21-1 Benzene and and the Concept of Aromaticity Chapter 21

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Benzene andBenzene and

and the Concept ofand the Concept of

AromaticityAromaticity

Chapter 21Chapter 21

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21.1 21.1 A.A. Benzene - Kekulé Benzene - Kekulé

Discovered by Michael Faraday in 1825. (#C = #H) The first structure for benzene was proposed by

August Kekulé in 1872.

This structure, however, did not account for the unusual chemical reactivity of benzene.

CH

CH

CH

CHC

H

CH

CCC

CCC

H

HHH

HH

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Benzene Benzene

The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure.• the carbon skeleton is a regular hexagon

• all C-C-C and H-C-C bond angles 120°.

sp2-sp2

sp2-1s109 pm

120°

120°120°

139 pm

C

C

C

C

C CH

H H

H

H H

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B.B. Benzene - Molecular Orbital Model Benzene - Molecular Orbital Model

The linear combination of six overlapping p orbitals must form six molecular orbitals.

Three will be bonding, three antibonding. Lowest energy MO will have all bonding

interactions, no nodes. As energy of MO increases, the number of

nodes increases.

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Benzene - Molecular Orbital ModelBenzene - Molecular Orbital Model

Orbitals of equal energy are degenerate.Antibonding orbitals are starred (*).Electrons in the orbitals of lowest energy are in the “ground state”.

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Benzene - Molecular Orbital ModelBenzene - Molecular Orbital Model

The pi system of benzene:• (a) the carbon framework with the six 2p orbitals.

• (b) overlap of the parallel 2p orbitals forms one torus above the plane of the ring and another. below it.

• this orbital represents the lowest-lying pi-bonding molecular orbital.

Figure 21.2

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Benzene - Molecular OrbitalsBenzene - Molecular Orbitals

Viewed from the topof each carbon atom

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C.C. Benzene - Resonance Model Benzene - Resonance Model

We often represent benzene as a hybrid of two equivalent Kekulé structures.• each makes an equal contribution to the hybrid

and thus the C-C bonds are neither double nor single, but something in between.

Benzene as a hybrid of two equivalentcontributing structures

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Benzene - ResonanceBenzene - Resonance

Resonance energy:Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds.• one way to estimate the resonance energy of

benzene is to compare the heats of hydrogenation of benzene and cyclohexene.

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Benzene, Fig 21.3Benzene, Fig 21.3

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21.2 21.2 A.A. Concept of AromaticityConcept of Aromaticity

The underlying criteria for aromaticityaromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations.

To be aromatic, a compound must:1. be cyclic.

2. have one p orbital on each atom of the ring (sp2).

3. be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring.

4. have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals.

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Frost Circles (Polygon Rule)Frost Circles (Polygon Rule)

Frost circle:Frost circle: a graphic method for determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds.• inscribe a polygon of the same number of sides as

the ring to be examined such that one of the vertices is at the bottom of the ring.

• the relative energies of the MOs in the ring are given by where the vertices touch the circle

Those MOs:• below the horizontal line through the center of the

ring are bonding MOs.• on the horizontal line are nonbonding MOs.• above the horizontal line are antibonding MOs.

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Frost Circles, Fig 21.4Frost Circles, Fig 21.4

• following are Frost circles describing the MOs for monocyclic, planar, fully conjugated four-, five-, and six-membered rings.

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B.B. Aromatic Hydrocarbons Aromatic Hydrocarbons

Annulene:Annulene: a cyclic hydrocarbon with a continuous alternation of single and double bonds.• [14]annulene is aromatic according to Hückel’s

criteria.

[14]Annulene (aromatic)

HH

HH

H

H

H

H

HH

H

H

H H

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Aromatic HydrocarbonsAromatic Hydrocarbons

• [18]annulene is also aromatic.

[18]Annulene (aromatic)

H

H

HH

HH

H

H

HH

H

HH

H

HH

H

H

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• according to Hückel’s criteria, [10]annulene should be aromatic; it has been found, however, that it is not.

• nonbonded interactions between the two hydrogens that point inward toward the center of the ring force the ring into a nonplanar conformation in which overlap of the ten 2p orbitals is no longer continuous.

[10]Annulene

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• what is remarkable relative to [10]annulene is that if the two hydrogens facing inward toward the center of the ring are replaced by a methylene (CH2) group, the ring is able to assume a conformation close enough to planar that it becomes aromatic.

CH2

Bridged [10]annulene

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C.C. Antiaromatic Hydrocarbons Antiaromatic Hydrocarbons

Antiaromatic hydrocarbon:Antiaromatic hydrocarbon: a monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...), it does not obey Huckel’s rule.• an antiaromatic hydrocarbon is especially unstable

relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms.

