Chapter 11 Arenes and Aromaticity

BenzeneToluene Naphthalene Examples of Aromatic Hydrocarbons HHH HH H CH 3 H H HH H HH H HH HHH

Another Example of an Aromatic Hydrocarbon Methyl paraben Common source: gum benzoin tree American queen bee pheromone Used as a preservative in food and cosmetics.

11.1 Benzene

Some History 1825 Michael Faraday isolates a new hydrocarbon from illuminating gas Eilhardt Mitscherlich isolates same substance 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 cyclic structure of benzene Kathleen Lonsdale confirms cyclic structure of benzene by X-ray crystallography.

11.2 Kekulé and the Structure of Benzene

Kekulé proposed a cyclic structure for C 6 H 6 with alternating single and double bonds. Kekulé Formulation of Benzene HHH HH H

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

Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. Kekulé Formulation of Benzene HXX HH H HXX HH H

Structural studies of benzene demonstrate that it does not have 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.

140 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.

11.3 A Resonance Picture of Bonding in Benzene

Instead of Kekulé's suggestion of a rapid equilibrium between two structures: HHH HH H HHH HH H Kekulé 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. Resonance Formulation of Benzene HHH HH H HHH HH H

Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures). Resonance Formulation of Benzene

11.4 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.

Heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene."  H° = –208 kJ/mol Thermochemical Measures of Stability + 3H 2 Pt

208 kJ/mol 231 kJ/mol 120 kJ/mol 360 kJ/mol 3 x cyclohexene

120 kJ/mol 360 kJ/mol 3 x cyclohexene "Expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene.

208 kJ/mol 360 kJ/mol 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.

Hydrogenation of 1,3- cyclohexadiene (2H 2 ) gives off more heat than hydrogenation of benzene (3H 2 )! 231 kJ/mol 208 kJ/mol

Heat of hydrogenation = 208 kJ/mol Heat of hydrogenation = 337 kJ/mol 3H 2 Pt Pt Cyclic Conjugation versus Noncyclic Conjugation

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

11.5 An Orbital Hybridization View of Bonding in Benzene

Orbital Hybridization Model of Bonding in Benzene Planar ring of 6 sp 2 hybridized carbons

Orbital Hybridization Model of Bonding in Benzene Each carbon contributes a p orbital. Six p orbitals overlap to give cyclic  system. Six  electrons delocalized throughout  system.

Orbital Hybridization Model of Bonding in Benzene High electron density above and below plane of ring.

11.6 The  Molecular Orbitals of Benzene

Energy Bonding orbitals Antibonding orbitals Benzene MOs 6 p AOs combine to give 6  MOs. 3 MOs are bonding; 3 are antibonding.

Energy Bonding orbitals Antibonding orbitals Benzene MOs All bonding MOs are filled. No electrons in antibonding orbitals.

Benzene MOs

11.7 Substituted Derivatives of Benzene and Their Nomenclature

1) Benzene is considered as the parent and comes last in the name. General Points Bromobenzene tert-Butylbenzene Nitrobenzene NO 2 C(CH 3 ) 3 Br

1. Benzene is considered as the parent and comes last in the name. 2. Number ring in direction that gives lowest locant at first point of difference. 3. List substituents in alphabetical order. General Points

2-Bromo-1-chloro-4-fluorobenzene Example Br Cl F

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-)

Examples NO 2 CH 2 CH 3 ClCl o-Ethylnitrobenzene m-Dichlorobenzene (1-Ethyl-2-nitrobenzene)(1,3-Dichlorobenzene)

Certain Monosubstituted Derivatives of Benzene Have Unique Names

Benzaldehyde CHO

Benzoic acid COHO

Styrene CH 2 CH

Acetophenone CCH 3 O

Phenol OH

Anisole OCH 3

Aniline NH 2

Names can be used as parent OCH 3 NO 2 OCH 3 Anisole p-Nitroanisole or 4-Nitroanisole

Names can be used as parent Benzoic acid o-Acetoxybenzoic acid or 2-Acetoxybenzoic acid (Acetylsalicylic acid = Aspirin)

Easily Confused Names PhenylPhenolBenzyl OH CH 2 — A group A compound

11.8 Polycyclic Aromatic Hydrocarbons

Resonance energy = 255 kJ/mol Most stable Lewis structure; both rings correspond to Kekulé benzene. Naphthalene

Anthracene Phenanthrene Resonance energy: 347 kJ/mol 381 kJ/mol Anthracene and Phenanthrene

11.9 Physical Properties of Arenes

Arenes (aromatic hydrocarbons) resemble other hydrocarbons. They are: Nonpolar Insoluble in water Less dense than water Physical Properties

11.10 Reactions of Arenes: A Preview 1. Some reactions involve the ring. 2. In other reactions the ring is a substituent.

a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) 1. Reactions involving the ring 2. The ring as a substituent (Sections )

Catalytic hydrogenation HH HH HH Birch reduction HH H HH H HH Reduction of Benzene Rings HH H HHHH HHH H H

11.11 The Birch Reduction

1,4-Cyclohexadiene(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. Benzene

