318 chemistry

Thin film of polyacetylene can be used as but in a majority of reactions of aromatic

electrodes in batteries. These films are good compounds, the unsaturation of benzene ring
conductors, lighter and cheaper than the is retained. However, there are examples of
metal conductors. aromatic hydrocarbons which do not contain a
(b) Cyclic polymerisation: Ethyne on benzene ring but instead contain other highly
passing through red hot iron tube at 873K unsaturated ring. Aromatic compounds
undergoes cyclic polymerization. Three containing benzene ring are known as
molecules polymerise to form benzene, which benzenoids and those not containing a
is the starting molecule for the preparation of benzene ring are known as non-benzenoids.
derivatives of benzene, dyes, drugs and large Some examples of arenes are given
number of other organic compounds. This is below:
the best route for entering from aliphatic to
aromatic compounds as discussed below:

Benzene Toluene Naphthalene

(9.69)

Problem 9.14 Biphenyl

How will you convert ethanoic acid into
benzene? 9.5.1 Nomenclature and Isomerism
The nomenclature and isomerism of aromatic
Solution hydrocarbons has already been discussed in
Unit 8. All six hydrogen atoms in benzene
are equivalent; so it forms one and only one
type of monosubstituted product. When two
hydrogen atoms in benzene are replaced by
two similar or different monovalent atoms or
groups, three different position isomers are
possible. The 1, 2 or 1, 6 is known as the ortho
(o–), the 1, 3 or 1, 5 as meta (m–) and the 1,
4 as para (p–) disubstituted compounds. A
few examples of derivatives of benzene are
given below:

9.5 Aromatic Hydrocarbon

These hydrocarbons are also known as
‘arenes’. Since most of them possess pleasant
odour (Greek; aroma meaning pleasant
smelling), the class of compounds was Methylbenzene 1,2-Dimethylbenzene
named as ‘aromatic compounds’. Most of such
compounds were found to contain benzene (Toluene) (o-Xylene)
ring. Benzene ring is highly unsaturated

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 Hydrocarbons 319

Friedrich August Kekulé,a German chemist was born in 1829 at Darmsdt in

Germany. He became Professor in 1856 and Fellow of Royal Society in 1875. He
made major contribution to structural organic chemistry by proposing in 1858 that
carbon atoms can join to one another to form chains and later in 1865,he found
an answer to the challenging problem of benzene structure by suggesting that
these chains can close to form rings. He gave the dynamic structural formula to
benzene which forms the basis for its modern electronic structure. He described
the discovery of benzene structure later as:
FRIEDRICH
“I was sitting writing at my textbook,but the work did not progress; my thoughts
AUGUST KEKULÉ
were elsewhere. I turned my chair to the fire, and dozed. Again the atoms were
(7th September
gambolling before my eyes. This time the smaller groups kept modestly in the
1829–13th July
background. My mental eye, rendered more acute by repeated visions of this
1896)
kind, could now distinguish larger structures of manifold conformations; long
rows,sometimes more closely fitted together; all twisting and turning in snake like motion. But look! What
was that? One of the snakes had seized hold of it’s own tail, and the form whirled mockingly before my
eyes. As if by a flash of lightning I woke;.... I spent the rest of the night working out the consequences of
the hypothesis. Let us learn to dream, gentlemen, and then perhaps we shall learn the truth but let us
beware of making our dreams public before they have been approved by the waking mind.”( 1890).
One hundred years later, on the occasion of Kekulé’s centenary celebrations a group of compounds
having polybenzenoid structures have been named as Kekulenes.

was further found to produce one and only one

monosubstituted derivative which indicated
that all the six carbon and six hydrogen atoms
of benzene are identical. On the basis of this
observation August Kekulé in 1865 proposed
the following structure for benzene having
cyclic arrangement of six carbon atoms with
1,3 Dimethylbenzene 1,4-Dimethylbenzene alternate single and double bonds and one
(m-Xylene) ( p-Xylene) hydrogen atom attached to each carbon atom.
9.5.2 Structure of Benzene
Benzene was isolated by Michael Faraday
in 1825. The molecular formula of benzene,
C6H6, indicates a high degree of unsaturation.
This molecular formula did not account for
its relationship to corresponding alkanes,
alkenes and alkynes which you have studied
in earlier sections of this unit. What do you The Kekulé structure indicates
think about its possible structure? Due to the possibility of two isomeric
its unique properties and unusual stability, 1, 2-dibromobenzenes. In one of the isomers,
it took several years to assign its structure. the bromine atoms are attached to the
Benzene was found to be a stable molecule doubly bonded carbon atoms whereas in the
and found to form a triozonide which indicates other, they are attached to the singly bonded
the presence of three double bonds. Benzene carbons.

