Benzene

L.S.T. Leung Chik Wai Memorial SchoolF.6 Chemistry

Chapter 32: Aromatic HydrocarbonBenzene and other Arenes

(A) Introduction

Benzene, C6H6 is the simplest member of the class of hydrocarbon called aromatic hydrocarbons or arenes. It is a colourless liquid with a characteristic smell. These aromatic compounds contain a ring of sp2 carbon atoms, with a delocalized system of electrons. Aromatic compounds (including aromatic hydrocarbons) are stabilized by the electron delocalization.

The delocalized formulae for benzene and other arenes are shown below:

CH3

CH3

CH3

__________________

___________________ ___________________

NO2

Cl

___________________

___________________ ___________________

NH2 SO3H OH

___________________

___________________ ___________________

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Chapter 32: Aromatic Hydrocarbon

NO2

NO2

NO2

NO2

___________________

___________________

(B) Reactions of Benzene: General consideration

The benzene ring is a planar hexagon with a cloud of delocalized electrons lying above and below the ring. The reactions of benzene could involve the attack of an electrophile on the cloud of electrons and undergo addition reaction.However, what is found is quite contrary to the expectation. The following table compares some reactions of cyclohexane, cyclohexene and benzene.

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Chapter 32: Aromatic HydrocarbonTable Some Reactions of Cyctohexane, Cyclohexene and Benzene

Reagent Cyclohexane Cydohexene BenzeneBr2 / CCl4

(in dark)

KMnO4/H+

Conc. H2SO4

H2 / finely divided Ni

No reaction Br2 decolorised readily, no HBr No reaction with Br2

alone, evolved, in the presence of Fe filings, Br2 decolorisedslowly and HBr fumes evolved.

No reaction MnO4- decolorised readily. No reaction

No reaction H2SO4 absorbed readily. Slow substitution reaction when heated, yellow oil formed.

No reaction One mole absorbs one mole of H2 One mole absorbs 3 moles at room temperature of H2 slowly at 1500C

Interpretation:

Benzene is LESS reactive than cyclohexene (an alkene) though it contains it electrons as alkenes do.

Reason :The delocalization of it electrons in the benzene ring leads to considerable aromatic

stabilization. Larger amount of energy is required to activate the ring before any reaction could occur. As a result, the benzene ring requires higher temperature and possibly catalyst before it can react due to the resonance stabilization.

(C) Electrophilic Aromatic Substitution of BenzeneUnlike alkenes. benzene undergoes electrophilic substitution rather than addition.Examples:

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Chapter 32: Aromatic HydrocarbonH

Cl2 / AlCl3

Cl

+ ClH

H

conc. H2SO4

SO3H

+ OH2

The substitution reactions can preserve the aromatic stabilization after electrophilic attack.

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Chapter 32: Aromatic HydrocarbonThe general reaction of electrophilic aromatic substitution can be represented as follow:

1. Formation of the reactive electrophile E+.

2. Attack by E+ on the cloud of the benzene ring. The formed carbocation intermediate involves a high activation energy, therefore this becomes the rate determining step.

+ E+

3. Formation of the final product by loss of the ring proton from the position of electrophilic attack.The aromaticity is restored to the ring.

The energy profile for this electrophilic aromatic substitution is shown below :

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Chapter 32: Aromatic Hydrocarbon(D) Nitration of Benzene

The substitution of a —H atom by a —NO2 group is called nitration.

To obtain nitrobenzene. C6H5NO2. benzene is refluxed on a water bath at 60°C with a ‘nitrating mixture” — a mixture of concentrated nitric acid and sulphuric acid.

nitrobenzene is a pale yellow liquid which can be separated from benzene by distillation under reduce pressure.

Mechanism:

1. The generation of the nitronium ion NO2+

Sulphuric acid serves as a strong acid and donate a proton to nitric acid which acts as a base.

2. The nitronium ion NO2+

adds to the benzene ring to form the short-lived intermediate, in which both the entering -NO2 group and the leaving —H atom are bonded to the ring.

3. The intermediate rapidly loses a proton, restoring the symmetry and stability of the benzene ring. The proton is immediately picked up by a hydrogen sulphate ion.

Note <1> The 2nd step is the rate determining step.

Reason:

<2> If the temperature is raised to 95°C, and the fuming nitric acid is used. 1,3—dinitrobenzene is formed.

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Chapter 32: Aromatic Hydrocarbon(E) Halogenation of benzene

The addition of halogens to the benzene ring takes place under conditions which favour the formation of free radicals, i.e., sunlight or high temperature. The first step in the addition is the homolysis of the halogen molecule, e.g.

