Chapter 5.3 Arenes Benzene

VIHS/DEPARTMENT OF CHEMISTRY Page 1

5.3 ARENES: BENZENE 2014

Syllabus specification

Arenes: benzene

a. use thermochemical, x-ray diffraction and infrared data as evidence for the structure and

stability of the benzene ring.

Students may represent the structure of benzene as

or

as appropriate in equations and mechanisms

b. describe the following reactions of benzene, limited to:

i) combustion to form a smoky flame.

treatment with:

ii) bromine.

iii) concentrated nitric and sulfuric acids.

iv) fuming sulfuric acid.

v) halogenoalkanes and acyl chlorides with aluminium chloride as catalyst (Friedel-Crafts

reaction).

vi) addition reactions with hydrogen.

c. describe the mechanism of the electrophilic substitution reactions of benzene in

halogenation, nitration and Friedel-Crafts reactions including the formation of the

electrophile.

d. carry out the reactions in 5.4.1b where appropriate (using methylbenzene or

methoxybenzene).

e. carry out the reaction of phenol with bromine water and dilute nitric acid and use these

results to illustrate the activation of the benzene ring.

Introduction:

Arenes are hydrocarbons with a ring or rings of carbon atoms in which there are delocalised

electrons. Benzene, the simplest arene with a molecular formula C6H6, is an important and

useful chemical which is obtained by the catalytic reforming of fractions from crude oil.

Arenes are sometimes called aromatic compounds.

Study of the structure of benzene is an another example that shows how scientific models

develop in response to new evidence. This links to further investigations of the models that

chemists use to describe the mechanisms of organic reactions.



General properties of benzene

It is a Colourless liquid with a characteristic odour.

Boils at 80oC and freezes at 6oC.

Immiscible with water but soluble in organic solvent.

Gives smoky luminous sooty flame on burning.

Structure of benzene:

Benzene, C6H6, is a cyclic compound that has six carbon atoms in a hexagonal ring. Several

structures for benzene have been proposed. Early theories suggested that there were

alternative single and double bonds between the carbon atoms(fig 5.3.1), but this did not fit

with later experimental evidence. It was shown that all the carbon-carbon bonds are the

same length and that the molecule is planar.

Two modern theories are used to explain the structure.

The Kekule version assumes that benzene is a resonance hybrid between

the two structures as given below. This model can be used to explain many

chemical properties and reaction of benzene.

Fig 5.3.2 The displayed formula of kekules benzene ring structure

The other theory assumes that each sp2 hybridized carbon atom is joined by

a - (sigma) bond to each of its two neighbours, and by a third - sigma bond

to s-orbital of hydrogen atom forming a hexagonal planar ring. The fourth

bonding electron is in p-orbital(called as non-hybrid porbital) in the right

angle to the planar of - (sigma) bonds. This p-orbital overlap side way, and

the six p-orbitals overlap above and below the plane of the ring of carbon

atoms. This produces a delocalised -(pi)bonding system of electrons, as in:

Fig. 5.3.1 Simplified structure of benzene



Fig. 5.3.3 The delocalisation of the electrons in the -bonds of the

symmetrical six-membered ring structure of benzene

Evidences for structure and extra stability of benzene

(i) Thermochemical evidence: via enthalpy of hydrogenation.

Benzene is more stable than cyclohexatriene, which is the theoretical compound

with three single and three localised double carbon-carbon bonds. The amount by

which it is stabilised can be calculated from the enthalpies of hydrogenation.

For example, the enthalpy of hydrogenation of one mole cyclohexene is -120 kJ.

+ H2(g) H = -120 kJ mol1

Cyclohexene Cyclohexane

Therefore , H for the addition to three localised double bonds in cyclohexatriene

would be 3 x (-120) = -360 kJ mol1. However for benzene:

+ 3H2(g) H = -208kJ mol1

Benzene cyclohexane

Thus,152 kJ less energy is given out because of benzenes unique structure. This is

called the delocalisation stabilisation energy or resonance energy and can be

shown in an enthalpy-level diagram.