HH HH HH Step 1: Electron transfer from sodium + Na + Na + H H HHH H – Benzene Benzene anion radical Mechanism of the Birch Reduction

Step 2: Proton transfer from methanol Mechanism of the Birch Reduction H H H H H H – OCH 3 H + H H HHH H H – Cyclohexadienylradical +

Step 3: Electron transfer from sodium H H HHH H H + Na H H HHH H H + Na + – Cyclohexadienylanion Mechanism of the Birch Reduction

Step 4: Proton transfer from methanol H H H H H H H – OCH 3 H + HH H HH H HH– 1,4-Cyclohexadiene + Mechanism of the Birch Reduction

tert-Butyl-1,4-cyclohexadiene (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. tert-Butylbenzene

11.12 Free-Radical Halogenation of Alkylbenzenes

allylic radical The Benzene Ring as a Substituent C C C C benzylic radical Benzylic carbon is analogous to allylic carbon.

The more stable the free radical R, the weaker the bond, and the smaller the bond-dissociation energy. Recall: R—HRH + Bond-dissociation energy for C—H bond is equal to  H° for: and is about 400 kJ/mol for alkanes ( kJ/mol, depending on degree of substitution).

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.

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.

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.

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.

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.

Spin Density in Benzyl Radical Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

Industrial process. Highly regioselective for benzylic position. Free-Radical Chlorination of Toluene CH 3 Cl 2 light or heat CH 2 Cl Toluene Benzyl chloride

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

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 p-Nitrotoluene + HBr NO 2 CH 2 Br p-Nitrobenzyl bromide (71%)

A convenient reagent for benzylic bromination. N-Bromosuccinimide (NBS) NBr OO CCl 4 benzoylperoxide,heat CH 2 CH 3 + CHCH 3 NH O O +Br(87%)

11.13 Oxidation of Alkylbenzenes

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 H 2 O, heat Potassium permanganate (KMnO 4 ) will also oxidize the benzylic carbon.

Example 2. HCl COHO CH 3 NO 2 p-Nitrotoluene NO 2 p-Nitrobenzoic acid (82-86%) 1.KMnO 4, H 2 O, heat

Example CH(CH 3 ) 2 CH 3 (45%) COHOCOH O Na 2 Cr 2 O 7, H 2 SO 4 H 2 O, heat

11.14 & Nucleophilic Substitution in Benzylic Halides

Primary Benzylic Halides acetic acid CH 2 Cl O2NO2NO2NO2N NaOCCH 3 O CH 2 OCCH 3 O2NO2NO2NO2NO Mechanism is S N 2. (78-82%)

Both are tertiary chlorides, but benzylic carbocation is formed more rapidly than tert-butyl carbocation; it is more stable. What about S N 1? C CH 3 Cl 6201 C Cl Relative solvolysis rates in aqueous acetone.

What about S N 1? C More stable Less stable C CH 3 Relative rates of reaction determined by stabilities of the carbocation intermediates: CH 3 + +

allylic carbocation Compare... + C C C + C benzylic carbocation Benzylic carbon is analogous to allylic carbon.

Resonance in Benzyl Cation C H H HH H H H + Positive charge is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

Resonance in Benzyl Cation C H H HH H H H + Positive charge is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

Resonance in Benzyl Cation C H H HH H H H + Positive charge is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

Resonance in Benzyl Cation C H H HH H H H + Positive charge is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

Solvolysis C CH 3 Cl CH 3 CH 2 OH C CH 3 OCH 2 CH 3 (87%)

11.16 Preparation of Alkenylbenzenes DehydrogenationDehydrationDehydrohalogenation

Industrial preparation of styrene. About 13 billion lbs. produced annually in U.S. Dehydrogenation CH 2 CH 3 630°C ZnO CH 2 CH + H 2

Acid-Catalyzed Dehydration of Benzylic Alcohols KHSO 4 heat (80-82%) CH 2 CH CHCH 3 OHOHOHOHClCl Cl+

Dehydrohalogenation H3CH3CH3CH3C CH 2 CHCH 3 Br NaOCH 2 CH 3 ethanol, 50°C (99%) CH H3CH3CH3CH3C CHCH 3

HydrogenationHalogenation Addition of hydrogen halides Addition Reactions of Alkenylbenzenes

Hydrogenation H2H2H2H2Pt (92%) Br C CH 3 CHCH 3 Br CHCH 2 CH 3 CH 3

Halogenation CH 2 CH Br 2 CH 2 CH BrBr (82%)

Addition of Hydrogen Halides HCl (75-84%)Cl via benzylic carbocation +

Free-Radical Addition of HBr CH 2 CH CH 2 CH 2 Br HBr peroxides via benzylic radical CH 2 Br CH Note: In the absence of peroxides, the benzylic position would be brominated instead.