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unhybridised p orbital perpendicular to the

plane of the ring as shown below:

However, benzene was found to form only

one ortho disubstituted product. This problem
was overcome by Kekulé by suggesting the
concept of oscillating nature of double bonds
in benzene as given below.

The unhybridised p orbital of carbon atoms

are close enough to form a π bond by lateral
overlap. There are two equal possibilities of
Even with this modification, Kekulé
forming three π bonds by overlap of p orbitals
structure of benzene fails to explain unusual
of C1 –C2, C3 – C4, C5 – C6 or C2 – C3, C4 – C5,
stability and preference to substitution
C6 – C1 respectively as shown in the following
reactions than addition reactions, which
figures.
could later on be explained by resonance.
Resonance and stability of benzene
According to Valence Bond Theory, the
concept of oscillating double bonds in benzene
is now explained by resonance. Benzene is a
hybrid of various resonating structures. The
two structures, A and B given by Kekulé are
the main contributing structures. The hybrid
structure is represented by inserting a circle
or a dotted circle in the hexagon as shown
in (C). The circle represents the six electrons
which are delocalised between the six carbon Fig. 9.7 (a)
atoms of the benzene ring.

(A) (B) (C)

The orbital overlapping gives us better
picture about the structure of benzene. All
2
the six carbon atoms in benzene are sp
2
hybridized. Two sp hybrid orbitals of each
2
carbon atom overlap with sp hybrid orbitals
of adjacent carbon atoms to form six C—C
sigma bonds which are in the hexagonal
2 Fig. 9.7 (b)
plane. The remaining sp hybrid orbital of
each carbon atom overlaps with s orbital Structures shown in Fig. 9.7(a) and (b)
of a hydrogen atom to form six C—H sigma correspond to two Kekulé’s structure with
bonds. Each carbon atom is now left with one localised π bonds. The internuclear distance

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 Hydrocarbons 321

between all the carbon atoms in the ring has (i) Planarity
been determined by the X-ray diffraction to (ii) Complete delocalisation of the π electrons
be the same; there is equal probability for the in the ring
p orbital of each carbon atom to overlap with
(iii) Presence of (4n + 2) π electrons in the ring
the p orbitals of adjacent carbon atoms [Fig.
where n is an integer (n = 0, 1, 2, . . .).
9.7 (c)]. This can be represented in the form
of two doughtnuts (rings) of electron clouds This is often referred to as Hückel Rule.
[Fig. 9.7 (d)], one above and one below the Some examples of aromatic compounds are
plane of the hexagonal ring as shown below: given below:

(electron cloud)

Fig. 9.7 (c) Fig. 9.7 (d)

The six π electrons are thus delocalised
and can move freely about the six carbon
nuclei, instead of any two as shown in
Fig. 9.6 (a) or (b). The delocalised π electron cloud
is attracted more strongly by the nuclei of the
carbon atoms than the electron cloud localised
between two carbon atoms. Therefore, presence
of delocalised π electrons in benzene makes
it more stable than the hypothetical
cyclohexatriene.
X-Ray diffraction data reveals that benzene
is a planar molecule. Had any one of the above
structures of benzene (A or B) been correct,
9.5.4 Preparation of Benzene
two types of C—C bond lengths were expected.
However, X-ray data indicates that all the Benzene is commercially isolated from coal
six C—C bond lengths are of the same order tar. However, it may be prepared in the
(139 pm) which is intermediate between laboratory by the following methods.
C— C single bond (154 pm) and C—C double (i) Cyclic polymerisation of ethyne:
bond (133 pm). Thus the absence of pure (Section 9.4.4)
double bond in benzene accounts for the (ii) Decarboxylation of aromatic acids:
reluctance of benzene to show addition Sodium salt of benzoic acid on heating
reactions under normal conditions, thus with sodalime gives benzene.
explaining the unusual behaviour of benzene.
9.5.3 Aromaticity
Benzene was considered as parent ‘aromatic’
compound. Now, the name is applied to all the
ring systems whether or not having benzene (9.70)
ring, possessing following characteristics.

