Chapter 11Chapter 11Arenes and AromaticityArenes and Aromaticity

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Benzene Toluene

Naphthalene

Examples of Aromatic Hydrocarbons

H

H H

HH

H

CH3

H

H

HH

H HH

H

HH

H

H H

Some history of Benzene

1825 Michael Faraday isolated a new hydrocarbon from illuminating gas.

1834 Eilhardt Mitscherlich isolated same substance and determines its empirical

formula to be CnHn. Compound comes to be called benzene.

1845 August W. von Hofmann isolated benzene from coal tar.

1866 August Kekulé proposed structure of benzene.

Kekulé (1866) proposed a cyclic structure for C6H6 with alternating single and double bonds.

The structure of Benzene: Kekulé

H

H H

HH

H

Later, Kekulé revised his proposal by suggestinga rapid equilibrium between two equivalentstructures.

Kekulé Formulation of Benzene

H

H H

HH

H

H

H

H

HH

HProposed

However, this proposal suggested isomers of thekind shown were possible. Yet, none were everfound.

Kekulé Formulation of Benzene

H

X X

HH

H

H

XX

HH

H

NeverObserved

Structural studies of benzene do not support theKekulé formulation. Instead of alternating singleand double bonds, all of the C—C bonds in benzene are the same length, 140 pm.

Structure of Benzene

Benzene has the shape of a regular hexagon.

140 pm 140 pm

140 pm 140 pm

140 pm140 pm

146 pm146 pm

134 pm134 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.

So, instead of Kekulé's suggestion of a rapidequilibrium between two structures:

H

H H

HH

H

H

H

H

HH

H

Kekulé Formulation of Benzene

Equilibriumnot valid

The structure of benzene is expressed 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

H

H H

HH

H

H

H

H

HH

HResonance model

The "circle-in-a-ring" notation is an acceptable resonance description of benzene (a hybrid of two Kekulé structures). But it does not show the Lewis structure electrons.

Resonance Formulation of Benzene

11.3The Stability of Benzene

Benzene is the best and most familiar example Benzene is the best and most familiar example of a substance that possesses "special stability" of a substance that possesses "special stability" or "aromaticity ".or "aromaticity ".

Aromaticity is a level of stability that is substantially Aromaticity is a level of stability that is substantially greater for a molecule than would be expected on greater for a molecule than would be expected on the basis of any of the Lewis structures written for it. the basis of any of the Lewis structures written for it.

Heat of hydrogenation: compare experimentalvalue with "expected" value for hypothetical"cyclohexatriene".

H°= – 208 kJ

Thermochemical Measures of Stability

+ 3H2

Pt

208 kJ/mol

231 kJ/mol

120 kJ/mol

360 kJ/mol

3 x cyclohexeneFigure 11.2 (p 433)

120 kJ/mol

360 kJ/mol 3 x cyclohexeneFigure 11.2 (p 433)

"Expected" heat of hydrogenation of benzene is 3x the heat of hydrogenation of cyclohexene.

208 kJ/mol

360 kJ/mol 3 x cyclohexeneFigure 11.2 (p 433)

The observed heat of hydrogenation is 152 kJ/mol (36 kcal/mol) less than "expected", so benzene is 152 kJ/mol more stable than expected.

This 152 kJ/mol is the resonance energy of benzene.

Hydrogenation of 1,3- cyclohexadiene (using + 2 H2) gives off more heat than hydrogenation of benzene (using + 3 H2) !

231 kJ/mol

208 kJ/mol Figure 11.2 (p 433)

heat of hydrogenation = 208 kJ/mol

heat of hydrogenation = 337 kJ/mol

3H2

Pt

3H2

Pt

Cyclic conjugation versus noncyclic conjugation

Compared to localized 1,3,5-cyclohexatriene

benzene is 152 kJ/mol more stable.

Compared to 1,3,5-hexatriene

benzene is 129 kJ/mol more stable.

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

11.4An Orbital Hybridization View

of Bonding in Benzene

Orbital Hybridization Model of Bonding in Benzene

Figure 11.3

Planar ring of 6 sp2 hybridized carbons.

Orbital Hybridization Model of Bonding in Benzene

Figure 11.3

Each carbon contributes a p orbital.Six p orbitals overlap to give cyclic system;six electrons are delocalized throughout the system.