11.18 Polymerization of Styrene

Polymerization of Styrene H2CH2CH2CH2C CHC 6 H 5 CH 2 CH C6H5C6H5C6H5C6H5 CH C6H5C6H5C6H5C6H5 CH C6H5C6H5C6H5C6H5 polystyrene

Mechanism RO H2CH2CH2CH2C CHC 6 H 5

Mechanism H2CH2CH2CH2C RO H2CH2CH2CH2C

Mechanism H2CH2CH2CH2C RO H2CH2CH2CH2C

Mechanism H2CH2CH2CH2C H2CH2CH2CH2C RO H2CH2CH2CH2C

Mechanism H2CH2CH2CH2C H2CH2CH2CH2C RO H2CH2CH2CH2C

Mechanism H2CH2CH2CH2C H2CH2CH2CH2C H2CH2CH2CH2C H2CH2CH2CH2C RO

11.19 Cyclobutadiene and Cyclooctatetraene

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

To give cyclooctane (kJ/mol): Heat of hydrogenation of cyclooctatetraene is more than 4 times heat of hydrogenation of cyclooctene

Structure of Cyclooctatetraene Cyclooctatetraene is not planar. It has alternating long (146 pm) and short (133 pm) bonds.

MO calculations give alternating short and long bonds for cyclobutadiene. Structure of Cyclobutadiene H H H H 135 pm 156 pm

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

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

Cyclic conjugation is necessary, but not sufficient. Requirements for Aromaticity Not aromatic Aromatic Antiaromatic when square Antiaromatic when planar

Conclusion There must be some factor in addition to cyclic conjugation that determines whether a molecule is aromatic or not.

11.20 Hückel's Rule The additional factor that influences aromaticity is the number of  electrons.

Among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2  electrons possess special stability (are aromatic): n4n Benzene! Hückel's Rule

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

Frost's circle is a mnemonic that allows us to draw a diagram showing the relative energies of the  orbitals of a planar monocyclic conjugated system. Hückel's Rule 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  electron energy level. 4.The middle of the circle separates bonding and antibonding orbitals.

Frost's Circle  MOs of Benzene BondingAntibonding

 -MOs of Benzene Benzene Antibonding Bonding 6 p orbitals give 6  orbitals. 3 orbitals are bonding; 3 are antibonding.

Benzene Antibonding Bonding 6  electrons fill all of the bonding orbitals. All  antibonding orbitals are empty.  -MOs of Benzene

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)

Cyclo- butadiene Antibonding Bonding 4  electrons; bonding orbital is filled; other 2  electrons singly occupy two nonbonding orbitals.  -MOs of Cyclobutadiene (square planar)

Antibonding Bonding 8 p orbitals give 8  orbitals. 3 orbitals are bonding, 3 are antibonding and 2 are nonbonding.  -MOs of Cyclooctatetraene (planar) Cyclo- octatetraene

Antibonding Bonding 8  electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled.  -MOs of Cyclooctatetraene (planar) Cyclo- octatetraene

 -Electron Requirement for Aromaticity Antiaromatic Aromatic Antiaromatic 4  electrons 6  electrons 8  electrons

Completely Conjugated Polyenes Aromatic 6  electrons; completely conjugated Not aromatic 6  electrons; not completely conjugated HH

11.21 Annulenes

Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule. Annulenes

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

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 van der Waals strain between these two hydrogens.

14  electrons satisfies Hückel's rule. van der Waals strain between hydrogens inside the ring. [14]Annulene HH HH

16  electrons does not satisfy Hückel's rule. Alternating short (134 pm) and long (146 pm) bonds. Is an antiaromatic 4n  -electron system. [16]Annulene

18  electrons satisfies Hückel's rule. Resonance energy = 418 kJ/mol. Bond distances range between pm. [18]Annulene H H H H H H

11.22 Aromatic Ions

6  electrons delocalized over 7 carbons. Positive charge dispersed over 7 carbons. Very stable carbocation. Also called tropylium cation. Cycloheptatrienyl Cation HHHH HH H +

HHHH HH H + +HHHH HH H

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

Cyclopentadienyl Anion HHHH H – 6  electrons delocalized over 5 carbons. Negative charge dispersed over 5 carbons. Stabilized anion.

Cyclopentadienide Anion HHHH H –HHHH H –

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 cyclopentadienyl anion.

Electron Delocalization in Cyclopentadienyl Anion –HHHH HHHHH H – HHHH H – HH HH H – HH HH H –

Compare Acidities of Cyclopentadiene and Cycloheptatriene HHHH HH pK a = 16 K a = pK a = 36 K a = HHHHH HH HH

HHHH H – Compare Acidities of Cyclopentadiene and Cycloheptatriene Aromatic anion 6  electrons Antiaromatic anion 8  electrons HHHH HH H –

n = 0 4n + 2 = 2  electrons Cyclopropenyl Cation +HHHHHH + also written as

n = 2 4n + 2 = 10  electrons Cyclooctatetraene Dianion HHHH H H H H H H H H HHHH 2– – – also written as

11.23 Heterocyclic Aromatic Compounds

Pyridine N Examples O S N H PyrroleFuranThiophene

Quinoline Examples N N Isoquinoline Adenine

11.24 Heterocyclic Aromatic Compounds and Hückel's Rule

N Pyridine 6  electrons in ring. Lone pair on nitrogen is in an sp 2 hybridized orbital; not part of  system of ring.

Pyrrole 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

Furan 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