Download pdf - 21 Benzene

7/27/2019 21 Benzene

1/62

21-1

Chapter 21, Benzeneand and the Concept of

Aromaticity

7/27/2019 21 Benzene

2/62

21-2

Benzene - Kekul

In 1872, August Kekul proposed the following

structure for benzene.

This structure, however, did not account for the

unusual chemical reactivity of benzene.

CH

CH

CH

CHC

H

C

HC

C

CC

C

C

H

HHH

HH

7/27/2019 21 Benzene

3/62

21-3

Benzene - Resonance

We often represent benzene as a hybrid of two

equivalent Kekul structures. Each makes an equal contribution to the hybrid and

thus the C-C bonds are neither double nor single, butsomething in between.

Benzene as a hybrid of two equ ivalen tcontributing structures

7/27/2019 21 Benzene

4/62

21-4

Benzene - Resonance Model

The concepts of hybridization of atomic orbitals

and the theory of resonance, developed in the1930s, provided the first adequate description ofbenzenes structure.

The carbon skeleton is a planar regular hexagon.

All C-C-C and H-C-C bond angles 120.

sp2-sp

2

sp2-1s109 pm

120

120

120

139 pm

C

C

C

C

C

C H

H H

H

H H

7/27/2019 21 Benzene

5/62

21-5

The Pi System of Benzene

(a) The carbon framework with the six 2p orbitals.

(b) Overlap of the parallel 2p orbitals forms one torusabove the plane of the ring and another below it

this orbital represents the lowest-lying pi-bondingmolecular orbital.

7/27/2019 21 Benzene

6/62

21-6

Benzene-Molecular Orbital Model

The molecular orbital representation of the pi

bonding in benzene.

7/27/2019 21 Benzene

7/62

21-7

Orbitals of the pi System of BenzeneNumber ofnodal surfaces

0

1

2

3

7/27/2019 21 Benzene

8/62

21-8

Benzene - Resonance

Resonance energy:The difference in energy

between a resonance hybrid in which theelectrons are delocalized

and

the most stable one of its hypotheticalcontributing structures in which electrons arelocalized on particular atoms and in particularbonds.

One way to estimate the resonance energy of benzeneis to compare the heats of hydrogenation of benzeneand cyclohexene.

7/27/2019 21 Benzene

9/62

21-9

Benzene- Resonance Energy

Experimentaldata

Model

7/27/2019 21 Benzene

10/62

21-10

Concept of Aromaticity

The underlying criteria for aromaticity were

recognized in the early 1930s by Erich Hckel,based on molecular orbital (MO) calculations.

To be aromatic, a compound must

Be cyclic. Have one p orbital on each atom of the ring.

Be planar or nearly planar so that there is continuousor nearly continuous overlap of all p orbitals of the

ring. Have a closed loop of (4n + 2) pi electrons in the cyclic

arrangement of p orbitals.

7/27/2019 21 Benzene

11/62

21-11

Frost Circles

Frost circle: A graphic method for determining

the relative order of pi MOs in planar, fullyconjugated monocyclic compounds.

Inscribe in a circle a polygon of the same number ofsides as the ring to be examined such that one of the

vertices of the polygon is at the bottom of the circle.

The relative energies of the MOs in the ring are givenby where the vertices of the polygon touch the circle.

Those MOs

Below the horizontal line through the center of the ringare bonding MOs.

on the horizontal line are nonbonding MOs.

above the horizontal line are antibonding MOs.

7/27/2019 21 Benzene

12/62

21-12

Frost Circles

Frost circles describing the MOs for monocyclic,

planar, fully conjugated four-, five-, and six-memberedrings.

7/27/2019 21 Benzene

13/62

21-13

Relationship of hexa-1,3,5-triene to benzene

How does the linear triene relate

to benzene?

7/27/2019 21 Benzene

14/62


?

R l i hi f h 3 i b

7/27/2019 21 Benzene

15/62


?