Cl2 2ClA different type of reaction takes place at room temperature in the presence of a Friedel—Crafts catalyst Substitution then occurs.e.g.

Mechanism of halogenation

Reaction : Benzene reacts with bromine to form bromobenzene (with the presence of FeBr3)

1 The electron cloud interacts with the bromine molecule, the Br—Br bond becomes polarised2. The friedel—Crafts catalyst accepts a pair of electrons from the 6—Br atom in the polarised Br2

molecule, and enables the Br—Br bond to split. A bromonium ion and a FeBr4-complex ion are

formed.

Br Br FeBr3

3. The bromonium ion rapidly loses a proton to form bromobenzene, with the generation of the catalyst, FeBr3.

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Note:lodination of benzene does not occur but some benzene derivatives, containing activating substituents, are iodinated.

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Chapter 32: Aromatic Hydrocarbon(F) Sulphonation of Benzene

Sulphonation is the substitution of a —H atom by an —SO3H group. Benzenesulphonic acid, C6H5SO3H. is obtained by refluxing benzene with concentrated sulphuric acid for many hours. or warming with fuming sulphuric acid (which contain SO3) at 40 0C for 20-30 minutes.

H

+ H2SO4

reflux with conc. H2SO4

Mechanism.

1. Sulphur trioxide is produced in the following equilibrium in which sulphuric acid acts as both an acid and a base.

2H2SO4 SO3 + H3O+ + HSO4-

2. Sulphur trioxide is a powerful electrophile and react with the aromatic n system to form a neutral but dipolar intermediate.

S

O

O

O

3. The intermediate is attacked by the hydrogen sulphate ion HSO4- to form the product.

Note : <1> Unlike nitration, halogenation, etc. sulphonation is reversible. By heating an0 aqueous solution of benzenesulphonic acid above 100 0C, benzene and sulphuric acid are formed.So, the choice of conditions determines the direction of the reaction.

<2> The —SO3H group in the product can be replaced by hydrolysis to give an —OH group.Benzenesulphonic acid is converted to phenol. C6H5OH. This is the easiest way ofpreparing phenol in the laboratory.

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Chapter 32: Aromatic Hydrocarbon<3> Sulphonation is important in the manufacture of detergents because detergents contain

—SO3H groups that they lathereven in hard water, as the calcium and magnesium salts of sulphonic acid are soluble.

(G) Alkylation of benzeneAn alkyl group can be introduced into the benzene ring by the reaction of a halogenoalkane with benzene.

Such type of reaction is called Friedel-Crafts reaction. The reaction takes place under the influence of a catalyst, such as aluminium chloride (AlCl3) , aluminium bromide (AlBr3) and iron(III) bromide (FeBr3)Such catalysts function as Lewis acid since the central atom can accept a pair of electrons. e.g. AlC13 can accept a pair of electrons from a chloride ion.

AlCl3 + Cl- AlCl4

-

Mechanism of alkylation

When a Friedel—Crafts catalyst (e.g. A1Br3) is dissolved in a halogenoalkane,

1. A1Br3 can accept the bromide ion from C2H5Br, forming a carbocation, C2H5+.

C2H5Br + AlBr3 C2H5+ + AlBr4

-+

2. The attack on the it electron cloud of the benzene ring by the C2H5+ or by the complex

C2H5+A1Br4

-, forming an intermediate.

C2H5+

3. The reactive intermediate quickly loses a proton to form the product, ethylbenzene. The catalyst, AlBr3 is regenerated, and hydrogen bromide is evolved:

Note:Alkylation can also be affected by the use of an alkene and a Friedel—Crafts catalyst together with and acid such as HCl or H3PO4:

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The first step in the reaction is protonation of the alkene by the acid to form a carbocation which attacks the benzene ring.

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(H) Methylbenzene (Toluene)

CH3

Methylbenzene (or toluene) C6H5CH3 . resembles benzene. Its physical properties are similar : the boiling tegiperature is higher (111 0C) and the melting temperature lower (-95 0C)

Reactions of the ring : Electrophilic substitutionMethylbenzene is more reactive than benzene towards the electrophilic reagents which substitute in the ring.

Reason : The methyl group can pushes electrons into the ring through positive inductive effect.

Milder conditions are employed than in the reactions of benzene.

<1> Suiphonation : fuming H2SO4 (containing SO3)

<2> Nitration : Conc. HNO3 + conc. H2SO4 at 30°C

A mixture of 1,2— and 1.4—substituted ,rnethylbenzenes obtained in each case.

CH3

H

+ NO2+

If the temperature is raised, two groups or three groups are introduced.

CH3

NO2

CH3

NO2

NO2

CH3

NO2

NO2

O2N

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Benzene