Cyclobutadiene is antiaromatic.• in the ground-state electron configuration of this

molecule, two electrons fill the 1 bonding MO.

• the remaining two electrons lie in the 2 and 3 nonbonding MOs.

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Cyclobutadiene, Fig. 21.5Cyclobutadiene, Fig. 21.5

• planar cyclobutadiene has two unpaired electrons, which make it highly unstable and reactive.

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CyclooctatetraeneCyclooctatetraene

• cyclooctatetraene, with 8 pi electrons is not aromatic; it shows reactions typical of alkenes.

• x-ray studies show that the most stable conformation is a nonplanar “tub’ conformation.

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CyclooctatetraeneCyclooctatetraene

2p orbital overlap forms each pi bond, there is essentially no overlap between adjacent alkenes.

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Cyclooctatetraene, Fig. 21.6Cyclooctatetraene, Fig. 21.6

• planar cyclooctatetraene, if it existed, would be antiaromatic.

• it would have unpaired electrons in the 4 and 5 nonbonding MOs.

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D.D. Heterocyclic Aromatics Heterocyclic Aromatics

Heterocyclic compound:Heterocyclic compound: a compound that contains more than one kind of atom in a ring. • in organic chemistry, the term refers to a ring with

one or more atoms are other than carbon. Pyridine and pyrimidine are heterocyclic analogs

of benzene; each is aromatic.

Pyridine

N

N

N••••

• •

Pyrimidine

12

3

4

5

6

12

3 4

5

6

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PyridinePyridine

• the nitrogen atom of pyridine is sp2 hybridized.

• the unshared pair of electrons lies in an sp2 hybrid orbital and is not a part of the six pi electrons of the aromatic system.

• pyridine has a resonance energy of 134 kJ (32 kcal)/mol, slightly less than that of benzene.

N•

•

•

•

•

•

this orbital is perpendicularto the six 2p orbitals of thepi system

this pair of electronsis not a part of the 4n + 2 pi electrons

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Furan, Fig. 21.7Furan, Fig. 21.7

• the oxygen atom of furan is sp2 hybridized.

• one unshared pairs of electrons on oxygen lies in an unhybridized 2p orbital and is a part of the aromatic sextet.

• the other unshared pair lies in an sp2 hybrid orbital and is not a part of the aromatic system.

• the resonance energy of furan is 67 kJ (16 kcal)/mol.

O•

this pair of electronsis not

•

• •

this pair of electronsis a part of the 4n + 2 pi electrons

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Other HeterocyclicsOther Heterocyclics

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Other HeterocyclicsOther Heterocyclics

Indole

N

H

N

H

CH2CH2NH2

Serotonin(a neurotransmitter)

HO

Caffeine

N

NN

N

O

O

H3CCH3

CH3Purine

N

NN

N

H

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E.E. Aromatic Hydrocarbon Ions Aromatic Hydrocarbon Ions

Any neutral, monocyclic unsaturated hydrocarbon with an odd number of carbons must have at least one CH2 group and, therefore, cannot be aromatic.

• cyclopropene, for example, has the correct number of pi electrons to be aromatic, 4(0) + 2 = 2, but does not have a closed loop of 2p orbitals.

Cyclopropene Cyclopentadiene Cycloheptatriene

CH2 CH2CH2

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Cyclopropenyl CationCyclopropenyl Cation

• if, however, the CH2 group of cyclopropene is transformed into a CH+ group in which carbon is sp2 hybridized and has a vacant 2p orbital, the overlap of orbitals is continuous and the cation is aromatic.

Cyclopropenyl cation represented as a hybrid of three equivalent contributing structures

+

H

H

H

H

H

H

H

H

H+

+

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Cyclopropenyl CationCyclopropenyl Cation

• when 3-chlorocyclopropene is treated with SbCl5, it forms a stable salt.

• this chemical behavior is to be contrasted with that of 5-chloro-1,3-cyclopentadiene, which cannot be made to form a stable salt.

H

ClSbCl5 H SbCl6

-+

Cyclopropenyl hexachloroantimonate

+

3-Chloro-cyclopropene

Antimony(V) chloride(a Lewis acid)

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Cyclopentadienyl CationCyclopentadienyl Cation

• if planar cyclopentadienyl cation existed, it would have 4 pi electrons and be antiaromatic.

• note that we can draw five equivalent contributing structures for the cyclopentadienyl cation; yet this cation is not aromatic because it has only 4 pi electrons.