Look at orbitals 2 and 3.

p2

p3

Curvearound

Antibonding,destabilizing

Bonding,stabilizing

7/27/2019 21 Benzene

16/62

21-16

Aromatic Hydrocarbons

Annulene:A cyclic hydrocarbon with a

continuous alternation of single and doublebonds.

[14]Annulene is aromatic according to Hckels

criteria.

[14]Annulene

(aromatic)

HH

H H

H

H

H

H

HH

H

H

H H

n = 3

7/27/2019 21 Benzene

17/62

21-17


[18]Annulene is also aromatic.

[18]Annulene

(aromatic)

H

H

H

H

H

H

H

H

HH

H

H

H

H

H

H

H

H

n = 4

7/27/2019 21 Benzene

18/62

21-18


According to Hckels 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 forcethe ring into a nonplanar conformation in which

overlap of the ten 2p orbitals is no longer continuous.

[10]Annulene

7/27/2019 21 Benzene

19/62

21-19


What is remarkable relative to [10]annulene is that if

the two hydrogens facing inward toward the center ofthe ring are replaced by a methylene (CH2) group, thering is able to assume a conformation close enough toplanar that it becomes aromatic.

CH2

Bridged [10]annulene

7/27/2019 21 Benzene

20/62

21-20

Antiaromatic Hydrocarbons

Antiaromatic hydrocarbon:A monocyclic, planar,

fully conjugated hydrocarbon with 4n pielectrons (4, 8, 12, 16, 20...).

An antiaromatic hydrocarbon is especially unstablerelative to an open-chain fully conjugated hydrocarbon

of the same number of carbon atoms.

Cyclobutadiene is antiaromatic.

In the ground-state electron configuration of thismolecule, two electrons fill the p

1

bonding MO.

The remaining two electrons lie in the p2 and p3nonbonding MOs.

7/27/2019 21 Benzene

21/62

21-21

Cyclobutadiene

The ground state of planar cyclobutadiene has two

unpaired electrons, which make it highly unstable andreactive.

7/27/2019 21 Benzene

22/62

21-22

Cyclooctatetraene

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.

Although overlap of 2p orbitals occurs to form pi

bonds, there is only minimal overlap between sets of2p orbitals because they are not parallel.

viewed from above viewed through an edge

7/27/2019 21 Benzene

23/62

21-23

Cyclooctatetraene

MO energy diagram for a planar conformation of

cyclooctatetraene.

7/27/2019 21 Benzene

24/62

21-24

Heterocyclic Aromatics

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 that differ from carbon.

Pyridine and pyrimidine are heterocyclic analogsof benzene; each is aromatic.

Pyridine

N

N

N

Pyrimidine

1

2

3

4

5

6

12

3 4

5

6

7/27/2019 21 Benzene

25/62

21-25

Pyridine

The nitrogen atom of

pyridine is sp2

hybridized.

The unshared pair ofelectrons lies in an sp2

hybrid orbital and is nota part of the six pielectrons of thearomatic system (thearomatic sextet).

Resonance energy ofpyridine is134 kJ (32kcal)/mol.

7/27/2019 21 Benzene

26/62

21-26

Furan and Pyrrole

The oxygen atom of furan is sp2 hybridized.

one unshared pairs of electrons on oxygen lies in anunhybridized 2p orbital and is a part of the aromaticsextet.

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.

7/27/2019 21 Benzene

27/62

21-27

Other Heterocyclics

Purine

Indole

N

N

NN

N

H

H

N

H

CH2 CH2 NH 2

Serotonin(a neurotransmitter)

HO

Caffeine

N

NN

N

O

O

H3 C

CH3

CH3

7/27/2019 21 Benzene

28/62

21-28

Aromatic Hydrocarbon Ions

Any neutral, monocyclic, unsaturated

hydrocarbon with an odd number of carbonsmust 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 nothave a closed loop of 2p orbitals.

Cyclopropene Cyclopentadiene Cycloheptatriene

CH2 CH2CH2

7/27/2019 21 Benzene

29/62

21-29

Cyclopropenyl 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 oforbitals is continuous and the cation is aromatic.

