F 2-bromo-1-chloro-4-fluorobenzene Dr. Wolf's CHM 201 & 202 3

alternative locants for disubstituted derivatives of benzene 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-) Dr. Wolf's CHM 201 & 202 4

(1-ethyl-2-nitrobenzene) Examples NO2 Cl CH2CH3 o-ethylnitrobenzene m-dichlorobenzene (1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene) Dr. Wolf's CHM 201 & 202 4

Certain monosubstituted derivatives of benzene have unique names Benzene Derivatives Certain monosubstituted derivatives of benzene have unique names Dr. Wolf's CHM 201 & 202 5

Benzene Derivatives CH O Benzaldehyde Dr. Wolf's CHM 201 & 202 1 2

Benzene Derivatives COH O Benzoic acid Dr. Wolf's CHM 201 & 202 1 2

Benzene Derivatives CH2 CH Styrene Dr. Wolf's CHM 201 & 202 6

Benzene Derivatives CH3 Toluene Dr. Wolf's CHM 201 & 202 9

Benzene Derivatives CCH3 O Acetophenone Dr. Wolf's CHM 201 & 202 7

Benzene Derivatives OH Phenol Dr. Wolf's CHM 201 & 202 8

Benzene Derivatives OCH3 Anisole Dr. Wolf's CHM 201 & 202 8

Benzene Derivatives NH2 Aniline Dr. Wolf's CHM 201 & 202 1 0

Benzene derivative names can be used as parent OCH3 OCH3 NO2 Anisole p-Nitroanisole or 4-Nitroanisole Dr. Wolf's CHM 201 & 202 13

Easily confused names OH CH2— phenyl phenol benzyl 14 Dr. Wolf's CHM 201 & 202 14

Polycyclic Aromatic Hydrocarbons Dr. Wolf's CHM 201 & 202 15

resonance energy = 255 kJ/mol Naphthalene resonance energy = 255 kJ/mol most stable Lewis structure; both rings correspond to Kekulé benzene Dr. Wolf's CHM 201 & 202 16

Anthracene and Phenanthrene resonance energy: 347 kJ/mol 381 kJ/mol Dr. Wolf's CHM 201 & 202 17

Physical Properties of Arenes Dr. Wolf's CHM 201 & 202 15

Resemble other hydrocarbons nonpolar insoluble in water Physical Properties Resemble other hydrocarbons nonpolar insoluble in water less dense than water Dr. Wolf's CHM 201 & 202 2

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

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) Dr. Wolf's CHM 201 & 202 2

Reduction of Benzene Rings catalytic hydrogenation (Section 11.4) H Birch reduction (Section 11.11) H H Dr. Wolf's CHM 201 & 202 3

The Birch Reduction Dr. Wolf's CHM 201 & 202 4

Birch Reduction of Benzene Na, NH3 CH3OH (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. Dr. Wolf's CHM 201 & 202 5

Mechanism of the Birch Reduction (Figure 11.8) Step 1: Electron transfer from sodium H H • •• – + + Na+ Na • Dr. Wolf's CHM 201 & 202 6

Mechanism of the Birch Reduction (Figure 11.8) Step 2: Proton transfer from methanol H H H • •• H H – OCH3 H •• • H Dr. Wolf's CHM 201 & 202 6

Mechanism of the Birch Reduction (Figure 11.8) Step 2: Proton transfer from methanol H H H H H H • • •• H H H H – H H OCH3 H •• • H – OCH3 •• • Dr. Wolf's CHM 201 & 202 6

Mechanism of the Birch Reduction (Figure 11.8) Step 3: Electron transfer from sodium H • + Na Dr. Wolf's CHM 201 & 202 6

Mechanism of the Birch Reduction (Figure 11.8) Step 3: Electron transfer from sodium H • + Na H – H H •• + Na+ H H H H Dr. Wolf's CHM 201 & 202 6

Mechanism of the Birch Reduction (Figure 11.8) Step 4: Proton transfer from methanol • OCH3 H •• H – H H •• H H H H Dr. Wolf's CHM 201 & 202 6