Cyclopentadienyltetrafluoroborate

++

5-Chloro-1,3-cyclopentadiene

H

ClHAgBF4 BF4

- + AgCl

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Cyclopentadienyl AnionCyclopentadienyl Anion

To convert cyclopentadiene to an aromatic ion, it is necessary to convert the CH2 group to a CH group in which carbon becomes sp2 hybridized and has 2 electrons in its unhybridized 2p orbital.

••

• • H

HH

H

H

the origin of the 6 pi electronsin the cyclopentadienyl anion

Cyclopentadienyl anion (aromatic)

HH

HH

H:

H

HH

HH

pKa = ~16

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• as seen in the Frost circle, the six pi electrons occupy the 1, 2, and 3 molecular orbitals, all of which are bonding.

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The pKa of cyclopentadiene is 16.

• in aqueous NaOH, it is in equilibrium with its sodium salt.

• it is converted completely to its anion by very strong bases such as NaNH2 , NaH, and LDA.

pKa 15.7pKa 16.0

Na+ + H2OH

H

H

H

H

CH2 + NaOH :

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MOs of Aromatic IonsMOs of Aromatic Ions

Cyclopropenyl cation and cyclopentadienyl anion.

cyclopropenyl cation cyclopentadienyl anion

+ +

+ ++

+ +

+

+

+++

+ ++

-

-

--

-- -

+

+

-

-+ +

+

- -

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pKa's of some hydrogens pKa's of some hydrogens Compound pKa Compound pKa Compound pKa

CH3CO2H 5 HOH 16 CH3-CO-NH2 25

NC-CH2-CO2CH2CH3 9 CH3CH2-OH 16 Ph-NH2 30

CH3-CO-CH2-CO-CH3 9 CH3-CHO 17 CH3-CO-N(CH3)2 30

Ph-CO-CH2-CO-CH3 9 (CH3)2-CH-OH 18 NH3 35

Ph-OH 10 (CH3)3-C-OH 19 Ph3-CH 40

CH3NO2 10 Ph-CO-CH3 19 CH3SOCH3 40

CH3CO-CH2-CO2CH2CH3 11 CH3COCH3 20 (CH3CH2)2-NH ~40

NC-CH2-CN 11 CH3SO2CH3 23 (isopropyl)2-NH ~40

CH3CH2O2C-CH2-CO2CH2CH3 13 CH3-CO2CH2CH3 24 CH3-CH=CH2 43

CH2- of cyclopentadiene 15 CH3CO2- 24 Ph-CH3 ~50

CH3-OH 16 CH3-CN 25 CH3CH2CH2CH3 ~50

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Cycloheptatrienyl CationCycloheptatrienyl Cation

Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with its sp2 carbon having a vacant 2p orbital.

+

Cycloheptatrienyl cation (Tropylium ion) (aromatic)

H

HH

H

H

HH

H

HH

H

H

HH

+

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21.3 21.3 A.A. NomenclatureNomenclature

Monosubstituted alkylbenzenes are named as derivatives of benzene.• many common names are retained.

Toluene CumeneEthylbenzene Styrene

Phenol Aniline Benzoic acid Anisole

COOHNH2 OCH3OH

Benzaldehyde

CHO

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NomenclatureNomenclature

Benzyl and phenyl groups.

(Z)-2-Phenyl-2-butene

4-(3-Methoxyphenyl)-2-butanone

1-Phenyl-1pentanone

O OH3CO

Ph

Benzene Phenyl group, Ph- Toluene Benzyl group, Bn-

CH3 CH2-

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B.B. Disubstituted Benzenes Disubstituted Benzenes

Locate two groups by numbers or by the locators orthoortho (1,2-), metameta (1,3-), and parapara (1,4-).• where one group imparts a special name, name

the compound as a derivative of that molecule.

CH3

Br

COOHNO2

Cl

NH2

CH3

CH3

2-Nitrobenzoic acid

(o-Nitrobenzoic acid)

3-Chloroaniline(m-Chloroaniline)

4-Bromotoluene(p-Bromotoluene)

m-Xylene

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Disubstituted BenzenesDisubstituted Benzenes

• where neither group imparts a special name, locate the groups and list them in alphabetical order.

CH2CH3

Cl

NO2Br

1-Bromo-2-nitrobenzene (o-Bromonitrobenzene)

1-Chloro-4-ethylbenzene (p-Chloroethylbenzene)

12

3

4 2

1

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C.C. Polysubstituted Derivatives Polysubstituted Derivatives

• if one group imparts a special name, name the molecule as a derivative of that compound.

• if no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers.

