Benzene and Aromatic Compounds - جامعة نزوى · Benzene and Aromatic Compounds ... presence...

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Benzene and Aromatic Compounds

• Benzene (C6H6) is the simplest aromatic hydrocarbon (or

arene).

• Benzene has four degrees of unsaturation, making it a

highly unsaturated hydrocarbon.

• Whereas unsaturated hydrocarbons such as alkenes,

alkynes and dienes readily undergo addition reactions,

benzene does not.

Background

3


• Benzene does react with bromine, but only in the presence of

FeBr3 (a Lewis acid), and the reaction is a substitution, not an

addition.

Background

• Proposed structures of benzene must account for its high

degree of unsaturation and its lack of reactivity towards

electrophilic addition.

• August Kekulé proposed that benzene was a rapidly

equilibrating mixture of two compounds, each containing a six-

membered ring with three alternating bonds.

• In the Kekulé description, the bond between any two carbon

atoms is sometimes a single bond and sometimes a double

bond.

4


• These structures are known as Kekulé structures.

Background

• Although benzene is still drawn as a six-membered ring with

alternating bonds, in reality there is no equilibrium between the

two different kinds of benzene molecules.

• Current descriptions of benzene are based on resonance and

electron delocalization due to orbital overlap.

• In the nineteenth century, many other compounds having

properties similar to those of benzene were isolated from natural

sources. Since these compounds possessed strong and

characteristic odors, they were called aromatic compounds. It

should be noted, however, that it is their chemical properties,

and not their odor, that make them special.

5


Any structure for benzene must account for the following facts:

1. It contains a six-membered ring and three additional

degrees of unsaturation.

2. It is planar.

3. All C—C bond lengths are equal.

The Structure of Benzene

The Kekulé structures satisfy the first two criteria but not the

third, because having three alternating bonds means that

benzene should have three short double bonds alternating with

three longer single bonds.

6


• The resonance description of benzene consists of two equivalent

Lewis structures, each with three double bonds that alternate

with three single bonds.

• The true structure of benzene is a resonance hybrid of the two

Lewis structures, with the dashed lines of the hybrid indicating

the position of the bonds.

• We will use one of the two Lewis structures and not the hybrid in

drawing benzene. This will make it easier to keep track of the

electron pairs in the bonds (the electrons).


7


• Because each bond has two electrons, benzene has six

electrons.


8


• In benzene, the actual bond length (1.39 Å) is

intermediate between the carbon—carbon single bond

(1.53 Å) and the carbon—carbon double bond (1.34 Å).


9

Molecular Orbital Model of Benzene

• The concepts of hybridization of atomic orbitals and resonance

provided the first adequate structure of benzene.

• Benzene has a six carbon skeleton in a regular hexagon with C-C-

C angles of 120o. All the carbons are the same length (1.39 Å) as

well as the hydrogens (1.09 Å).

• The hybridization of the C-C bonds is sp2-sp2 whereas the C-H

bond is sp2-1s.

CC

C CC

CH

H

H

H

HH

120o

120o

sp2-sp2

sp2-1s

1.39 Å

1.09 Å

10


• To name a benzene ring with one substituent, name the

substituent and add the word benzene.

Nomenclature of Benzene Derivatives

• Many monosubstituted benzenes have common names

which you must also learn.

11


• There are three different ways that two groups can be

attached to a benzene ring, so a prefix—ortho, meta, or

para—can be used to designate the relative position of

the two substituents.


ortho-dibromobenzene

or

o-dibromobenzene

or 1,2-dibromobenzene

meta-dibromobenzene

or

m-dibromobenzene


para-dibromobenzene

or

p-dibromobenzene


12


• If the two groups on the benzene ring are different,

alphabetize the names of the substituents preceding the

word benzene.

• If one substituent is part of a common root, name the

molecule as a derivative of that monosubstituted

benzene.


13


For three or more substituents on a benzene ring:

1. Number to give the lowest possible numbers around the ring.

2. Alphabetize the substituent names.

3. When substituents are part of common roots, name the

molecule as a derivative of that monosubstituted benzene. The

substituent that comprises the common root is located at C1.


14


• A benzene substituent is called a phenyl group, and it can be

abbreviated in a structure as “Ph-”.


