benzene and electrophilic aromatic substitution

New Way Chemistry for Hong Kong A-Level Book 3A1New Way Chemistry for Hong Kong A-Level

3A

1

Benzene Benzene

and and

Electrophilic Electrophilic Aromatic Aromatic

SubstitutionSubstitution

New Way Chemistry for Hong Kong A-Level Book 3A2New Way Chemistry for Hong Kong A-Level 3A2

Aromatic Hydrocarbons (Arenes)Aromatic Hydrocarbons (Arenes)

• Simplest member : Benzene, C6H6

• Characteristic aroma

• Alkylbenzene, Cn+6H2n+6

E.g. CH3

C6H5CH3 C6H5C2H5

New Way Chemistry for Hong Kong A-Level Book 3A33


• Generally less dense than water

• Insoluble in water but soluble in many organic solvents

New Way Chemistry for Hong Kong A-Level 3A



• PAHs : Polycyclic aromatic hydrocarbons

• Fused aromatic rings




• No heteroatoms

N

B

Not PAH




• PAHs : Polycyclic aromatic hydrocarbonsSources : - incomplete combustion of

Q.38benzocyclobutadiene


wood, coal, diesel, fat, or tobacco


PAHs : Polyaromatic hydrocarbons

Toxic and carcinogenic

Benzo(a)pyrene : - first carcinogen discovered


Found in : - tobacco smoke,

- char-grilled food

- burnt toast,

- edible oils


Preparation of Preparation of BenzeneBenzene



1. Destructive Distillation of Coal1. Destructive Distillation of Coal• Gives coal gas, ammoniacal

liquor, coal tar and coke as products

• The coal tar produced is a mixture of many organic compounds (mainly aromatic ones)

• benzene and methylbenzene can be obtained by fractional distillation



1. Destructive Distillation of Coal1. Destructive Distillation of Coal

A laboratory set-up of the destructive

distillation of coal



Industrial Industrial PreparationPreparation

2. Catalytic trimerization of ethyne2. Catalytic trimerization of ethyne

HC CH3organonickel catalyst

70oC, under pressure



Industrial Industrial PreparationPreparation

3. Catalytic Reforming of Petroleum3. Catalytic Reforming of Petroleum• Converts alkanes and

cycloalkanes into aromatic hydrocarbons

C6H14 C6H6 + 4H2500 oC, 10 – 20

atm

Pt



Laboratory Laboratory SynthesisSynthesis1. Decarboxylation of Sodium Salt of Benzoic Acid1. Decarboxylation of Sodium Salt of Benzoic Acid

• When sodium benzoate is fused with sodium hydroxide

the carboxylate group is removed

volatileBenzene is separated by fractional distillation


2. Reduction of Phenol2. Reduction of Phenol• Passing phenol vapour over heated

zinc dust (reducing agent)

produce benzene and zinc(II) oxide

Benzene is separated by fractional distillation

volatile


Reactions of Reactions of BenzeneBenzene



Reactivity of Benzene

Unreactive towards addition reactions due to

stabilization of the system by delocalization of -electrons




Not oxidized by KMnO4




Resistant to electrophilic addition

Br2 / HBr / H2O




Resistant to catalytic hydrogenation



Addition reactions occur only under drastic conditions.

excess H2, high T&P

Ni or Pt or Pd



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Br2 / HBr / H2O

Incomplete combustiondue to high C content

No reaction when cold.slow sulphonation whenheated.Occur only

under drastic conditions

No reaction


The -electron cloud is susceptible to electrophilic attack.

Substitution is preferred to addition since the former retains aromaticity.