CH3

NO2

OHBr

Br

NO2

CH2CH3

Br

4

2

1

6

4

21

4

1 2

4-Chloro-2-nitro-toluene

2,4,6-Tribromo-phenol

2-Bromo-1-ethyl-4-nitrobenzene

Br

Cl

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21.4 21.4 A.A. PhenolsPhenols

The functional group of a phenolphenol is an -OH group bonded to a benzene ring.

1,2-Benzenediol(Catechol)

1,4-Benzenediol(Hydroquinone)

3-Methylphenol(m-Cresol)

Phenol

OH OHOHOH

OHCH3

OH

1,3-Benzenediol is resorcinol

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PhenolsPhenols

• hexylresorcinol is a mild antiseptic and disinfectant.

• eugenol is used as a dental antiseptic and analgesic.

• urushiol is the main component of the oil of poison ivy.

OH

OH

OHOCH3 OH

OH

Hexylresorcinol Eugenol Urushiol

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B.B. Acidity of Phenols Acidity of Phenols

Phenols are significantly more acidic than alcohols, compounds that also contain the OH group.

OH H2O

CH3CH2OH H2O

O-

CH3CH2O-

H3O+

H3O+

pKa = 9.95+

+

+

+ pKa = 15.9

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Acidity of PhenolsAcidity of Phenols

• the greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.

These 2 Kekuléstructures areequivalent

HH

OO O O

H

O

These three contributing structuresdelocalize the negative chargeonto carbon atoms of the ring

H

OO O O

H

O

21-21-4747


Alkyl and halogen substituents effect acidities by inductive effects.• alkyl groups are electron-releasing.

• halogens are electron-withdrawing.

p-ChororophenolpKa 9.18

m-ChlorophenolpKa 8.85

PhenolpKa 9.95

m-CresolpKa 10.01

p-CresolpKa 10.17

OH OH OH OH OH

CH3

CH3

ClCl

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• nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.

OH

NO2

OH OH

NO2PhenolpKa 9.95

p-NitrophenolpKa 7.15

m-NitrophenolpKa 8.28

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• part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect.

• in addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge.

+ +

delocalization of negativecharge onto oxygen furtherincreases the resonancestabilization of phenoxide ion

O

O ON

O

O

NO

– – –

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C.C. Acid-Base Reactions of Phenols Acid-Base Reactions of Phenols

Phenols are weak acids and react with strong bases to form water-soluble salts.• water-insoluble phenols dissolve in NaOH(aq).

OH NaOH O- Na+ H2O

PhenolpKa 9.95

(stronger acid)

Sodium hydroxide

Sodiumphenoxide

WaterpKa 15.7

(weaker acid)

+ +

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Acid-Base Reactions of PhenolsAcid-Base Reactions of Phenols

• most phenols do not react with weak bases such as NaHCO3; they do not dissolve in aqueous NaHCO3

OH NaHCO3 O- Na+H2CO3

PhenolpKa 9.95

(weaker acid)

Sodium bicarbonate

Sodiumphenoxide

Carbonic acidpKa 6.36

(stronger acid)

+ +Noreaction

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D.D. Alkyl-Aryl Ethers Alkyl-Aryl Ethers

Alkyl-aryl ethers can be prepared by the Williamson ether synthesis.• but only using phenoxide salts and haloalkanes.

• haloarenes are unreactive to SN2 reactions.

The following two examples illustrate:• the use of a phase-transfer catalyst.

• the use of dimethyl sulfate as a methylating agent.

no reaction+X RO-Na+

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Alkyl-Aryl EthersAlkyl-Aryl Ethers

OH CH2=CHCH2ClNaOH, H2O, CH2Cl2

Bu4N+Br -

OCH2CH=CH2

Phenyl 2-propenyl ether(Allyl phenyl ether)

+

Phenol 3-Chloropropene(Allyl chloride)

OH

O

O

CH3OSOCH3NaOH, H2O, CH2Cl2

Bu4N+Br -

OCH3 Na2SO4

Methyl phenyl ether(Anisole)

+

Phenol Dimethyl sulfate+

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E.E. Kolbe Carboxylation Kolbe Carboxylation

Phenoxide ions react with carbon dioxide to give a carboxylic salt.

OH

NaOHH2O

O-Na

+

CO2

H2O

OHCO

-Na

+O

HClH2O

OH O

COH

Phenol Sodiumphenoxide

Sodium salicylate Salicylic acid

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Kolbe CarboxylationKolbe Carboxylation

• the mechanism begins by nucleophilic addition of the phenoxide ion to a carbonyl group of CO2.

O

C

O

O

OC

H

OO OH

CO

O

A cyclohexadienoneintermediate

+

Sodium phenoxide

Salicylate anion

keto-enoltautomerism

(1) (2)

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F.F. Quinones Quinones

Because of the presence of the electron-donating -OH group, phenols are susceptible to oxidation by a variety of strong oxidizing agents.