Mechanism of the Birch Reduction (Figure 11.8) Step 4: Proton transfer from methanol • OCH3 H •• – • OCH3 •• H H – H H •• H H H H Dr. Wolf's CHM 201 & 202 6

Birch Reduction of an Alkylbenzene C(CH3)3 H C(CH3)3 Na, NH3 CH3OH (86%) If an alkyl group is present on the ring, it ends up as a substituent on the double bond. Dr. Wolf's CHM 201 & 202 5

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) Dr. Wolf's CHM 201 & 202 2

Free-Radical Halogenation of Alkylbenzenes Dr. Wolf's CHM 201 & 202 12

The Benzene Ring as a Substituent • C • C allylic radical benzylic radical benzylic carbon is analogous to allylic carbon Dr. Wolf's CHM 201 & 202 2

Bond-dissociation energy for C—H bond is equal to DH° for: Recall: Bond-dissociation energy for C—H bond is equal to DH° for: R—H R• + •H 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. Dr. Wolf's CHM 201 & 202 13

Bond-dissociation energies of propene and toluene H H2C CH C H C • 368 kJ/mol H2C CH -H• H C H C • 356 kJ/mol -H• Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals. Dr. Wolf's CHM 201 & 202 14

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 Dr. Wolf's CHM 201 & 202 18

Resonance in Benzyl Radical H • unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it Dr. Wolf's CHM 201 & 202 18

Resonance in Benzyl Radical H • unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it Dr. Wolf's CHM 201 & 202 18

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 Dr. Wolf's CHM 201 & 202 18

Free-radical chlorination of toluene industrial process highly regioselective for benzylic position CH3 Cl2 CH2Cl light or heat Toluene Benzyl chloride Dr. Wolf's CHM 201 & 202 16

Free-radical chlorination of toluene Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives: CHCl2 CCl3 (Dichloromethyl)benzene (Trichloromethyl)benzene Dr. Wolf's CHM 201 & 202 16

Benzylic Bromination is used in the laboratory to introduce a halogen at the benzylic position CH3 NO2 CH2Br CCl4, 80°C light + Br2 + HBr NO2 p-Nitrotoluene p-Nitrobenzyl bromide (71%) Dr. Wolf's CHM 201 & 202 22

N-Bromosuccinimide (NBS) is a convenient reagent for benzylic bromination Br NBr O CH2CH3 CHCH3 NH O CCl4 + + benzoyl peroxide, heat (87%) Dr. Wolf's CHM 201 & 202 23

Oxidation of Alkylbenzenes Dr. Wolf's CHM 201 & 202 24

Site of Oxidation is Benzylic Carbon CH3 or Na2Cr2O7 H2SO4 COH O CH2R H2O heat or CHR2 Dr. Wolf's CHM 201 & 202 25

p-Nitrobenzoic acid (82-86%) Example COH O CH3 NO2 Na2Cr2O7 H2SO4 H2O heat NO2 p-Nitrotoluene p-Nitrobenzoic acid (82-86%) Dr. Wolf's CHM 201 & 202 26

Example O CH(CH3)2 COH Na2Cr2O7 H2SO4 H2O heat CH3 (45%) 26 Dr. Wolf's CHM 201 & 202 26

SN1 Reactions of Benzylic Halides Dr. Wolf's CHM 201 & 202 24

Relative solvolysis rates in aqueous acetone What about SN1? Relative solvolysis rates in aqueous acetone C CH3 Cl C CH3 Cl 600 1 tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; therefore, more stable Dr. Wolf's CHM 201 & 202 30

Relative rates of formation: What about SN1? Relative rates of formation: CH3 + CH3 + C CH3 C more stable less stable Dr. Wolf's CHM 201 & 202 30

benzylic carbon is analogous to allylic carbon Compare. + C + C allylic carbocation benzylic carbocation benzylic carbon is analogous to allylic carbon Dr. Wolf's CHM 201 & 202 2

Resonance in Benzyl Cation 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 Dr. Wolf's CHM 201 & 202 18

Resonance in Benzyl Cation 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 Dr. Wolf's CHM 201 & 202 18