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(iii) Reduction of phenol: Phenol is reduced (ii) Halogenation: Arenes react with halogens
to benzene by passing its vapours over in the presence of a Lewis acid like anhydrous
heated zinc dust FeCl3, FeBr3 or AlCl3 to yield haloarenes.

(9.71) Chlorobenzene
9.5.5 Properties (9.73)
Physical properties (iii) Sulphonation: The replacement of a
Aromatic hydrocarbons are non- polar hydrogen atom by a sulphonic acid group in
molecules and are usually colourless liquids a ring is called sulphonation. It is carried out
or solids with a characteristic aroma. You are by heating benzene with fuming sulphuric
also familiar with naphthalene balls which are acid (oleum).
used in toilets and for preservation of clothes
because of unique smell of the compound
and the moth repellent property. Aromatic
hydrocarbons are immiscible with water but
are readily miscible with organic solvents.
They burn with sooty flame.
Chemical properties (9.74)
Arenes are characterised by electrophilic
substitution reactions. However, under (iv) Friedel-Crafts alkylation reaction:
special conditions they can also undergo When benzene is treated with an alkyl halide
addition and oxidation reactions. in the presence of anhydrous aluminium
chloride, alkylbenene is formed.
Electrophilic substitution reactions
The common electrophilic substitution
reactions of arenes are nitration, halogenation,
sulphonation, Friedel Craft’s alkylation and
acylation reactions in which attacking reagent
+
is an electrophile (E )
(i) Nitration: A nitro group is introduced (9.75)
into benzene ring when benzene is heated
with a mixture of concentrated nitric acid
and concentrated sulphuric acid (nitrating
mixture).
(9.76)

Why do we get isopropyl benzene on

treating benzene with 1-chloropropane
instead of n-propyl benzene?

(v) Friedel-Crafts acylation reaction: The

reaction of benzene with an acyl halide or
(9.72) acid anhydride in the presence of Lewis acids
(AlCl3) yields acyl benzene.
Nitrobenzene

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 Hydrocarbons 323

(9.77)

In the case of nitration, the electrophile,

nitronium ion, is produced by transfer of
a proton (from sulphuric acid) to nitric acid
in the following manner:
(9.78) Step I
If excess of electrophilic reagent is used,
further substitution reaction may take place
in which other hydrogen atoms of benzene Step II
ring may also be successively replaced
by the electrophile. For example, benzene
on treatment with excess of chlorine in
the presence of anhydrous AlCl 3 can be Protonated Nitronium
chlorinated to hexachlorobenzene (C6Cl6) nitric acid ion
It is interesting to note that in the process
of generation of nitronium ion, sulphuric acid
serves as an acid and nitric acid as a base.
Thus, it is a simple acid-base equilibrium.
(b) F o r m a t i o n o f C a r b o c a t i o n
(arenium ion): Attack of electrophile
results in the formation of σ-complex or
(9.79) arenium ion in which one of the carbon is sp
3

Mechanism of electrophilic substitution hybridised.

reactions:
According to experimental evidences, SE (S =
substitution; E = electrophilic) reactions are
supposed to proceed via the following three
steps:
(a) Generation of the eletrophile sigma complex (arenium ion)
(b) Formation of carbocation intermediate The arenium ion gets stabilised by
(c) Removal of proton from the carbocation resonance:
intermediate
⊕
(a) Generation of electrophile E : During
chlorination, alkylation and acylation of
benzene, anhydrous AlCl3, being a Lewis acid
⊕ ⊕
helps in generation of the elctrophile Cl , R ,
⊕
RC O (acylium ion) respectively by combining
with the attacking reagent.