• Therefore, benzene can be represented as PhH, and phenol

would be PhOH.

15


• The benzyl group, another common substituent that contains a

benzene ring, differs from a phenyl group.


• Substituents derived from other substituted aromatic rings are

collectively known as aryl groups.

16


Four structural criteria must be satisfied for a compound

to be aromatic.

The Criteria for Aromaticity—Hückel’s Rule

[1] A molecule must be cyclic.

To be aromatic, each p orbital must overlap with p orbitals

on adjacent atoms.

17



[2] A molecule must be planar.

All adjacent p orbitals must be aligned so that the

electron density can be delocalized.

Since cyclooctatetraene is non-planar, it is not aromatic,

and it undergoes addition reactions just like those of other

alkenes.

18

19



[3] A molecule must be completely conjugated.

Aromatic compounds must have a p orbital on every atom.

20



[4] A molecule must satisfy Hückel’s rule, and contain

a particular number of electrons.

Benzene is aromatic and especially stable because it

contains 6 electrons. Cyclobutadiene is antiaromatic and

especially unstable because it contains 4 electrons.

Hückel's rule:

21



Note that Hückel’s rule refers to the number of electrons,

not the number of atoms in a particular ring.

22



1. Aromatic—A cyclic, planar, completely conjugated

compound with 4n + 2 electrons.

2. Antiaromatic—A cyclic, planar, completely conjugated

compound with 4n electrons.

3. Not aromatic (nonaromatic)—A compound that lacks

one (or more) of the following requirements for

aromaticity: being cyclic, planar, and completely

conjugated.

Considering aromaticity, a compound can be classified in

one of three ways:

23



Note the relationship between each compound type and a similar

open-chained molecule having the same number of electrons.

24



• 1H NMR spectroscopy readily indicates whether a

compound is aromatic.

• The protons on sp2 hybridized carbons in aromatic

hydrocarbons are highly deshielded and absorb at 6.5-8

ppm, whereas hydrocarbons that are not aromatic

absorb at 4.5-6 ppm.

25


Examples of Aromatic Rings

• Completely conjugated rings larger than benzene are

also aromatic if they are planar and have 4n + 2

electrons.

• Hydrocarbons containing a single ring with alternating

double and single bonds are called annulenes.

• To name an annulene, indicate the number of atoms in

the ring in brackets and add the word annulene.

26

O

N

i)ii) iii)

vii)vii

v)iv)

vi) ix) x)

Aromatic Heterocyclic Compound

Aromatic Compound

Which of the following compounds are Aromatic, anti Aromatic and non aromatic Compounds

27

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Examples of Aromatic Rings

• [10]-Annulene has 10 electrons, which satisfies

Hückel's rule, but a planar molecule would place the two

H atoms inside the ring too close to each other. Thus,

the ring puckers to relieve this strain.

• Since [10]-annulene is not planar, the 10 electrons

can’t delocalize over the entire ring and it is not

aromatic.

29 29

Electrophilic Aromatic Substitution

• The characteristic reaction of benzene is electrophilic aromatic

substitution—a hydrogen atom is replaced by an electrophile.

Background

30 30

• Benzene does not undergo addition reactions like other

unsaturated hydrocarbons, because addition would yield

a product that is not aromatic.

• Substitution of a hydrogen keeps the aromatic ring

intact.

• There are five main examples of electrophilic aromatic

substitution.

31 31

32 32

• Regardless of the electrophile used, all electrophilic aromatic

substitution reactions occur by the same two-step mechanism—

addition of the electrophile E+ to form a resonance-stabilized

carbocation, followed by deprotonation with base, as shown

below:

• * The intermediate cyclohexadienyl cation thus formed is, in general called arenium ion (or some times a σ sigma

complex

σ sigma complex

33 33

• The first step in electrophilic aromatic substitution forms a

carbocation, for which three resonance structures can be

drawn. To help keep track of the location of the positive

charge:

34 34

• The energy changes in electrophilic aromatic substitution are

shown below:

35 35

• In halogenation, benzene reacts with Cl2 or Br2 in the

presence of a Lewis acid catalyst, such as FeCl3 or

FeBr3, to give the aryl halides chlorobenzene or

bromobenzene respectively.