Orbital Hybridization Model of Bonding in Benzene

Figure 11.3

High electron density above and below plane of ring.

Requirements for Aromaticity

1. The compound must be cyclic.

2. Each ring atom must have a p-orbital.

3. The ring must be planar. ( electrons are completely conjugated)

4. An additional factor that is required for a molecule to be aromatic is Huckel’s Rule. (4n + 2 electrons).

Requirements for Aromaticity

Aromatic compounds meet all of these four conditions.

A nonaromatic compound does not obey one or more of conditions 1-3.

An antiaromatic obeys conditions 1-3 but does not meet condition 4.

Annulenes: cyclic compounds that have completely alternating double bonds and may or may not be aromatic.

Definitions

Heat of hydrogenation of benzene is 152 kJ/mol, less than 3 times heat of hydrogenation of cyclohexene (resonance energy).

To give cyclohexane (kJ/mol):

Heats of Hydrogenation 120 231 208

Heats of Hydrogenation

To give cyclooctane (kJ/mol):

Heat of hydrogenation of cyclooctatetraene is roughly four times the heat of hydrogenation of cyclooctene (no stabilization).

97 205 303 410

Structure of Cyclooctatetraene

Cyclooctatetraene is not planar. It has alternating long (146 pm)

and short (133 pm) bonds.

Structure of Cyclobutadiene

H H

H H

135 pm

156 pm

MO calculations give alternating short and longbonds for cyclobutadiene (no resonance or bonds would all be equal).

The actual structure of a stabilized derivative is characterized by alternating short bonds and long bonds.

Structure of Cyclobutadiene

C(CH3)3(CH3)3C

CO2CH3(CH3)3C

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 when meeting rules 1-3.

An antiaromatic substance is one that is destabilized by cyclic conjugation.

Stability of Cyclobutadiene

Requirements for Aromaticity not

aromatic aromaticnot

aromatic

Antiaromatic when square.

Antiaromatic when planar.

11.19Huckel’s Rule

The additional factor that influences The additional factor that influences aromaticity is the number of aromaticity is the number of electrons. electrons.

Among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2 electrons possess special stability (are aromatic).

n 4n+2

0 2

1 6 Benzene!

2 10

3 14

4 18

Hückel's Rule

Hückel restricted his analysis to planar,completely conjugated, monocyclic polyenes.

He found that the molecular orbitals ofthese 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 highestand the lowest.

Hückel's Rule

-Electron Requirement for Aromaticity not

aromaticaromatic

notaromatic

4 electrons 6 electrons 8 electrons

Completely Conjugated Polyenes aromatic

6 electrons;completely conjugated.

notaromatic

6 electrons; not completely conjugated, one C lacks a p orbital.

H H

11.20Annulenes

Annulenes are planar, monocyclic, completely conjugated polyenes (alternating double and single bonds) kind of hydrocarbons treated by Hückel's rule.

They may or may not be aromatic.

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

Not planar

Incorporating two trans double bonds intothe ring relieves angle strain but introducesvan der Waals strain into the structure andcauses the ring to be distorted from planarity.

[10]Annulene

van der Waalsstrain between

these two hydrogens

Not planar

14 electrons satisfies Hückel's rule.

van der Waals strain between hydrogens inside the ring.

[14]Annulene

H H

H HAromatic

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 Antiaromatic

18 electrons satisfies Hückel's rule.

Resonance energy = 418 kJ/mol.

Bond distances range between 137-143 pm.

[18]Annulene

H HH

HH

HAromatic

11.21Aromatic Ions

It has 6 electrons (4n + 2) delocalized over 7 carbons.The positive charge is dispersed over 7 carbons.Is a very stable carbocation (aromatic) also called a tropylium cation.

Cycloheptatrienyl Cation(the tropylium ion) H H

HH

H H

H

+

+

H H

HH

H H

H

The tropylium cation is so stable that tropyliumbromide is ionic rather than covalent. mp 203°C; soluble in water; insoluble in diethyl ether

Cycloheptatrienyl Cation H Br

+ Br–

Ionic Covalent

Cyclopentadienide Anion

It has 6 electrons delocalized over 5 carbons.The negative charge is dispersed over 5 carbons.It is a stabilized anion (aromatic).