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 324 chemistry

Sigma complex or arenium ion loses its chemical equation:

aromatic character because delocalisation of
3 CxHy + (x + ) O2 → x CO2 + H2O n (9.83)
electrons stops at sp hybridised carbon.
(c) Removal of proton: To restore the 9.5.6 Directive influence of a functional
aromatic character, σ -complex releases group in monosubstituted benzene
3
proton from sp hybridised carbon on attack
– When monosubstituted benzene is subjected
by [AlCl4] (in case of halogenation, alkylation
– to further substitution, three possible
and acylation) and [HSO 4 ] (in case of
disubstituted products are not formed in
nitration).
equal amounts. Two types of behaviour are
observed. Either ortho and para products or
meta product is predominantly formed. It
has also been observed that this behaviour
depends on the nature of the substituent
already present in the benzene ring and not
on the nature of the entering group. This is
known as directive influence of substituents.
Reasons for ortho/para or meta directive
nature of groups are discussed below:
Addition reactions
Ortho and para directing groups: The
Under vigorous conditions, i.e., at high
groups which direct the incoming group to
temperature and/ or pressure in the presence
ortho and para positions are called ortho and
of nickel catalyst, hydrogenation of benzene para directing groups. As an example, let us
gives cyclohexane. discuss the directive influence of phenolic
(–OH) group. Phenol is resonance hybrid of
following structures:

Cyclohexane
(9.80)
Under ultra-violet light, three chlorine
molecules add to benzene to produce benzene
hexachloride, C6H6Cl6 which is also called
gammaxane.

Benzene hexachloride,
It is clear from the above resonating
(BHC) structures that the electron density is more on
(9.81) o – and p – positions. Hence, the substitution
Combustion: When heated in air, benzene takes place mainly at these positions. However,
burns with sooty flame producing CO2 and it may be noted that –I effect of – OH group also
H2O operates due to which the electron density on
15 ortho and para positions of the benzene ring
C H6 + O2 → 6CO2 +3H2O is slightly reduced. But the overall electron
6 2 (9.82) density increases at these positions of the
General combustion reaction for any ring due to resonance. Therefore, –OH group
hydrocarbon may be given by the following activates the benzene ring for the attack by

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 Hydrocarbons 325

an electrophile. Other examples of activating In this case, the overall electron density
groups are –NH2, –NHR, –NHCOCH3, –OCH3, on benzene ring decreases making further
–CH3, –C2H5, etc. substitution difficult, therefore these groups
In the case of aryl halides, halogens are are also called ‘deactivating groups’. The
moderately deactivating. Because of their electron density on o – and p – position
strong – I effect, overall electron density on is comparatively less than that at meta
benzene ring decreases. It makes further position. Hence, the electrophile attacks on
substitution difficult. However, due to comparatively electron rich meta position
resonance the electron density on o– and resulting in meta substitution.
p – positions is greater than that at the
9.6 Carcinogenicity and Toxicity
m-position. Hence, they are also o – and p –
directing groups. Resonance structures of Benzene and polynuclear hydrocarbons
chlorobenzene are given below: containing more than two benzene rings
fused together are toxic and said to
possess cancer producing (carcinogenic)
property. Such polynuclear hydrocarbons
are formed on incomplete combustion of
organic materials like tobacco, coal and
petroleum. They enter into human body
and undergo various biochemical reactions
and finally damage DNA and cause cancer.
Some of the carcinogenic hydrocarbons are
given below (see box).

Meta directing group: The groups which

direct the incoming group to meta position are
called meta directing groups. Some examples
of meta directing groups are –NO2, –CN, –CHO,
–COR, –COOH, –COOR, –SO3H, etc.
Let us take the example of nitro group.
Nitro group reduces the electron density in
the benzene ring due to its strong–I effect.
Nitrobenzene is a resonance hybrid of the
following structures.

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SUMMARY

Hydrocarbons are the compounds of carbon and hydrogen only. Hydrocarbons are mainly
obtained from coal and petroleum, which are the major sources of energy. Petrochemicals
are the prominent starting materials used for the manufacture of a large number of
commercially important products. LPG (liquefied petroleum gas) and CNG (compressed
natural gas), the main sources of energy for domestic fuels and the automobile industry, are
obtained from petroleum. Hydrocarbons are classified as open chain saturated (alkanes)
and unsaturated (alkenes and alkynes), cyclic (alicyclic) and aromatic, according to their
structure.
The important reactions of alkanes are free radical substitution, combustion, oxidation
and aromatization. Alkenes and alkynes undergo addition reactions, which are mainly
electrophilic additions. Aromatic hydrocarbons, despite having unsaturation, undergo
mainly electrophilic substitution reactions. These undergo addition reactions only under
special conditions.
Alkanes show conformational isomerism due to free rotation along the C–C sigma
bonds. Out of staggered and the eclipsed conformations of ethane, staggered conformation
is more stable as hydrogen atoms are farthest apart. Alkenes exhibit geometrical
(cis-trans) isomerism due to restricted rotation around the carbon–carbon double bond.
Benzene and benzenoid compounds show aromatic character. Aromaticity, the
property of being aromatic is possessed by compounds having specific electronic structure
characterised by Hückel (4n+2)π electron rule. The nature of groups or substituents attached
to benzene ring is responsible for activation or deactivation of the benzene ring towards
further electrophilic substitution and also for orientation of the incoming group. Some of
the polynuclear hydrocarbons having fused benzene ring system have carcinogenic property.