• Analogous reactions with I2 and F2 are not synthetically

useful because I2 is too unreactive and F2 reacts too

violently.

Halogenation

36 36

• Chlorination proceeds by a similar mechanism.

37 37

• Nitration and sulfonation introduce two different functional

groups into the aromatic ring.

• Nitration is especially useful because the nitro group can be

reduced to an NH2 group.

Nitration and Sulfonation

38 38

• Generation of the electrophile in nitration requires

strong acid.

39 39

• Generation of the electrophile in sulfonation requires

strong acid.

40 40

• In Friedel-Crafts alkylation, treatment of benzene with an

alkyl halide and a Lewis acid (AlCl3) forms an alkyl

benzene.

Friedel-Crafts Alkylation and Friedel-Crafts Acylation

41 41

• In Friedel-Crafts acylation, a benzene ring is treated with

an acid chloride (RCOCl) and AlCl3 to form a ketone.

• Because the new group bonded to the benzene ring is

called an acyl group, the transfer of an acyl group from

one atom to another is an acylation.

42 42

Friedel-Crafts Alkylation and Friedel-Crafts Acylation

43 43

44 44

• In Friedel-Crafts acylation, the Lewis acid AlCl3 ionizes the

carbon-halogen bond of the acid chloride, thus forming a

positively charged carbon electrophile called an acylium ion,

which is resonance stabilized.

• The positively charged carbon atom of the acylium ion then goes

on to react with benzene in the two step mechanism of

electrophilic aromatic substitution.

45 45

Three additional facts about Friedel-Crafts alkylation

should be kept in mind.

[1] Vinyl halides and aryl halides do not react in Friedel-

Crafts alkylation.

46 46

[2] Rearrangements can occur.

These results can be explained by carbocation

rearrangements.

47 47

48 48

Rearrangements can occur even when no free carbocation

is formed initially.

49 49

[3] Other functional groups that form carbocations can

also be used as starting materials.

50 50

Each carbocation can then go on to react with benzene to

form a product of electrophilic aromatic substitution. For

example:

51 51

Starting materials that contain both a benzene ring and an

electrophile are capable of intramolecular Friedel-Crafts

reactions.

52 52

Formylation

It is the substitution of formyl group -CHO in the benzen ring.

Formylation can be brought about by

• Gattermann Koch Reaction

• Formyl Fuoride

Gattermann Koch Reaction

When carbon monoxide is passed through a solution of

Benzene containing anhydrous AlCl3, CuCl and HCl

formylation CO + HCl Cl H

O

H Cl

O

AlCl3 H

O

H

O

+

+

AlCl4

O

H

53

Formylation of benzene occurs when formyl fluorde reacts with it in the presence of BF3 in

ether. The reaction is carried out at low temperature

Formylation Using Formyl Fluoride

H

O

+

O

H

F

BF3HBF4+

54

Carboxylation

It is the substation of carboxylic group (-COOH) in the

benzene ring. For example carboxylation of benzene takes

place on treatment with Phosgene gas in the presence of

AlCl3

Cl Cl

O

AlCl3 Cl

O

+

+

AlCl4

O

ClBF3

Cl Cl

O

O

OHAlCl3

H2O

Cl

OH

O

Cl

O

ClH2O

O

OH

Mechanism

55

56 56

1) Why is benzene less reactive than an alkene?

The pi electrons of benzene are delocalized over 6

atoms, thus making benzene more stable and less

available for electron donation.

While an alkene’s electrons are localized between two

atoms, thus making it more nucleophillc and more

reactive toward electrophiles.

57 57

2) Show how the other two resonance structures can be

deprotonated in step two of electrophillic aromatic

substitution. H

E

BE

H

E

BE

58 58

H

E

BE

59 59

3) Draw a detailed mechanism of the chlorination of

benzene.

Cl ClFeCl3 Cl Cl FeCl3

Formation of Electrophile

Cl Cl FeCl3

HH

Cl

H

Cl

H

Cl

+ FeCl4

Electrophillic Additon

60 60

H

Cl

FeCl4

Cl

+ HCl + FeCl3

Deprotonation

61 61

4) Draw stepwise mechanism for the sulfonation of A.