Cyclohexatriene

H = -360 kJ mol-1 H = -152 kJ (resonance energy)

Benzene

H = -208 kJ mol-1

Cyclohexane, C6H12

Fig.5.3.4 Enthalpy-level diagram for the hydrogenation of benzene and cyclohexatriene.

Thermo-chemical evidence: via bond enthalpies

The amount by which benzene is stabilised can also be calculated from average

bond enthalpies. The enthalpy of formation of gaseous benzene is +83 kJ mol-1.

The value for the theoretical molecule cyclohexatriene can be found using the

Hesss law cycle below:

6C(s) + 3H2(g) C6H6(g)

6C(g) + 3H2(g) 6C(g) + 6H(g)

Step 1 equals 6 x enthalpy of atomisation of carbon(Hatm[C(s)]) = 6 x (+715)

= +4290 kJ

Step 2 equals 3 x HH bond enthalpy = 3 x (+436) = + 1308kJ

Step 3 equals enthalpy change of bonds made, which is calculated as below

Three CC = 3 x (-348) = -1044 kJ

Three C=C = 3 X (-612) = -1836 kJ

Six CH = 6 x (-412) = -2472 kJ

Total = - 5362 kJ

Enthalpy

kJmol-1

Hf

Step 1

Step 2

Step 3



Hence the Hf of cyclohexatriene = Hstep 1 + Hstep 2 + Hstep 3

= +4290 + 1308 +(-5352)

= +246 kJ mol-1.

The actual enthalpy of formation of gaseous benzene is +83 kJ mol-1. The value

calculated above is 163 kJ more and approximately equals the resonance energy of

benzene. Hence, the structure with the delocalised electron system is energetically

more stable.

X-ray diffraction evidence

X-ray diffraction shows the position of the centre of atoms. If the diffraction pattern of

benzene is analysed, it clearly shows that all the bond lengths between the carbon

atoms are the same. Which is not the same in the case of cyclohexene.

Table 1. comparison of bond length in benzene and cyclohexene.

Bond Bond length/nm

All the six carbon-carbon bonds in

benzene

0.140

Carbon-carbon single bond in

cyclohexene

0.154

Carbon-carbon double bond in

cyclohexene

0.134

Fig.5.3.5 Electron density map of

benzene.

Electrons are equally distributed

over six carbon atoms due to

delocalisation of the pi- bonding

electron system.

If benzene has cyclohexatriene

structure, equal distribution of

electrons cannot be seen on the

carbon ring.

Thus, benzene is thermodynamically

more stable due to its delocalized pi-

bonding system.

0.140 nm



Infra red evidence:

Comparison of the infrared spectrum of aromatic compounds with those of aliphatic

compounds containing a C=C group showed slight differences. The CH stretching

vibration in benzene is at 3036cm-1 and the C=C stretching is at 1479cm-1, whereas

the equivalent vibrations in an aliphatic compound such as cyclohexene are at 3023

and 1438cm-1.

Naming benzene derivatives.

The derivatives of benzene are named either as substituted products of benzene or

as compounds containing the phenyl group, C6H5. The names and structures of

some derivatives of benzene are given below.

Systematic name Substituent group Structure

Chlorobenzene Chloro, -Cl C6H5-Cl

Nitrobenzene Nitro, -NO2 C6H5-NO2

Methylbenzene Methyl,-CH3 C6H5-CH3

Phenol Hydroxyl, -OH C6H5-OH

Phenylamine Amine, -NH2 C6H5-NH2

Phenylethanone Ethanone,-COCH3 C6H5-COCH3

Phenylmethanol Methanol,-CH2OH C6H5-CH2OH

When more than one hydrogen atom is substituted, numbers are used to indicate the

positions of substituent on the benzene ring. The ring is usually numbered clockwise

and the numbers used are the lowest ones possible. In some cases, the ring is

numbered anticlockwise to get the lowest possible numbers.

Fig. 5.3.6 IR spectra for (a)

cyclohexzene and (b) benzene.



In phenyl compounds, such as phenol and phenylamine, the OH and NH2, groups

are assumed to occupy the 1 position.

Fig. 5.3.7 Naming substituted benzene compounds.

Reactions of benzene

(i) combustion:

Benzene burns in a limited amount of air with a smoky flame. Combustion is

incomplete and particles of carbon are formed.