H H

H H

H

••

–

H H

H H

H

–

Acidity of Cyclopentadiene H H

H H

H H

H H

H H

H

••

–

H+ +

pKa = 16, Ka = 10-16

Cyclopentadiene is unusually acidic for a hydrocarbon.

Increased acidity is due to stability of cyclopentadienyl anion (becomes aromatic).

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

••–

••••

Resonance structures

Compare Acidities ofCyclopentadiene and Cycloheptatriene H H

H H

H H

pKa = 16

Ka = 10-16pKa = 36

Ka = 10-36

H H

HHH

H H

H H

By losing H+ itbecomes aromatic.

H H

H H

H

••

–

Compare Acidities ofCyclopentadiene and Cycloheptatriene

An aromatic anion6 electrons

An antiaromatic anion8 electrons

H H

HH

H H

H

••–

n = 0

4n +2 = 2 electrons

Cyclopropenyl Cation +

H H

H

H H

H

+also written as:

An aromatic cation.

n = 24n +2 = 10 electrons

Cyclooctatetraene Dianion H H H H

HHHH

H H H H

HHHH

2–••

••

–

–

also written as

An aromatic dianion.

11.5The Molecular Orbitals

of Benzene

Energy

Bondingorbitals

Antibondingorbitals

Benzene MOs

6 p AOs combine to give 6 MOs3 MOs are bonding; 3 are antibonding

Energy

Bondingorbitals

Antibondingorbitals

Benzene MOs

All bonding MOs are filled.No electrons in antibonding orbitals.

Benzene MOs

Figure 11.4

p 435

Frost's Circle (Polygon Rule)

MOs of Benzene

Bonding

Antibonding

Frost's Circle (Polygon Rule)

+

+

··

6 es 6 es 8 es

2 es 4 es 6 es

-MOs of Benzene

Benzene

Antibonding

Bonding6 p orbitals give 6 orbitals.

3 orbitals are bonding; 3 are antibonding.

Benzene

Antibonding

Bonding6 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; 2nonbonding orbitals are each half-filled.

-MOs of Cyclooctatetraene(Planar)

Cyclo-octatetraene

11.6Substituted Derivatives of Benzene

and their Nomenclature

1. Benzene is considered as the parent andcomes last in the name.

2. Substituents are listed in alphabetical order.

3. Number ring in direction that gives lowest numbering.

General Points on Naming

Benzene is considered as the parent and comes last in the name.

General Points on Naming Bromobenzene tert-Butylbenzene Nitrobenzene

NO2C(CH3)3Br

2-bromo-1-chloro-4-fluorobenzene

Example

Br

Cl

F

Substituents are listed in alphabetical order.

Number ring in direction that gives lowest numbering.

Ortho, Meta, and Para Can use ortho, meta and para for disubstituted derivatives of benzene.

1,2 = ortho(abbreviated o-)

1,3 = meta

(abbreviated m-)

1,4 = para

(abbreviated p-)

o-xylene m-xylene p-xylene

Examples NO2

CH2CH3

Cl

Cl

o-ethylnitrobenzene m-dichlorobenzene(1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene)

Benzaldehyde

Table 11.1

CH

O

Benzoic acid

COH

O

Certain monosubstituted derivatives of benzene have unique names.

Phenol

Table 11.1 OH

Acetophenone

CCH3

O

Styrene

CH2CH

Anisole

Table 11.1 OCH3

Aniline

NH2

OCH3

NO2

OCH3

p-Nitroanisoleor

4-Nitroanisole

Easily Confused Names

phenyl phenol benzyl OH

CH2—

a group(substituent)

a group(substituent)

a compound

11.7Polycyclic Aromatic Hydrocarbons

Napthtalene resonance energy = 255 kJ/mol (~127 kJ/mol per ring). The resonance energy per ring decreases as the number of rings increases or with fewer possible Kekule structures (benzene = 152 kJ/m).

Most stable Lewis structure;here both rings correspond to Kekulé benzene.

Naphthalene

Anthracene Phenanthrene

resonance energy:347 kJ/mol

(~116 kJ/mol per ring)381 kJ/mol

(~127 kJ/mol per ring)

Anthracene and Phenanthrene

11.8Physical Properties of Arenes

Arenes (aromatic hydrocarbons) resembleother hydrocarbons. They are:

nonpolar,

insoluble in water and

less dense than water.