SO3

H2SO4

SO3H

a

b

O

S

O O

H OSO3H SO3H + HSO4


62 62

H

SO3H

RR

H

SO3H

R

H

SO3H

R

H

SO3H

Electrophillic Addition

R

H

SO3H

HSO4R

SO3H

+ H2SO4

Deprotonation

63 63

5) What product is formed when benzene is reacted with

each of the following alkyl halides?

+ (CH3)2CHClAlCl3

a) CH(CH3)2

+

Cl

AlCl3

b)

64 64

+AlCl3

H3CH2C

Cl

O

c) O

CH2CH3

65 65

6) What acid chloride is necessary to produce each

product from benzene using a Friedal-Crafts acylation?

O

CH2CH2CH(CH3)2

a)

Cl

O

CH2CH2CH(CH3)2

O

b) O

Cl

66 66

O

c)

O

Cl

67 67

7) Draw a stepwise mechanism for the following

friedal-Crafts alkylation?

+ CH3CH2Cl

AlCl3

CH2CH3

CH3CH2Cl AlCl3 H3CH2C Cl AlCl3


68 68

H3CH2C Cl AlCl3

H

H

CH2CH3

H

CH2CH3

H

CH2CH3

Electrophillic Additoon

H

CH2CH3

AlCl3Cl

CH2CH3

+ HCl + AlCl3

Protonation

69 69

8) Which of these halides are reactive in a Friedal-Crafts

alkylation?

Br

A

Br

B

Br

CBr

D

Br

B

Br

D

Look at the carbon to which the halogen is

attached and determine its hybridization. If sp2 its

unreactive, while sp3 is reactive.

70 70

9) Draw a stepwise mechanism for the following

reaction.

+ (CHe)2CHCH2Cl

AlCl3

C(CH3)3

+ HCl + AlCl3

H3C

CH3

H

H2C Cl

AlCl3H3C

CH3

H

H2C Cl AlCl3

H3C

CH3

CH3+ AlCl4


1,2 H shift

71 71

H3C

CH3

CH3

H H C(CH3)3 H C(CH3)3 H C(CH3)3

Electrophillic Additon

H C(CH3)3

AlCl4

C(CH3)3

+ AlCl3 + HCl

Deprotonation

72 72

10) Draw the product of each reaction

+H2SO4

a)

+H2SO4

(H3C)2C CH2

b)

C(CH3)3

73 73

+H2SO4

OH

c)

+H2SO4

OHd)

74 74

Substituted Benzenes

Many substituted benzene rings undergo electrophilic

aromatic substitution.

Each substituent either increases or decreases the

electron density in the benzene ring, and this affects the

course of electrophilic aromatic substitution.

75 75

Considering inductive effects only, the NH2 group

withdraws electron density and CH3 donates electron

density.

76 76

Resonance effects are only observed with substituents

containing lone pairs or bonds.

An electron-donating resonance effect is observed

whenever an atom Z having a lone pair of electrons is

directly bonded to a benzene ring.

77 77

• An electron-withdrawing resonance effect is observed in

substituted benzenes having the general structure

C6H5-Y=Z, where Z is more electronegative than Y.

• Seven resonance structures can be drawn for

benzaldehyde (C6H5CHO). Because three of them place a

positive charge on a carbon atom of the benzene ring,

the CHO group withdraws electrons from the benzene

ring by a resonance effect.

78 78

• To predict whether a substituted benzene is more or less

electron rich than benzene itself, we must consider the

net balance of both the inductive and resonance effects.

• For example, alkyl groups donate electrons by an

inductive effect, but they have no resonance effect

because they lack nonbonded electron pairs or bonds.

• Thus, any alkyl-substituted benzene is more electron

rich than benzene itself.

79 79

• The inductive and resonance effects in compounds having the

general structure C6H5-Y=Z (with Z more electronegative than Y)

are both electron withdrawing.

80 80

• These compounds represent examples of the general

structural features in electron-donating and electron

withdrawing substituents.

81 81

Electrophilic Aromatic Substitution and Substituted

Benzenes.

• Electrophilic aromatic substitution is a general reaction

of all aromatic compounds, including polycyclic

aromatic hydrocarbons, heterocycles, and substituted

benzene derivatives.