The complete combustion of benzene requires large volume of oxygen.

2C6H6(l) + 15O2(g) 12CO2(g) + 6H2O(l)

(ii) Addition:

The double bond in benzene is not as susceptible to addition as is the double bond in

alkenes. However, it does react with hydrogen in the presence of a hot nickel catalyst

to form cyclohexane.

+ 3H2

Benzene cyclohexane

Reagents: Hydrogen gas.

Conditions: In the presence of Raney nickel(finely divided with a very large surface

area and very high catalytic activity)catalyst at high temperature(about 150oC).

Reaction type: Electrophilic addition.



Electrophilic substitution:

(iv) Halogenation

Dry benzene reacts with chlorine gas in the presence of iron (or a catalyst of

anhydrous iron(III) chloride). Steamy fumes of hydrogen chloride are given off and

chlorobenzene(C6H5Cl) is formed.

Cl

+ Cl2(g) + HCl(g)

Benzene chlorobenzene

Reagents: Chlorine gas.

Conditions: Room temperature and pressure, in the presence of anhydrous FeCl3.

Reaction type: Electrophilic substitution.

Mechanism: Heterolytic electrophilic substitution.

The mechanism for this reaction is as follows.

Step 1: The catalyst, anhydrous iron(III) chloride , is made by the reaction of iron with

chlorine

Fe + 1 Cl2 FeCl3

This reacts with more chlorine, forming the electrophile Cl+

Cl2 + FeCl3 Cl+ + [FeCl4]

electrophile

Step 2: The Cl+ attacks the -electrons in the benzene ring, forming an intermediate

with a positive charge. Finally, the [FeCl4] ion removes an H+ ion from benzene,

producing chlorobenzene(C6H5Cl) and reforming the catalyst(FeCl3)

H+ + [FeCl4] HCl + FeCl3



Reaction with nitric acid: Nitration.

When benzene is warmed with a mixture of concentrated nitric and sulfuric acid, a

nitro-group(NO2) replaces a hydrogen atom in the benzene ring. Nitrobenzene and

water are produced.

NO2

+ HNO3(conc.) + H2O

Benzene nitrobenzene

Reagents: A mixture of Conc.H2SO4 and Conc.HNO3(nitrating mixture)

Conditions: Warm under reflux at 50oC.

Reaction Type: Electrophilic substitution.



Step 1:The sulfuric acid reacts with the nitric acid to form the electrophile NO2+. The

temperature must not go above 50oC or some dinitrobenzene(C6H4(NO2)2) is formed.

2H2SO4 + HNO3 2HSO4 + H3O+ + NO2

+

Acid base electrophile

Step 2: The NO2+ attacks the -electrons in the benzene ring, forming an

intermediate with a positive charge. Finally, the HSO4 ion removes an H+ ion from

benzene, producing nitrobenzene(C6H5NO2) and reforming the catalyst(H2SO4).

Note: The addition of Cl+ to benzene is similar to the first step of the addition of

chlorine to ethene. The difference arises at the next step. The benzene

intermediate loses an H+, thus regaining the stability of the delocalised system,

whereas the intermediate with ethene adds Clion.

A catalyst must be present for the addition of Cl+ to benzene, because the

activation energy of the first step is higher than that for the addition to ethene.

Conc.H2SO4

50oC



HSO4 + H+ H2SO4

Role of H2SO4

Acts as a catalyst, as it increases the rate of reaction and remains chemically

unchanged as it is being regenerated at the end of the reaction.

Acts as an acid(proton donor), as it donates protons in the reaction.

Role of HNO3

It generates nitronium ion,NO2+, which acts as an electrophile in the

mechanism.

It acts as a base by accepting protons.

Exercise

(01) Benzene prefers to undergo substitution reaction rather than addition reactions.

Explain.

(02) In the nitration of benzene sulphuric acid acts as an acid whereas nitric acid acts

as a base. Show by an equation how this is so.



(03) Why Raney nickel is used in the manufacture of cyclohexane from benzene?

(04) Explain why smoky flame are seen during the combustion of benzene.