Physical Properties

11.9Reactions of Arenes:

A Preview

1. Some reactions involve the ring.1. Some reactions involve the ring.

2. Some reactions involve groups attached to 2. Some reactions involve groups attached to the ring.the ring.

a) Reduction:

Catalytic hydrogenation (Section 11.3)Birch reduction (Section 11.10)

b) Electrophilic aromatic substitution:(Chapter 12)

c) Nucleophilic aromatic substitution:(Chapter 12)

1. Reactions involving the ring:

2. Reactions involving a substituent: (Sections 11.11-11.16)

Reactions of Aromatics

catalytic hydrogenation(Section 11.3)

H

H

H H

HH

Birch reduction(Section 11.10)

H H

H

H H

H

H H

Reduction of Benzene Rings

HH

HH H

H

H

HH

H H

H

High T High P Dissolving Metal

11.10The Birch Reduction

A reaction involving the ring.A reaction involving the ring.

(80%)

H

H

H H

HH

H H

H

H H

H

H H

Na, NH3

CH3OH

Birch Reduction of Benzene

Product is non-conjugated diene.Reaction stops here. There is no further reduction.

Reaction is not hydrogenation (H2 is not involved),

it is a dissolving metal reduction.

H

H

H H

HH

Step 1: Electron transfer from sodium

+ Na•

Mechanism of the Birch Reduction

+ Na+

H

H

H

H

H

H•

••–

Step 2: Proton transfer from methanol

Mechanism of the Birch Reduction

H

H

H

H

H

H

–

•

••

••

OCH3

H

••

H

H

H

H

H

H

H

•

OCH3••••••–

Step 3: Electron transfer from sodium

Mechanism of the Birch Reduction H

H

H

H

H

H

H

•+ Na•

H

H

H

H

H

H

H

••+ Na+

–

Step 4: Proton transfer from methanol

Mechanism of the Birch Reduction H

H

H

H

H

H

H

••–

H H

H

H H

H

H H

–• OCH3••

•••

• OCH3

H

•••

(86%)

H

H

H C(CH3)3

HH

H H

H

H C(CH3)3

H

H H

Na, NH3

CH3OH

Birch Reduction of an Alkylbenzene

If an alkyl group is present on the ring, it ends up asa substituent on the double bond.

11.11Free-Radical Halogenation

of Alkylbenzenes

A reaction involving an alkyl substituent.A reaction involving an alkyl substituent.

allylic radical

A Substituent on the Benzene Ring

•CC

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 (allylic is more stable than 3o, Chapter 4).

Recall:

R—H R• •H+

Bond-dissociation energy for C—H bond is equal to H° for:

and is about 400 kJ/mol for alkanes.

Bond-dissociation Energies of Propene and Toluene

368 kJ/mol

356 kJ/mol

H

H2C CH C H

H H

C H

H

H2C CH-H•

-H•

H

C

H

•

H

C

H

•

Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals.

Resonance in Benzyl Radical

CH

H

HH

H

H

H

•

Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

CH

H

HH

H

H

H•

Resonance in Benzyl Radical

CH

H

HH

H

H

H

•

Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

CH

H

HH

H

H

H

•

An industrial process that is highly regioselective for benzylic position. Bromine is commonly used in lab.

Free-radical Chlorination of Toluene CH3

Cl2

lightor

heat

CH2Cl

Toluene Benzyl chloride

Similarly, dichlorination and trichlorination areselective for the benzylic carbon. Furtherchlorination gives:

Free-radical Chlorination of Toluene CCl3

(Dichloromethyl) benzene

CHCl2

(Dichloromethyl) benzene

Br2 is used in the laboratory to introduce a halogen at the benzylic position.

Benzylic Bromination CH3

NO2

+ Br2

CCl4, 80°C

light

p-Nitrotoluene

+ HBr

NO2

CH2Br

p-Nitrobenzylbromide (71%)

As was the case in allylic bromination, NBS is a convenient reagent for benzylic bromination.

