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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.

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5.3 ARENES: BENZENE 2014

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

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5.3 ARENES: BENZENE 2014

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.

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5.3 ARENES: BENZENE 2014

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

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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

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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 usua