• A substituent affects two aspects of the electrophilic

aromatic substitution reaction:

1. The rate of the reaction—A substituted benzene

reacts faster or slower than benzene itself.

2. The orientation—The new group is located either

ortho, meta, or para to the existing substituent.

The identity of the first substituent determines the

position of the second incoming substituent.

82 82

• Consider toluene—Toluene reacts faster than benzene

in all substitution reactions.

• The electron-donating CH3 group activates the benzene

ring to electrophilic attack.

• Ortho and para products predominate.

• The CH3 group is called an ortho, para director.

83 83

• Consider nitrobenzene—It reacts more slowly than

benzene in all substitution reactions.

• The electron-withdrawing NO2 group deactivates the

benzene ring to electrophilic attack.

• The meta product predominates.

• The NO2 group is called a meta director.

84 84

All substituents can be divided into three general types:

85 85

86 86

• Keep in mind that halogens are in a class by

themselves.

• Also note that:

87 87

• To understand how substituents activate or deactivate the ring,

we must consider the first step in electrophilic aromatic

substitution.

• The first step involves addition of the electrophile (E+) to form a

resonance stabilized carbocation.

• The Hammond postulate makes it possible to predict the relative

rate of the reaction by looking at the stability of the carbocation

intermediate.

88 88

• The principles of inductive effects and resonance effects can

now be used to predict carbocation stability.

89 89

90 90

Orientation Effects in Substituted Benzenes

• There are two general types of ortho, para directors and

one general type of meta director.

• All ortho, para directors are R groups or have a

nonbonded electron pair on the atom bonded to the

benzene ring.

• All meta directors have a full or partial positive charge

on the atom bonded to the benzene ring.

91 91

To evaluate the effects of a given substituent, we can use

the following stepwise procedure:

92 92

• A CH3 group directs electrophilic attack ortho and para to itself

because an electron-donating inductive effect stabilizes the

carbocation intermediate.

93 93

• An NH2 group directs electrophilic attack ortho and para to itself

because the carbocation intermediate has additional resonance

stabilization.

94 94

• With the NO2 group (and all meta directors) meta attack occurs

because attack at the ortho and para position gives a

destabilized carbocation intermediate.

95 95

Figure 18.7 The reactivity and directing

effects of common substituted

benezenes

96

12)Identify each group as having an electron

donating or electron withdrawing inductive effect.

a) CH3CH2CH2CH2-

Electron donating

b) Br-

Electron withdrawing

c) CH3CH2O-

Electron withdrawing

97

14)Identify as electron donating or electron withdrawing.

a) OCH3

Lone pair on oxygen, electron

donating

b) I Halogen, electron

withdrawing

c) C(CH3)3 Alkyl group, electron

donating

98

OCH3

CH3CH2Cl

AlCl3

15)Predict the products.

a) OCH3

CH2CH3

+

OCH3

H3CH2C

b) Br

HNO3

H2SO4

Br

NO2

+

Br

O2N

99

c)

NO2

Cl2

FeCl3

NO2

Cl

100

16)Predict the products when reacted with HNO3 and

H2SO4. Also state whether the reactant is more or less

reactive than benzene.

a) OO

NO2

N

b) N

O2N

Less

Less

101

Cl

c)

Cl

NO2

+

Cl

O2N

Less

d)

OHOH

NO2

+

OH

O2N

More

102

CH2CH3

CH2CH3

NO2

+

CH2CH3

O2N

More

d)

103

17)Label each compound as less or more reactive

than benzene.

a)

C(CH3)3

more

OH

OH

b)

more

104

c) O

OCH2CH3 less

N(CH3)3

d)

less

105

18) Rank each group in order of increasing reactivity.

Cl OCH3

a)

2 3 1

NO2 CH3

b)

2 3 1

106 106

Disubstituted Benzenes

1. When the directing effects of two groups reinforce, the

new substituent is located on the position directed by

both groups.

107 107

2. If the directing effects of two groups oppose each

other, the more powerful activator “wins out.”

108 108

Synthesis of Benzene Derivatives

In a disubstituted benzene, the directing effects indicate

which substituent must be added to the ring first.

Let us consider the consequences of bromination first

followed by nitration, and nitration first, followed by

bromination.

109

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