New Way Chemistry for Hong Kong A-Level Book 3A1 New Way Chemistry for Hong Kong A-Level 3A 1...

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

1 New Way Chemistry for Hong Kong A-Level

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Benzene Benzene and and

Electrophilic AromatElectrophilic Aromatic Substitutionic Substitution



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Aromatic Hydrocarbons (Arenes)Aromatic Hydrocarbons (Arenes)

• Simplest member : Benzene, C6H6

• Characteristic aroma

• Alkylbenzene, Cn+6H2n+6

E.g.CH3

C6H5CH3 C6H5C2H5



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• Generally less dense than water

• Insoluble in water but soluble in many organic solvents



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• PAHs : Polyaromatic hydrocarbons

• Fused aromatic rings



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• No heteroatoms

N

B

Not PAH



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• PAHs : Polyaromatic hydrocarbons

From incomplete combustion of

wood, coal, diesel, fat, or tobacco

Q.38benzocyclobutadiene



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PAHs : Polyaromatic hydrocarbons

Toxic and carcinogenic

Benzo(a)pyrene : - first carcinogen discovered

Found in : - tobacco smoke,

- char-grilled food

- burnt toast,

- edible oils



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Preparation of Preparation of BenzeneBenzene



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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



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1. Destructive Distillation of Coal1. Destructive Distillation of Coal

A laboratory set-up of the destructive

distillation of coal



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Industrial Industrial PreparationPreparation

2. Catalytic trimerization of ethyne2. Catalytic trimerization of ethyne

HC CH3organonickel catalyst

70oC, under pressure



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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



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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



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

dust

produce benzene and zinc(II) oxide

Benzene is separated by fractional distillation



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Reactions of Reactions of BenzeneBenzene



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Reactivity of Benzene

Unreactive towards addition reactions due to

stabilization of the system by delocalization of -electrons



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Not oxidized by KMnO4



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Resistant to electrophilic addition

Br2 / HBr / H2O



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Resistant to catalytic hydrogenation



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Addition reactions occur only under drastic conditions.

excess H2, high T&P

Ni or Pt or Pd



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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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Electrophilic Aromatic Substitution Electrophilic Aromatic Substitution ReactionsReactions

where E+ denotes an electrophile



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General Reaction MechanismGeneral Reaction MechanismStep 1:

• Benzene reacts with the electrophile

• A carbocation intermediate is formed

• Rate determining step



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General Reaction MechanismGeneral Reaction Mechanism• The carbocation formed has a positive

charge on the carbon atom of benzene

• Stabilized by delocalization of electrons



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General Reaction MechanismGeneral Reaction MechanismStep 2:

• The carbocation loses a hydrogen ion

forms the substitution product



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1. Halogenation1. Halogenation

• Benzene does not react with Cl2 and Br2 in 1,1,1-trichloroethane

• When catalysts (e.g. AlCl3, FeCl3 or FeBr3) are present

benzene react readily with Cl2 and Br2

form chlorobenzene and bromobenzene



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No apparent reaction



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fast

slow



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1. Halogenation - Mechanism1. Halogenation - Mechanism

Step 1:

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

Br BrFeBr3

Br [FeBr4]



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Step 2:

• Formation of carbocation intermediate

• Rate determining step

Br [FeBr]

Br

H

+

Br

H

Br

H

Stabilized by delocalization of -electrons



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Step 3:

• The loss of a proton from the carbocation intermediate

• Forms the bromination product

• The catalyst (FeBr3) is regenerated

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



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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

+



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H2SO4 + N

O

O

HO O N

O

O

H

H

O N O + H2O

acid base

electrophile

2HNO3 NO3 + H2O + NO2

+



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NO2

conc. HNO3

conc. H2SO4, 110oC

NO2

NO2NO2O2N

NO2

conc. HNO3

conc. H2SO4, 95oC

NO2

NO2

Optional



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3. Sulphonation3. Sulphonation• Benzene reacts with fuming sulphuric

acid at room temp

form benzenesulphonic acid

+

(H2SO4 + SO3)



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4. Sulphonation4. Sulphonation

• Sulphonation is a reversible process

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

benzene and sulphuric acid are formed



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5. Friedel-Crafts Alkylation5. Friedel-Crafts Alkylation

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

alkylbenzene is formed



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

Example 29-6Example 29-6 Check Point 29-6Check Point 29-6

5. Friedel-Crafts Alkylation5. Friedel-Crafts Alkylation



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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


New Way Chemistry for Hong Kong A-Level Book 3A1 New Way Chemistry for Hong Kong A-Level 3A 1...

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