Resonance in Benzyl Cation 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 Dr. Wolf's CHM 201 & 202 18

Resonance in Benzyl Cation 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 Dr. Wolf's CHM 201 & 202 18

Solvolysis CH3 C Cl CH3CH2OH CH3 C OCH2CH3 (87%) 37 Dr. Wolf's CHM 201 & 202 37

SN2 Reactions of Benzylic Halides Dr. Wolf's CHM 201 & 202 24

Primary Benzylic Halides CH2Cl O2N NaOCCH3 O Mechanism is SN2 acetic acid CH2OCCH3 O2N O (78-82%) Dr. Wolf's CHM 201 & 202 29

Preparation of Alkenylbenzenes dehydrogenation dehydration dehydrohalogenation Dr. Wolf's CHM 201 & 202 1

Dehydrogenation industrial preparation of styrene 630°C CH2CH3 CH CH2 ZnO + H2 Dr. Wolf's CHM 201 & 202 2

Acid-Catalyzed Dehydration of Benzylic Alcohols CHCH3 OH Cl Cl KHSO4 CH2 CH heat + H2O (80-82%) Dr. Wolf's CHM 201 & 202 3

Acid-Catalyzed Dehydration of Benzylic Alcohols CHCH3 OH Cl Cl KHSO4 CH2 CH heat (80-82%) CHCH3 Cl + Dr. Wolf's CHM 201 & 202 3

Dehydrohalogenation H3C CH2CHCH3 Br NaOCH2CH3 ethanol, 50°C CH H3C (99%) Dr. Wolf's CHM 201 & 202 5

Addition Reactions of Alkenylbenzenes hydrogenation halogenation addition of hydrogen halides Dr. Wolf's CHM 201 & 202 6

Hydrogenation Br C CH3 CHCH3 Br CHCH2CH3 CH3 H2 Pt (92%) 7 Dr. Wolf's CHM 201 & 202 7

Halogenation Br2 CH2 CH CH CH2 Br Br (82%) Dr. Wolf's CHM 201 & 202 8

Addition of Hydrogen Halides Cl HCl (75-84%) Dr. Wolf's CHM 201 & 202 9

Addition of Hydrogen Halides Cl HCl + via benzylic carbocation Dr. Wolf's CHM 201 & 202 9

Free-Radical Addition of HBr CH2 CH CH2CH2Br peroxides Dr. Wolf's CHM 201 & 202 10

Free-Radical Addition of HBr CH2 CH CH2CH2Br peroxides CH CH2Br • via benzylic radical Dr. Wolf's CHM 201 & 202 10

Polymerization of Styrene Dr. Wolf's CHM 201 & 202 11

Polymerization of Styrene H2C CHC6H5 CH2 CH C6H5 polystyrene Dr. Wolf's CHM 201 & 202 12

Mechanism •• • RO H2C CHC6H5 • Dr. Wolf's CHM 201 & 202 16

Mechanism •• • RO H2C CHC6H5 • Dr. Wolf's CHM 201 & 202 16

Mechanism •• • RO H2C CHC6H5 • H2C CHC6H5 Dr. Wolf's CHM 201 & 202 16

Mechanism •• • RO H2C CHC6H5 H2C CHC6H5 • Dr. Wolf's CHM 201 & 202 16

Mechanism •• • RO H2C CHC6H5 H2C CHC6H5 • Dr. Wolf's CHM 201 & 202 16

Mechanism •• • RO H2C CHC6H5 H2C CHC6H5 • H2C CHC6H5 16 Dr. Wolf's CHM 201 & 202 16

Mechanism •• • RO H2C CHC6H5 H2C CHC6H5 H2C CHC6H5 • 16 Dr. Wolf's CHM 201 & 202 16