Cyclopropenyl cation represented as a hybridof three equ ivalen t contributin g structures

+

H

H

H

H

H

H

H

H

H+

+

7/27/2019 21 Benzene

30/62

21-30

Cyclopropenyl Cation

When 3-chlorocyclopropene is treated with SbCl5, it

forms a stable salt.

This chemical behavior is to be contrasted with that of5-chloro-1,3-cyclopentadiene, which cannot be made

to form a stable salt.

+

Cyclopropenyl

hexachloroantimonate

+

3-Chloro-

cyclopropene

H

H

ClSbCl5 SbCl6

-

Antimony(V)

chloride(a Lewis acid)

7/27/2019 21 Benzene

31/62

21-31

Cyclopentadienyl Cation

If planar cyclopentadienyl cation were to exist, it wouldhave 4 pi electrons and be antiaromatic.

Note that we can draw five equivalent contributingstructures for the cyclopentadienyl cation. Yet this

cation is not aromatic because it has only 4 pielectrons.

Cyclope ntadienylte trafluoroborate

++

5-Chloro-1,3-cyclopentadiene

H

ClHAgBF4 BF4 - + AgCl

C C

7/27/2019 21 Benzene

32/62

21-32

Cyclopentadienyl Anion, C5H5-

To convert cyclopentadiene to an aromatic ion, it

is necessary to convert the CH2 group to a CHgroup in which carbon becomes sp2 hybridizedand has 2 electrons in its unhybridized 2p orbital.

H

HH

HH

the origin of the 6 pi electronsin the cyclopentadienyl anion

Cyclopentadienyl anion(aromatic)

HH

HH

H:

H

H

H

HH

n = 1

C l di l A i C H

7/27/2019 21 Benzene

33/62

21-33


As seen in the Frost circle, the six pi electrons of

cyclopentadienyl anion occupy the p1, p2, and p3molecular orbitals, all of which are bonding.

C l di l A i C H

7/27/2019 21 Benzene

34/62

21-34


The pKa of cyclopentadiene is 16.

In aqueous NaOH, it is in equilibrium with its sodiumsalt.

It is converted completely to its anion by very strongbases such as NaNH2 , NaH, and LDA.

pKa15.7pKa16.0

Na+

+ H2OH

H

H

H

H

CH2 + NaOH :

C l h i l C i C H

7/27/2019 21 Benzene

35/62

21-35

Cycloheptatrienyl Cation, C7H7+

Cycloheptatriene forms an aromatic cation by

conversion of its CH2 group to a CH+ group withits sp2 carbon having a vacant 2p orbital.

+

Cyclohep tatrienyl cation(Tropylium ion)

(aromatic)

H

HH

H

H

HH

H

HH

H

H

HH

+

N l t

7/27/2019 21 Benzene

36/62

21-36

Nomenclature

Monosubstituted alkylbenzenes are named as

derivatives of benzene. Many common names are retained.

Toluene CumeneEthylbe nzene Styrene

Phenol Aniline Benzoic acid Anisole

COOHNH2 OCH3OH

Benzaldehyde

CHO

N l t

7/27/2019 21 Benzene

37/62

21-37

Nomenclature

Benzyl and phenyl groups

(Z)-2-Phenyl-

2-butene

4-(3-Me thoxyphenyl)-

2-butanone

1-Phenyl-1-pentanone

O OH3CO

Ph

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

CH3 CH2

Di b tit t d B

7/27/2019 21 Benzene

38/62

21-38

Disubstituted Benzenes

Locate two groups by numbers or by the

locators ortho (1,2-), meta (1,3-), and para (1,4-). Where one group imparts a special name, name the

compound as a derivative of that molecule.

CH3

Br

COOH

NO2

Cl

NH2

CH3

CH3

2-Nitrobenzoic acid

(o-Nitrobenzoic acid )

3-Chloroaniline

(m-Chloroaniline)

4-Bromotoluene

(p-Bromotoluene)

m-Xylene

Di b tit t d B

7/27/2019 21 Benzene

39/62

21-39

Disubstituted Benzenes

Where neither group imparts a special name, locate

the groups and list them in alphabetical order.CH2 CH3

Cl

NO2Br

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

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

1

2

3

4 2

1

P l b tit t d D i ti

7/27/2019 21 Benzene

40/62

21-40

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 ofnumbers.