Electrophilic aromatic substitution (SE)



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X

X : Cl, Br

(Halogenation)

NO2 (Nitration)

SO3H (Sulphonation)

R (Alkylation)

C

R

O

(Acylation)

conc. H2SO4 or fuming H2SO4

R

X

O


Electrophilic Aromatic Substitution Electrophilic Aromatic Substitution ReactionsReactions

E+ : electrophile



General Reaction MechanismGeneral Reaction Mechanism

Step 1:


• Rate determining step


General Reaction MechanismGeneral Reaction Mechanism• Stabilized by delocalization of

electrons


+


General Reaction MechanismGeneral Reaction MechanismStep 2:


• The carbocation loses a hydrogen ion

forms the substitution product


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Cl

Br

+ HCl (slow)

+ HBr (fast)

No apparent reation

1. Halogenation1. Halogenation

Catalysts : AlCl3, FeCl3 or FeBr3


1. Halogenation - Mechanism1. Halogenation - Mechanism

Step 1:

• The catalyst (FeBr3) combines with bromine to give a complex

Br BrFeBr3

Br [FeBr4]



Step 2:

Br [FeBr]

Br

H

+

Br

H

Br

H

Stabilized by delocalization of -electronsNew Way Chemistry for Hong Kong A-Level 3A

• Formation of carbocation intermediate

• Rate determining step


Step 3:

• The catalyst (FeBr3) is regenerated

Fumes of HBr(g) are produced, indicating substitution rather than addition has occurred.



2. Nitration2. Nitration

• Benzene reacts readily with a mixture of conc. HNO3 and conc. H2SO4

• Conc. H2SO4 increases the rate of reaction by increasing the concentration of the electrophile, NO2

+



H2SO4 + N

O

O

HO O N

O

O

H

H

O N O + H2O

acid base

electrophile

2HNO3 NO3 + H2O + NO2

+



NO2

conc. HNO3

conc. H2SO4, 110oC

NO2

NO2NO2O2N

NO2

conc. HNO3

conc. H2SO4, 95oC

NO2

NO2

Optional New Way Chemistry for Hong Kong A-Level 3A

meta-directing


N

O O

O N O


35

Q.39

H2SO4 + HNO3 NO2+ + H2O +

HSO4


N

O O

N

O O

N

O O


N

O O

H

HSO4

NO2

+ H2SO4


3. Sulphonation3. Sulphonation

• Benzene reacts with fuming sulphuric acid at room temp

form benzenesulphonic acid

+

(H2SO4 + SO3)



3. Sulphonation3. Sulphonation• Sulphonation is a reversible process

• By heating an aqueous solution of benzenesulphonic acid to above 100 oC

benzene and sulphuric acid are formed


heat


S

H

O

O

O


40

Q.40

H2SO4 + H2SO4 SO3 + H3O+ + HSO4

S

O

O O


S

H

O

O

O S

H

O

O

O S

H

O

O

O


HSO4

SO3S

H

O

O

O

H+

SO3H

+ H2SO4


4. Friedel-Crafts Alkylation4. Friedel-Crafts Alkylation

• When benzene is warmed with a haloalkane in the presence of AlCl3 as a catalyst

alkylbenzene is formed



Important starting step in the manufacture of styrene, phenol and detergents

4. Friedel-Crafts Alkylation4. Friedel-Crafts Alkylation


The alkyl group introduced activates the ring towards further alkylation by +ve I-effect. Friedel-Crafts alkylation is not a good way to prepare alkylbenzene.

New Way Chemistry for Hong Kong A-Level Book 3A4545 New Way Chemistry for Hong Kong A-Level 3A

C Cl

R

O

AlCl3

C

R OCH2

R

Hg/Zn

conc. HCl

Friedel-Craft acylation

Clemensen reduction

deactivating

+


Devise a reaction pathway of not more than four steps for the following conversion.

COOH CH3

46

6B 09/07/2011


CH3

COOH

sodalime

fusion

C Cl

H

O

AlCl3

C

O H

47


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Chlorination

Bromination or Friedel-Craft reaction

Soda lime to absorb HBr or HCl fumes

Br2, RCl or RCOCl

or AlCl3


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Q.41

CH3Cl + AlCl3 [CH3]+

[AlCl4]

electrophile


C

H

H H

CH3

H

two more structures

AlCl4

CH3

+ HCl + AlCl3


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Q.42(a)

2C6H6 + X C6H5CH2C6H5 + 2HClAlCl3

X : CH2Cl2


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-CH2Cl deactivate the ring

Susceptibility to SE reaction :