N-Bromosuccinimide (NBS) NBr

O

O

CCl4

benzoylperoxide,

heat

CH2CH3 +

CHCH3 NH

O

O

+

Br

(87%)

11.12Oxidation of Alkylbenzenes

A reaction involving an alkyl substituent.A reaction involving an alkyl substituent.

Site of Oxidation is Benzylic Carbon CH3 CH2R CHR2

or

or

COH

ONa2Cr2O7

H2SO4

H2O

heat

Example

Na2Cr2O7

H2SO4

H2O

heat

COH

O CH3

NO2

p-Nitrotoluene

NO2

p-Nitrobenzoicacid (82-86%)

Example

1. conc. KMnO4

NaOH, heat

2. H+ , HOH

CH(CH3)2

CH3

(45%)

COH

O COH

OBasic KMnO4 is also an effective oxidant for this reaction.

11.13SN1 Reactions of Benzylic Halides

Tertiary benzylic carbocation is formedmore rapidly than tertiary carbocation;therefore, more stable.

Benzylic SN1 Reactions C

CH3

CH3

Cl

620 1

C

CH3

CH3

ClCH3

Relative solvolysis rates in aqueous acetone:

Benzylic SN1 Reactions C

more stable less stable

CCH3

Relative rates of formation:

CH3

CH3

+

CH3

CH3

+

allylic carbocation

Compare

+CC

C +C

benzylic carbocation

Benzylic carbon is analogous to allylic carbon.

Resonance in Benzyl Cation CH

H

HH

H

H

H+

Positive charge is delocalized between benzylic carbon and the ring carbons that are ortho and para to it as shown by resonance.

CH

H

HH

H

H

H

+

Resonance in Benzyl Cation

CH

H

HH

H

H

H

+

Additional resonance structures give this benzylic carbocation greater stability.

CH

H

HH

H

H

H

+

Solvolysis C

CH3

CH3

Cl

CH3CH2OH

C

CH3

CH3

OCH2CH3

(87%)

Mechanism is SN1

Solvolysis of a 3o benzylic halide is faster than a normal tertiary halide.

11.14SN2 Reactions of Benzylic Halides

Primary Benzylic Halides

acetic acid

CH2Cl

O2N

NaOCCH3

O

CH2OCCH3

O2N

O

Mechanism is SN2

Like allylic halide, substitution is faster than a normal primary halide.

(78-82%)

11.15Preparation of Alkenyl Benzenes

•dehydrogenationdehydrogenation

•dehydrationdehydration

•dehydrohalogenationdehydrohalogenation

•Industrial preparation of styrene

•Almost 12 billion lbs. produced annually in U.S.

Dehydrogenation CH2CH3

630°C

ZnO

CH2CH

+ H2

Acid-Catalyzed Dehydration of Benzylic Alcohols

KHSO4

heat (80-82%)

CH2CHCHCH3

OH

Cl Cl CHCH3

Cl +

Dehydrohalogenation

H3C CH2CHCH3

Br

NaOCH2CH3 ethanol, 50°C

(99%)

CHH3C

CHCH3

11.16Addition Reactions of Alkenyl Benzenes

•hydrogenationhydrogenation

•halogenationhalogenation

•addition of hydrogen halidesaddition of hydrogen halides

Hydrogenation

H2

Pt

(92%)

Br

C

CH3

CHCH3

Br

CHCH2CH3

CH3

Halogenation CH2CH

Br2

CH2CH

BrBr

(82%)

HCl

Addition of Hydrogen Halides (75-84%)

Cl via benzylic carbocation

+

Free-Radical Addition of HBr CH2CH

CH2CH2Br

HBr

peroxides

via benzylic radical

CH2BrCH

•

11.22Heterocyclic Aromatic Compounds

Pyridine

N••

Examples

••

O••

••

S••

N

H

••

Pyrrole Furan Thiophene N••

N ••

Quinoline Isoquinoline

11.23Heterocyclic Aromatic Compounds

and Huckel’s Rule

Furan

Two lone pairs on oxygen.One pair is in a p orbital and is part of ring system; other is in an sp2 hybridized orbital and is not part of ring system.

••

O••

Pyridine N••

6 electrons in ring.Lone pair on nitrogen is in an sp2

hybridized orbital; not part of system of ring.

Pyrrole N

H

••

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.

End of Chapter 11End of Chapter 11Arenes and AromaticityArenes and Aromaticity