EXERCISES

9.1 How do you account for the formation of ethane during chlorination of methane ?
9.2 Write IUPAC names of the following compounds :
(a) CH3CH=C(CH3)2 (b) CH2=CH-C≡C-CH3

(c) (d) –CH2–CH2–CH=CH2

(f) CH3(CH2)4 CH (CH2)3 CH3

(e) CH2 –CH (CH3)2

(g) CH3 – CH = CH – CH2 – CH = CH – CH – CH2 – CH = CH2
|
C2H5
9.3 For the following compounds, write structural formulas and IUPAC names for all
possible isomers having the number of double or triple bond as indicated :
(a) C4H8 (one double bond) (b) C5H8 (one triple bond)
9.4 Write IUPAC names of the products obtained by the ozonolysis of the following
compounds :
(i) Pent-2-ene (ii) 3,4-Dimethylhept-3-ene
(iii) 2-Ethylbut-1-ene (iv) 1-Phenylbut-1-ene

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 Hydrocarbons 327

9.5 An alkene ‘A’ on ozonolysis gives a mixture of ethanal and pentan-3-one.

Write structure and IUPAC name of ‘A’.
9.6 An alkene ‘A’ contains three C – C, eight C – H σ bonds and one C – C π
bond. ‘A’ on ozonolysis gives two moles of an aldehyde of molar mass 44
u. Write IUPAC name of ‘A’.
9.7 Propanal and pentan-3-one are the ozonolysis products of an alkene? What
is the structural formula of the alkene?
9.8 Write chemical equations for combustion reaction of the following
hydrocarbons:
(i) Butane (ii) Pentene
(iii) Hexyne (iv) Toluene
9.9 Draw the cis and trans structures of hex-2-ene. Which isomer will have
higher b.p. and why?
9.10 Why is benzene extra ordinarily stable though it contains three double
bonds?
9.11 What are the necessary conditions for any system to be aromatic?
9.12 Explain why the following systems are not aromatic?

(i) (ii) (iii)

9.13 How will you convert benzene into
(i) p-nitrobromobenzene (ii) m- nitrochlorobenzene
(iii) p - nitrotoluene (iv) acetophenone?
9.14 In the alkane H3C – CH2 – C(CH3)2 – CH2 – CH(CH3)2, identify 1°,2°,3° carbon
atoms and give the number of H atoms bonded to each one of these.
9.15 What effect does branching of an alkane chain has on its boiling point?
9.16 Addition of HBr to propene yields 2-bromopropane, while in the presence
of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain
and give mechanism.
9.17 Write down the products of ozonolysis of 1,2-dimethylbenzene (o-xylene).
How does the result support Kekulé structure for benzene?
9.18 Arrange benzene, n-hexane and ethyne in decreasing order of acidic
behaviour. Also give reason for this behaviour.
9.19 Why does benzene undergo electrophilic substitution reactions easily and
nucleophilic substitutions with difficulty?
9.20 How would you convert the following compounds into benzene?
(i) Ethyne (ii) Ethene (iii) Hexane
9.21 Write structures of all the alkenes which on hydrogenation give
2-methylbutane.
9.22 Arrange the following set of compounds in order of their decreasing relative
+
reactivity with an electrophile, E
(a) Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
(b) Toluene, p-H3C – C6H4 – NO2, p-O2N – C6H4 – NO2.
9.23 Out of benzene, m–dinitrobenzene and toluene which will undergo nitration
most easily and why?
9.24 Suggest the name of a Lewis acid other than anhydrous aluminium chloride
which can be used during ethylation of benzene.
9.25 Why is Wurtz reaction not preferred for the preparation of alkanes
containing odd number of carbon atoms? Illustrate your answer by taking
one example.

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