(05) Write an equation for the bromination of benzene. By using appropriate arrow

draw the mechanism of this reaction.



Reaction with fuming sulphuric acid: Sulfonation.

When benzene is warmed with fuming sulfuric acid, benzenesulfonic acid is

produced. Fuming sulphuric acid is a solution of sulphur trioxide in sulphuric acid.

The electrophile is the SO3 molecule.

SO3H

+ SO3

Benzene benzenesulfonic acid

Reagents: fuming sulphuric acid

Conditions: Heat under reflux




Step 1

Step 2

This reaction is important in the manufacture of detergents, where a substituted

benzene ring is sulfonated and the final product is neutralised.

Friedel-Crafts reaction: (i) Reaction with halogenoalaknes

In the presence of an anhydrous aluminium chloride catalyst, alkyl group(eg C2H5)

can be substituted into the ring.



For example, In the reaction between benzene and chloroethane, the products are

ethylbenzene and hydrogen chloride.

C2H5

+ C2H5 Cl + HCl

benzene ethylbenzene

Reagents: Halogenoalkanes

Conditions: Heat under reflux at 50oC, in the presence of anhydrous AlCl3 as a

catalyst.



Note:The reaction mixture must be dry.


Step 1: The electrophile, +CH2CH3, is produced by the reaction of the catalyst with

the halogenoalkane:

CH3CH2Cl + AlCl3 +CH2CH3 + [AlCl4]

Chloroethane electrophile

Step 2: The positive carbon atom attacks the system in the benzene ring:

Step 3: The intermediate loses a H+ ion so as to regain the stability of the benzene

ring.

Finally, the catalyst is regenerated by the reaction:

H+ + [AlCl4] HCl + AlCl3.



In this reaction, a catalyst is used to increase the positive nature of the electrophile

and make it better at attacking benzene rings. AlCl3 acts as a Lewis Acid and helps

break the CCl bond.

Friedel-Crafts reaction: (ii) Reaction with acyl(acid) chlorides.

In the presence of an anhydrous aluminium chloride catalyst, benzene reacts with

acylchlorides to form ketones.

For example, In the reaction between benzene and ethanoyl chloride, the products

are phenylethanone and hydrogen chloride.

COCH3

+ CH3COCl + HCl

Benzene phenylethanone

Reagents: Acyl(acid) chlorides

Conditions: Heat under reflux at 50oC, in the presence of anhydrous AlCl3 as a

catalyst.




Step 1: The electrophile, CH3C+O is produced by the reaction of the acylchloride with

the catalyst:

CH3COCl + AlCl3 CH3C+O + [AlCl4]

ethanoyl chloride electrophile

Step 2: The positive carbon atom attacks the -system in the benzene ring.

Step 3: The intermediate loses a H+ ion so as to regain the stability of the benzene

ring.



Finally, the catalyst is regenerated by the reaction:

H+ + [AlCl4] HCl + AlCl3.

Phenol

Phenol(C6H5OH) contains an OH group on a benzene ring. A lone pair of electron

on the oxygen atom becomes part of the delocalised -system and makes phenol

much more susceptible to attack by electrophiles.

simple structure of phenol.

Fig. 5.3.8 Orbital structure of phenol.

Properties of phenol.

Phenol is less acidic than carboxylic acid but more acidic than alcohol(-COOH >

phenol > -OH). Therefore it can easily loses a proton and form stable phenoxide ion.

C6H5OH(aq) C6H5O(aq) + H+(aq)

Phenoxide ion

It is a solid at room temperature.



It is partially soluble in water due to the formation of hydrogen bond with water.

It is more reactive than benzene.

It can be used as antiseptic compounds.

Reactions of phenol.

(i) Reaction with aqueous sodium hydroxide.

Phenol reacts with sodium hydroxide to form a salt - sodium phenoxide. it is ionic and

water soluble

C6H5OH(aq) + NaOH(aq) C6H5O Na+(aq) + H2O(l)

This reaction is an evidence for the acidic character of phenol.

(ii) Reaction with sodium metal.

Phenol reacts with sodium to form an ionic salt - sodium phenoxide and hydrogen.