Mechanism H2C CHC6H5 RO •• • • Dr. Wolf's CHM 201 & 202 16

Mechanism H2C CHC6H5 RO •• • • H2C CHC6H5 Dr. Wolf's CHM 201 & 202 16

Mechanism H2C CHC6H5 RO •• • • H2C CHC6H5 Dr. Wolf's CHM 201 & 202 16

Cyclobutadiene and Cyclooctatetraene Dr. Wolf's CHM 201 & 202 1

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 3

Heats of Hydrogenation to give cyclooctane (kJ/mol) 97 205 303 410 heat of hydrogenation of cyclooctatetraene is more than 4 times heat of hydrogenation of cyclooctene Dr. Wolf's CHM 201 & 202 4

Structure of Cyclooctatetraene cyclooctatetraene is not planar has alternating long (146 pm) and short (133 pm) bonds Dr. Wolf's CHM 201 & 202 5

Structure of Cyclobutadiene structure of a stabilized derivative is characterized by alternating short bonds and long bonds C(CH3)3 (CH3)3C CO2CH3 138 pm 151 pm Dr. Wolf's CHM 201 & 202 5

Stability of Cyclobutadiene 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. 5

Requirements for Aromaticity cyclic conjugation is necessary, but not sufficient not aromatic aromatic Antiaromatic when square Antiaromatic when planar 2

Conclusion there must be some factor in addition to cyclic conjugation that determines whether a molecule is aromatic or not Dr. Wolf's CHM 201 & 202 5

Hückel's Rule: Annulenes the additional factor that influences aromaticity is the number of p electrons Dr. Wolf's CHM 201 & 202 6

Hückel's Rule among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 p electrons possess special stability (are aromatic) n 4n+2 0 2 1 6 2 10 3 14 4 18 Dr. Wolf's CHM 201 & 202 7

Hückel's Rule among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 p electrons possess special stability (are aromatic) n 4n+2 0 2 1 6 benzene! 2 10 3 14 4 18 Dr. Wolf's CHM 201 & 202 7

Hückel's Rule Hückel restricted his analysis to planar, completely conjugated, monocyclic polyenes he found that the p molecular orbitals of these compounds had a distinctive pattern one p orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highest and the lowest Dr. Wolf's CHM 201 & 202 7

6 p orbitals give 6 p orbitals p-MOs of Benzene Antibonding Benzene Bonding 6 p orbitals give 6 p orbitals 3 orbitals are bonding; 3 are antibonding Dr. Wolf's CHM 201 & 202 8

6 p electrons fill all of the bonding orbitals p-MOs of Benzene Antibonding Benzene Bonding 6 p electrons fill all of the bonding orbitals all p antibonding orbitals are empty Dr. Wolf's CHM 201 & 202 8

p-MOs of Cyclobutadiene (square planar) Antibonding Cyclo- butadiene Bonding 4 p orbitals give 4p orbitals 1 orbital is bonding, one is antibonding, and 2 are nonbonding Dr. Wolf's CHM 201 & 202 8

p-MOs of Cyclobutadiene (square planar) Antibonding Cyclo- butadiene Bonding 4 p electrons; bonding orbital is filled; other 2 p electrons singly occupy two nonbonding orbitals Dr. Wolf's CHM 201 & 202 8

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

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

p-Electron Requirement for Aromaticity 4 p electrons 6 p electrons 8 p electrons not aromatic not aromatic aromatic Dr. Wolf's CHM 201 & 202 2

Completely Conjugated Polyenes 6 p electrons; not completely conjugated 6 p electrons; completely conjugated H not aromatic aromatic Dr. Wolf's CHM 201 & 202 2

Annulenes 1

Annulenes Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule. Dr. Wolf's CHM 201 & 202 15

10-sided regular polygon has angles of 144° [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° Dr. Wolf's CHM 201 & 202 16

[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 Dr. Wolf's CHM 201 & 202 16

van der Waals strain between these two hydrogens [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 Dr. Wolf's CHM 201 & 202 16

14 p electrons satisfies Hückel's rule [14]Annulene H H H H 14 p electrons satisfies Hückel's rule van der Waals strain between hydrogens inside the ring Dr. Wolf's CHM 201 & 202 16

16 p electrons does not satisfy Hückel's rule [16]Annulene 16 p electrons does not satisfy Hückel's rule alternating short (134 pm) and long (146 pm) bonds not aromatic Dr. Wolf's CHM 201 & 202 16