>

CH2Cl

CH2Cl2

AlCl3

(excess)

CH2Cl

CH2

AlCl3


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Q.42(b)

3C6H6 + Y (C6H5)3CH + 3HClAlCl3

Y : CHCl3


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CHCl3

AlCl3

(excess)

CHCl2

CHCl

C H

AlCl3

AlCl3


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Q.42(c)

2C6H6 + Z C6H5CH2CH2C6H5 + 2HClAlCl3

Z : CH2ClCH2Cl


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AlCl3

(excess)

AlCl3

ClH2C CH2Cl

ClH2C


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1. Coal tarFractional

distillation

CH3

2. PetroleumFractional

distillation

CH3

catalyst

high T & P

CH3

+ 3H2

Industrial preparation of toluene


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Laboratory Preparation of Methylbenzene1. Friedel-Crafts acylation of benzene followed by Clemensen reduction.

2. Friedel-Crafts alkylation of benzene is not recommended as further alkylation will take place giving a mixture of alkylbenzenes that are difficult to be

separated.


Reactions of methylbenzene and Other Alkylbenzenes

1. Electrophilic aromatic substitution rxs

more susceptible to electrophilic attacks ∵ alkyl groups activate the ring by

positive inductive effect.


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2. Reactions of the side chain

(a) Free radical substitution vs electrophilic substitution at the ring

CH3

CH2Cl CHCl2 CCl3

+ +

CH3CH3

Cl

Cl

+

Free radical substitution

Electrophilic substitution


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Benzylic hydrogens are much more susceptible to free radical substitution than hydrogens at other positions.

H2CCH3

CHCH3

Br

H2CCH2Br

CCH3

BrBr

+

H2CCHBr2

H2CCBr3

+ +

Major products


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(b) Oxidation at the benzylic carbon

CHR

R

CR3

COOH

CH3

H2C

R

Absence of benzylic

H


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If alkaline KMnO4 is used, the resulting mixture must be acidified in order to obtain the benzoic acid.

CH3

KMnO4 / OH- H+

COO- COOH

CH3

1. KMnO4 / OH-, heat

2. H+

COOH

heat

Or,


H3C

CH3

CH3

CH3

CH2CH3

COOH

COOH

COOH

COOH

COOH

COOH

COOH

CH3

CH3

m.p. 231oC m.p. 348oC m.p. 300oC m.p. 122oC

b.p. 144oC b.p. 139oC b.p. 138oC b.p. 136oC

alkylbenzenes with very close boiling points

acids with different melting points

Structural determination of isomeric alkylbenzene


C8H10


H3C

CH3

CH3

CH3

CH2CH3

COOH

COOH

COOH

COOH

COOH

COOH

COOH

CH3

CH3

m.p. 231oC m.p. 348oC m.p. 300oC m.p. 122oC

b.p. 144oC b.p. 139oC b.p. 138oC b.p. 136oC

alkylbenzenes with very close boiling points

acids with different melting points

Structural determination of isomeric alkylbenzene



Q.43(a)

COOH

1. MnO4- / OH-

2. H++

HO

C

CH3

O

New Way Chemistry for Hong Kong A-Level 3A66


Q.43(b)

COOH

COOH

1. MnO4- / OH-

2. H++ CO2


Terminal alkene gives CO2

COOH

COOH

1. MnO4- / OH-

2. H+

COOH

1. MnO4- / OH-

2. H+HOOC

+ H2O


Q.43(b)


COOH1. MnO4

- / OH-

2. H+


Q.43(b)

COOH

COOH

1. MnO4- / OH-

2. H+


COOH1. MnO4

- / OH-

2. H+

COOH


Q.43(b)


COOH1. MnO4

- / OH-

2. H+

COOH1. MnO4

- / OH-

2. H+


Q.43(c)

1. MnO4- / OH-

2. H++ CO2

O

71 New Way Chemistry for Hong Kong A-Level 3A

+ H2O


The END



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29.2 Nomenclature of the Derivatives of Benzene (SB p.191)

Draw the structural formula for each of the following compounds:

(a) 1,3,5-Trichlorobenzene

(b) 2,5-Dibromophenol

(c) 2,4-Dinitrobenzoic acid Answer

Back

(a) (b) (c)


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Give the IUPAC name for each of the following compounds:

(a)

(b)

Answer

(a) 1,2-Dimethylbenzene

(b) 1-Methyl-2-nitrobenzene or 2-

nitrotoluene


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Give the IUPAC name for each of the following compounds:

(c)

(d)

Answer

Back

(c) 3-Bromo-5-chlorobenzoic acid

(d) 4-Bromo-2,6-dinitrophenol


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29.3 Structure of Benzene and Aromaticity (SB p.195)

The basic structural requirement for aromatic compounds is that the molecule must be planar,cyclic and with (4n + 2) electrons delocalized inthe ring. n must be a natural number (i.e. n = 1, 2,

3, and so on).There are aromatic compounds without benzene

ring. An example is the 1,3-cyclopentadienyl anion. Can you draw its structure and explain its

aromaticity?Answer


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29.3 Structure of Benzene and Aromaticity (SB p.195)

Back

Cyclopentadienyl anion is an aromatic anion. It has six π

electrons delocalized over a completely conjugated planar

monocyclic system of five sp2 hybridized carbon atoms.

Cyclopentadienyl anion


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29.4 Physical Properties of Aromatic Hydrocarbons (SB p.197)

PAHs are formed from partial combustion and pyrolysis of aromatic compounds. They are in

common occurrence in our environment. List some important uses of aromatic hydrocarbons and how

they release PAHs to our environment.

Answer


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29.4 Physical Properties of Aromatic Hydrocarbons (SB p.197)

Aromatic hydrocarbons are the raw materials for the manufacture of

monomers and plasticizers in polymers, commonly used as

solvents and important constituents of lead-free gasoline.

Incomplete combustion and pyrolysis process favour the production

of PAHs. These compounds are encountered abundantly in the

atmosphere, soil and elsewhere in the environment from sources

that include engine exhaust, wood stove smoke, cigarette smoke

and charbroiled food. Coal tar and petroleum residues such as road

and roofing asphalt have high levels of PAHs.

Back


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29.6 Reactions of Benzene (SB p.203)

Complete each of the following by supplying the missing reactant or product as indicated by the question mark:

(a)

(b)

(c)

Answer

Back

(a)

(b) conc. H2SO4, conc. HNO3

(c) fuming H2SO4


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(a) One mole of benzene reacts with three moles of chlorine under special conditions. What are the conditions required for the reaction?

(a) UV radiation or diffuse sunlight must

be present for the free radical addition

reaction to take place.

Answer


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(b) Draw the structure of the reaction product in (a).

Answer(b)


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(c) Methylbenzene undergoes two different types of chlorination reaction by different mechanisms. Compare the two different types of chlorination reaction in terms of reaction conditions as well as the products formed. Answer


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(c) The two different types of chlorination reaction of methylbenzene

are:

Type I: free radical substitution reaction

Type II: electrophilic aromatic substitution reaction

Back


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Reaction Cyclohexane (a saturated

alicyclic hydrocarbon)

Cyclohexene (an unsaturated


Methylbenzene (an aromatic hydrocarbon)

Action of bromine in 1,1,1-trichloro-ethane (in dark)

No reaction Bromine is decolourized and no hydrogen bromide is evolved

No reaction with bromine alone. In the presence of iron(III) bromide, bromine is decolourized and hydrogen bromide fumes are evolved

Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene


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Action of hydrogen (with nickel catalyst)

No reaction One mole of cyclohexene reacts with one mole of hydrogen at room temperature

One mole of methylbenzene reacts with three moles of hydrogen at high temperature



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Action of acidified potassium manganate(VII)

No reaction Acidified potassium manganate(VII) solution is decolourized

No reaction



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Action of concentrated nitric acid and concentrated sulphuric acid

No reaction Cyclohexene is oxidized and the colour darkens

A yellow liquid is formed


benzene and electrophilic aromatic substitution

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