QuinonesQuinones are six-membered rings with two C=O.

H2 CrO4

Phenol 1,4-Benzoquinone(p-Quinone)

O

O

OH

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QuinonesQuinones

OHOH K2Cr2O7

OH

OH

H2SO4

K2Cr2O7

H2SO4

O

O

OO

1,4-Benzoquinone (p-Quinone)

1,2-Benzenediol (Catechol)

1,2-Benzoquinone (o-Quinone)


21-21-5858

QuinonesQuinones

Perhaps the most important chemical property of quinones is that they are readily reduced to hydroquinones by sodium hydrosulfite.

1,4-Benzoquinone(p-Quinone)

(reduction)


O

O

OH

OH

Na2S2O4, H2O

21-21-5959

Coenzyme QCoenzyme Q

Coenzyme Q is a carrier of electrons in the respiratory chain.

O

O

MeO

HMeO MeO

MeO

OH

OH

Hn n

Coenzyme Q(oxidized form)

Coenzyme Q(reduced form)

reduction

oxidation

21-21-6060

Vitamin KVitamin K

• both natural and synthetic vitamin K (menadione) are 1,4-naphthoquinones.

CH3 CrO3

H

CH3

O

O

CH3

O

O

oxidation

2-Methylnaphthalene 2-Methyl-1,4-naphthoquinone(Menadione)

Vitamin K2

7

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21.5 21.5 A.A. Benzylic OxidationBenzylic Oxidation

Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4.

• halogen and nitro substituents are also unaffected by these reagents.

• an alkyl group with at least one hydrogen on its benzylicbenzylic carboncarbon is oxidized to a carboxyl group.

2-Chloro-4-nitrotoluene 2-Chloro-4-nitrobenzoic acid

H2CrO4

O2N Cl

CH3

O2N Cl

COOH

21-21-6262

Benzylic OxidationBenzylic Oxidation

• if there is more than one alkyl group on the benzene ring, each is oxidized to a -COOH group.

• an aryl-COOH is the oxidation product regardless of the alkyl group that was attached to the aromatic ring (may be Me, Et, Pr, Bu, vinyl, etc.).

1,4-Dimethylbenzene (p-xylene)

1,4-Benzenedicarboxylic acid (terephthalic acid)

CH3 H2SO4

K2Cr2O7H3C COH

O

HOCO

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B.B. Benzylic Chlorination Benzylic Chlorination

Chlorination (and bromination) occurs by a radical mechanism.

CH3

Cl2+

CH2Cl

HCl+

Toluene

heat or light

Benzyl chloride

(PhCO2)2, CCl4

NBS

Br

Ethylbenzene 1-Bromo-1-phenylethane(racemic)

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Benzylic ReactionsBenzylic Reactions

Benzylic radicals (and cations also) are easily formed because of the resonance stabilization of these intermediates.• the benzyl radical is a hybrid of five contributing

structures.

C

C

C

C C

21-21-6565

Benzylic HalogenationBenzylic Halogenation

• benzylic bromination is highly regioselective.

• benzylic chlorination is less regioselective.

(PhCO2)2, CCl4

NBS

Br

Ethylbenzene 1-Bromo-1-phenylethane(the only product formed)

Cl

Cl2 +

heat or light

1-Chloro-2-phenylethane

(10%)

Ethylbenzene

+

1-Chloro-1-phenylethane

(90%)

Cl

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C.C. Hydrogenolysis Hydrogenolysis

Hydrogenolysis:Hydrogenolysis: Cleavage of a single bond by H2:

• among ethers, benzylic ethers are unique in that they are cleaved under conditions of catalytic hydrogenation.

O H2Pd/C

OHMe

+

Benzyl butyl ether Toluene1-Butanol

+

this bondis cleaved

21-21-6767

Benzyl EthersBenzyl Ethers

Benzyl ethers are used as protecting groups for the OH groups of alcohols and phenols.• to carry out hydroboration/oxidation of this

alkene, the phenolic -OH must first be protected; it is acidic enough to react with BH3 and destroy the reagent.

OH

1. ClCH2Ph

O Ph

2. BH3•THF

Et3N 3. H2O2/NaOH

H2

Pd/CO Ph

OH

OH

OH

2-(3-Hydroxypropyl)phenol

2-(2-Propenyl)phenol(2-Allylphenol)

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End of Chapter 21End of Chapter 21

Benzene andBenzene andand the Concept ofand the Concept ofAromaticityAromaticity

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21-1 Benzene and and the Concept of Aromaticity Chapter 21