This reaction is similar to that with aliphatic alcohols such as ethanol

2C6H5OH(s) + 2Na(s) 2C6H5O Na+(s) + H2(g)

(iii) Reaction with carbonates and hydrogen carbonates.

Phenol does not react with carbonates and hydrogen carbonates as is is weakly

acidic.

(iv) Electrophilic substitution:

The OH group in phenol is electron releasing therefore it increases the electron

density of the delocalised system which makes substitution much easier compared to

benzene as a p orbital on the oxygen overlaps with the p orbitals in benzene

Fig. 5.3.9 p-orbitals in the system. The p orbital on the Oxygen

overlaps with the p orbitals in

the ring.



The electron density is greatest at the 2,4 and 6 positions which results in the substitution

takes place at the 2,4 and 6 positions.

Reaction with aqueous bromine.

The electron rich ring in phenol is attacked by bromine water, in an electrophilic substitution

reaction. The brown bromine water is decolorised and a white precipitate of 2,4,6-

tribromophenol and a solution of hydrogen bromide are formed. No catalyst is needed.

Phenol 2,4,6-tribromophenol

(white precipitate)

Reagents : Aqueous bromine

Conditions: Room temperature and pressure.

Reaction type: Electrophilic substitution

Observation: Orange colour decolourises/ formation of white ppt./ misty fumes.

Reaction with nitric acid:

The ring is sufficiently activated for nitration to take place with dilute nitric acid. At room

temperature, the organic product is a mixture of 2-nitrophenol and 4-nitrophenol.

OH OH OH

NO2

+ HNO3(aq) + + H2O

NO2

Phenol 2-nitrophenol 4-nitrophenol

6

6

6

6

6

2

4



Reagents : Dilute nitric acid

Conditions: Room temperature and pressure.

Reaction type: Electrophilic substitution

If the mixture is heated 2,4 and 2,6 dinitrophenol are formed as well. If concentrated nitic

acid is used, 2,4,6-trinitrophenol is the product.

Checklist

After studying this topic, you should be able to:

Define electrophile.

Estimate resonance energy of benzene from hydrogenation and bond enthalpy data.

Write equations and state conditions for the reactions of benzene and phenol with

bromine and nitric acid and benzene with sulphuric acid and the friedel- crafts

reactions.

Draw mechanisms for the halogenations, nitration and friedel- crafts reactions of

benzene.

Explain why the ring in methylbenzene is slightly activated and that in phenol very

activated.

Practice questions

(01) Explain why phenol can be nitrated under much milder conditions than those

required to nitrate benzene.

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(02) In the reaction shown below, the aromatic compound 1,4-dimethylbenzene reacts

with 2-bromobutane. The reaction is catalysed by aluminium chloride, AlCl3, which

dissolves in the reaction mixture.



(a) (i) Name the type of reaction and the mechanism.

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(ii) Write the equation to show how the attacking species forms and give the mechanism for

the reaction.

Equation:

Mechanism:



(03) Some reactions of benzene are shown below.

(a) (i) Write the equation to show how the catalyst, AlCl3, reacts with reagent A to form the

species which attacks the benzene ring.

(ii) Draw the structure of the intermediate ion formed when the species in (ii) attacks the

benzene ring.

(b) The methylbenzene formed in reaction 1 generally reacts in a similar way to benzene

but faster, as the ring is said to be activated.

(i) Explain how the presence of a methyl group activates the benzene ring.

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(ii) Use your answer to (i) to explain why methylbenzene reacts faster.

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(c) (i) Draw the structural formula of compound X, formed in reaction 2.

(ii) The organic product of reaction 2 is also formed when the same reactants, but with an

aluminium catalyst, are heated using microwave radiation. Suggest two reasons why this

technique may be considered greener.

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(d) Name reagent B needed for reaction 3.



(04) Explain, in terms of the bonding in the benzene ring, why the enthalpy of hydrogenation

is less exothermic than would be expected from a molecule with three double bonds.

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(05)(i) Explain why phenol, C6H5OH, and methoxybenzene, C6H5OCH3, are much more

reactive than benzene with bromine.

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(ii) Write the equation for the reaction between phenol and bromine water. State symbols are

not required.

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Chapter 5.3 Arenes Benzene

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