18 p electrons satisfies Hückel's rule resonance energy = 418 kJ/mol [18]Annulene H 18 p electrons satisfies Hückel's rule resonance energy = 418 kJ/mol bond distances range between 137-143 pm Dr. Wolf's CHM 201 & 202 16

Aromatic Ions Dr. Wolf's CHM 201 & 202 21

Cycloheptatrienyl Cation + 6 p electrons delocalized over 7 carbons positive charge dispersed over 7 carbons very stable carbocation also called tropylium cation Dr. Wolf's CHM 201 & 202 22

Cycloheptatrienyl Cation + + H Dr. Wolf's CHM 201 & 202 22

Cycloheptatrienyl Cation Br + Br– Ionic Covalent Tropylium cation is so stable that tropylium bromide is ionic rather than covalent. mp 203 °C; soluble in water; insoluble in diethyl ether Dr. Wolf's CHM 201 & 202 23

Cyclopentadienide Anion H •• – 6 p electrons delocalized over 5 carbons negative charge dispersed over 5 carbons stabilized anion Dr. Wolf's CHM 201 & 202 22

Cyclopentadienide Anion H •• – H – Dr. Wolf's CHM 201 & 202 22

Acidity of Cyclopentadiene H H •• – H+ + pKa = 16 Ka = 10-16 Cyclopentadiene is unusually acidic for a hydrocarbon. Increased acidity is due to stability of cyclopentadienide anion. Dr. Wolf's CHM 201 & 202 22

Electron Delocalization in Cyclopentadienide Anion H •• – Dr. Wolf's CHM 201 & 202 26

Electron Delocalization in Cyclopentadienide Anion H H •• – •• – Dr. Wolf's CHM 201 & 202 26

Electron Delocalization in Cyclopentadienide Anion H H •• – •• – H • – Dr. Wolf's CHM 201 & 202 26

Electron Delocalization in Cyclopentadienide Anion H H •• – •• – H H – • • – Dr. Wolf's CHM 201 & 202 26

Electron Delocalization in Cyclopentadienide Anion H H •• – •• – H H H H – • •• • – H H H Dr. Wolf's CHM 201 & 202 26

Compare Acidities of Cyclopentadiene and Cycloheptatriene pKa = 16 Ka = 10-16 pKa = 36 Ka = 10-36 Dr. Wolf's CHM 201 & 202 27

Compare Acidities of Cyclopentadiene and Cycloheptatriene •• H •• – – Aromatic anion 6 p electrons Anion not aromatic 8 p electrons Dr. Wolf's CHM 201 & 202 27

Cyclopropenyl Cation + H H + also written as n = 0 4n +2 = 2 p electrons Dr. Wolf's CHM 201 & 202 28

Cyclooctatetraene Dianion H H H H – H •• H H H also written as 2– •• H H H H – H H H H n = 2 4n +2 = 10 p electrons Dr. Wolf's CHM 201 & 202 29

Heterocyclic Aromatic Compounds Dr. Wolf's CHM 201 & 202 30

Examples N N H O S Pyridine Pyrrole Furan Thiophene •• •• •• •• 31 Dr. Wolf's CHM 201 & 202 31

Examples N •• N • Quinoline Isoquinoline Dr. Wolf's CHM 201 & 202 31

Heterocyclic Aromatic Compounds and Hückel's Rule Dr. Wolf's CHM 201 & 202 36

Pyridine 6 p electrons in ring lone pair on nitrogen is in an sp2 hybridized orbital; not part of p system of ring N •• Dr. Wolf's CHM 201 & 202 31

Pyrrole lone pair on nitrogen must be part of ring p system if ring is to have 6 p electrons lone pair must be in a p orbital in order to overlap with ring p system N H •• Dr. Wolf's CHM 201 & 202 31

two lone pairs on oxygen Furan two lone pairs on oxygen one pair is in a p orbital and is part of ring p system; other is in an sp2 hybridized orbital and is not part of ring p system O •• Dr. Wolf's CHM 201 & 202 31

End of Chapter 11 Dr. Wolf's CHM 201 & 202 31