CH3

NO2

OH

Br

Br

NO2

CH2 CH3

Br

4

2

1

6

4

21

4

1 2

4-Chloro-2-nitro-

toluene

2,4,6-Tribromo-

phenol2-Bromo-1-ethyl-4-

nitrobenzene

Br

Cl

Ph l

7/27/2019 21 Benzene

41/62

21-41

Phenols

The functional group of a phenol 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

Acidit of Phenols

7/27/2019 21 Benzene

42/62

21-43

Acidity of Phenols

Phenols are significantly more acidic than

alcohols.

OH H2 O

CH3CH2 OH H2 O

O-

CH3CH2 O-

H3 O+

H3 O+

pKa = 9.95+

+

+

+ pKa = 15.9

Acidity of Phenols

7/27/2019 21 Benzene

43/62

21-44

Acidity of Phenols

Separation of water-

insoluble phenolsfrom water-insolublealcohols.

Acidity of Phenols (Resonance)

7/27/2019 21 Benzene

44/62

21-45

Acidity of Phenols (Resonance)

The greater acidity of phenols compared with alcohols

is due to the greater stability of the phenoxide ionrelative to an alkoxide ion.

These 2 Kekulstructures are

equivalent

HH

OO O O

H

O

These three contrib uting s tructuresdelocalize the negative charge

onto carbon atoms of the rin g

H

OO O O

H

O

Phenol Subsitituents (Inductive Effect)

7/27/2019 21 Benzene

45/62

21-46

Phenol Subsitituents (Inductive Effect)

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 9.95m-

CresolpKa 10.01p-

CresolpKa 10.17

OH OH OH OH OH

CH3CH3

Cl

Cl

Phenol Subsitituents(Resonance Inductiion)

7/27/2019 21 Benzene

46/62

21-47

Phenol Subsitituents(Resonance, Inductiion)

Nitro groups increase the acidity of phenols by both an

electron-withdrawing inductive effect and a resonanceeffect.OH

NO2

OH OH

NO2PhenolpKa 9.95

p-NitrophenolpKa 7.15

m-NitrophenolpKa 8.28

Acidity of Phenols

7/27/2019 21 Benzene

47/62

21-48

Acidity of Phenols

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 negative

charge onto oxygen furtherincreases the resonancestabilization of phenoxide ion

O

O O

N

O

O

NO

Synthesis: Alkyl Aryl Ethers

7/27/2019 21 Benzene

48/62

21-51

Synthesis: Alkyl-Aryl Ethers

Alkyl-aryl ethers can be prepared by the

Williamson ether synthesis: but only using phenoxide salts and haloalkanes.

haloarenes cannot be used because they areunreactive to SN2 reactions.

no reaction+X RO-Na

+

Synthesis: Alkyl Aryl Ethers

7/27/2019 21 Benzene

49/62

21-52

Synthesis: Alkyl-Aryl Ethers

OH CH2 =CHCH2 ClNaOH, H2 O, CH2 Cl2

OCH2 CH=CH2

Phenyl 2-prope nyl ether

(Allyl phenyl ether)

+

Phenol 3-Chloropropene(Allyl chloride)

OH

O

O

CH3 OSOCH3NaOH, H2 O, CH2 Cl2

OCH3 Na2 SO4+

Methyl phenyl e ther(Anisole)

+

Phenol Dimethyl sulfate

Synthesis: Kolbe Carboxylation

7/27/2019 21 Benzene

50/62

21-53

Synthesis: Kolbe Carboxylation

Phenoxide ions react with carbon dioxide to give

a carboxylate salt.

OH

NaOH

H2

O

O-Na

+

CO2

H2

O

OH

CO-Na

+O

HCl

H2 O

OH O

COH

Phenol Sodiumphenoxide

Sodium salicylate S alicylic acid

Mechanism: Kolbe Carboxylation

7/27/2019 21 Benzene

51/62

21-54

Mechanism: Kolbe 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 cyclohexadienone

intermediate

+

Sodium

phenoxide

Salicylate anion

keto-enol

tautomerism

(1) (2)

Go back to aromatic

structure

Synthesis: Quinones

7/27/2019 21 Benzene

52/62

21-55

Synthesis: Quinones

Because of the presence of the electron-donating

-OH group, phenols are susceptible to oxidationby a variety of strong oxidizing agents.

H2 CrO4

Phenol 1,4-Benzoquinone(p-Quinone)

O

O

OH

Quinones

7/27/2019 21 Benzene

53/62

21-56

Quinones

OH

OH K2Cr2O7

OH

OH

H2SO

4

K2Cr2O7

H2SO

4

O

O

O

O

1,4-Benzoquinone(p-Quinone)

1,2-Benzenediol(Catechol)

1,2-Benzoquinone(o-Quinone)

1,4-Benzenediol(Hydroquinone)

Quinones

7/27/2019 21 Benzene

54/62

21-57

Quinones

Readily reduced to hydroquinones.

1,4-Benzoquinone(p-Quinone)

(reduction)

1,4-Benzenediol(Hydroquinone)

O

O

OH

OH

Na2 S2O4 , H2 O

Coenzyme Q

7/27/2019 21 Benzene

55/62

21-58

Coenzyme 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

Benzylic Oxidation

7/27/2019 21 Benzene

56/62

21-60

Benzylic 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

benzylic carbon is oxidized to a carboxyl group.

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

H2 CrO4

O2N Cl

CH3

O2N Cl

COOH

Benzylic Oxidation

7/27/2019 21 Benzene

57/62

21-61

Benzylic Oxidation

If there is more than one alkyl group on the benzene

ring, each is oxidized to a -COOH group.

1,4-Dimethylbenzene(p-xylene)

1,4-Be nzenedicarboxylic acid(terephthalic acid)

CH3H2 SO4

K2 Cr2 O7H3 C COH

O

HOC

O

Benzylic Chlorination

7/27/2019 21 Benzene

58/62

21-62

Benzylic Chlorination

Chlorination and bromination occur by a radical

chain mechanism.CH3

Cl2+

CH2Cl

HCl+

Toluene

heator light

Benzyl chloride

( PhCO2) 2 , CCl4

NBS

Br

Ethylbenzene 1-Bromo-1-phenylethane(racemic)

Mechanism: Benzylic Reactions

7/27/2019 21 Benzene

59/62

21-63

Mechanism: Benzylic Reactions

Benzylic radicals (and cations also) are easily

formed because of the resonance stabilization ofthese intermediates.

The benzyl radical is a hybrid of five contributingstructures.

C

C

C

C C

Benzylic Halogenation

7/27/2019 21 Benzene

60/62

21-64

Benzylic Halogenation

Benzylic bromination is highly regioselective.

Benzylic chlorination is less regioselective.

(PhCO2 )2 , CCl4

NBSBr

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

Cl

Cl2 +

heator light

1-Chloro-2-phenylethane

(10%)

Ethylbenzene

+

1-Chloro-1-phenylethane

(90%)

Cl

Hydrogenolysis

7/27/2019 21 Benzene

61/62

21-65

Hydrogenolysis

Hydrogenolysis:Cleavage of a single bond by H2

Benzylic ethers are unique in that they are cleavedunder conditions of catalytic hydrogenation.

O H2Pd/ C

OH

Me+

Benzyl butyl ether Toluene1-Butanol

+

this bondis cleaved

Synthesis, Protecting Group: Benzyl Ethers

7/27/2019 21 Benzene

62/62

Synthesis, Protecting Group: Benzyl Ethers

The value of benzyl ethers is as protecting

groups for the OH groups of alcohols andphenols.

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. ClCH2 Ph

O Ph

2. BH3 THF

Et 3N 3. H2 O2/ NaOH

H2Pd/ C

O Ph

OH

OH

OH

2-(2-Propenyl)phenol

(2-Allylphenol)