Hydrocarbon arjav patel

Welcome

Anandalaya Study Corner.
By-Arjav Patel
11-C

HYDROCARBONS are the compounds containing
carbon and hydrogen only.

Depending upon the types of carbon-carbon bonds present, they can
be classified into tree main categories:

1) Saturated Hydrocarbon
2) Unsaturated Hydrocarbon
3) Aromatic Hydrocarbon

The hydrocarbon that contain only carbon-carbon single bond is called
Saturated Hydrocarbon. These include open chain hydrocarbon as
well as closed chain hydrocarbons. These compounds are called
saturated because they have maximum number of bonded
hydrogen
If different carbon atoms are joined together to form an
open chain of carbon atoms with single bonds, they are called
Alkanes.
For example: 2-Methylpropane (Isobutane)

If carbon atoms form a closed chain or ring, they are called
Cycloalkanes.
For example: Cyclopentane

The hydrocarbons which contain carbon-carbon multiple bond (Double
bonds or triple bond) are called unsturated hydrocarbon.
Depending upon multiple bond they are
further classified as alkenes and alkynes.

o Alkenes : These are hydrocarbon which contain at least one
carbon-carbon double bond. For example: Ethene

o Alkynes: These are hydrocarbons which contain at least one
carbon-carbon triple bond. For example: Ethyle

The hydrocarbons which contain at least one special type of
hexagonal ring of carbon atoms with three double bond in the
alternate positions are called aromatic hydrocarbon. The ring is
called aromatic ring.

For example: i) Toluene ii) o-Xylene

The aromatic compounds may also contain more than one benzene
rings. For example: i) Naphthalene ii)
Anthracene

Hydrocarbon Type Characteristic Group Example

Saturated Hydrocarbon: No double or Triple Bond CH3CH2CH3
Propane
Alkanes

Unsaturated Hydrocarbon: Double Bond CH3–CH═CH2
Alkenes Propene

Alkynes Triple Bond CH3−C≡CH
Propyne

Aromatic Hydrocarbons: Benzene ring

Methyl Benzene

Alkanes are saturated hydrocarbon containing only carbon-carbon
single bond in their molecule. They are also called Paraffins. At high
temperatures and pressure do undergo some reaction. The alkanes
may be divided as:

1) Open chain or Acyclic alkanes .
2) Cycloalkanes or cyclic alkanes.

These are simple alkanes without any close chains and have
the general formula where CnH2n + 2 n is the number of
carbon atoms.

For example: i) Methane - CH4

These contain a closed chain or ring in their molecules. They
have the general formula CnH2n.

For example: i) Cyclopropane- or

ii)Cyclobutane- or

Methane is the first member of the family. It has Tetrahedral Structure
involving sp3 Hybridisation. The four sigma bond is formed by the
overlapping of sp3 hybrid orbitals of carbon and 1s orbital of hydrogen.
In this, carbon atom lies at the centre and the four hydrogen atoms lies at
the corners of a regular tetrahedron. Making H-C-H bond angle of 109.5˚.

a) b) c)

1.4 Nomenclature Of
Alkanes
Nomenclature implies assigning proper name to the basis of
certain standard rules so that the study of these compounds
may become standard. The rules for naming them are as
follows:

i)
First of all, select the longest continues chain of carbon atoms in a
molecule.
1 2 3 4 5 6 7 8 9
For eg. CH3– CH– CH2– CH2– CH2–CH– CH2– CH2–CH3

The carbon atoms of the parent chain are numbered to identify the
parent alkane and to locate the positions of the carbon atom at
which branching take place due to the substitution of alkyle group
in place of hydrogen atom. The numbering is done in such a way that
the branched carbon atoms get the lowest possible number.

For eg: 9 8 7 6 5 4 3 2 1
C−C−C−C−C−C−C−C−C

C C−C

When two or more substituents are present, then end of the parent chain
which gives the lowest set of the locants is preferred for numbering. This
rule is called lowest set of locants.
This means that when two or more different sets of locants are
possible, that set of locants which when compared term with other
sets, each in order of increasing magnitude, has the lowest term at the
first point of difference.

For eg: 6 5 4 3 2 1
H3C−CH−CH3−CH−CH−CH3

CH3 CH3 CH3

Set of
locants: 2,3,5

If the same substituent or side chain occurs more than once, the prefix di(for
2), tri(for 3), tetra(for 4), penta(for 5),hexa(for 6)…etc., are attached to
the names of the substituents. The positions of the substituents are
indicated separately and the numerals representing their positions are
separated by commas.

For eg: 1 2 3 4 5
CH3–CH–CH2–CH–CH3

CH3 CH3

2,4-Dimethylpentane

If two or more different substituents or side chains are present in the
molecule, they are named in the alphabetical order along with their
appropriate positions. Prefix are ignored while comparing the
substituents.

For eg: CH3CH3
5 4 3 2 1
CH3−CH3−C−CH3−CH3

CH3CH3

3 -Ethyl-2,3-dimethylpentane

If two different substituents are in equivalent positions from the two ends
of the chain, then the numbering of the chain is done in such a way that
the group which comes first in the alphabetical order gets lower down.
For eg: 1 2 3 4 5 6 7 7 6 5 4
3 2 1
CH3−CH2−CH−CH2−CH−CH2−CH3 CH3−CH2−CH−CH2−CH−CH2−CH3

CH3 CH2CH3 CH3 CH3CH3

( Methyl at C-3) (Ethyl
at C-3)
The carbon bearing ethyl group gets lower position because it is
cited first in the name according to alphabetical order of

If the substituent on the parent chain is complex it is named
as substituted alkyl group by numbering the carbon atom
of this group attached to the parent chain as 1.the name of
such substituents is given in brackets in order to avoid
confusion with the numbering of the parent chain.
For eg: 1 2 3 4 5 6 7 8 9
CH3−CH3−CH3−CH3−CH3−CH3−CH3−CH3−CH3
1
CH3
2
CH3 Complex Substituent
3

Petroleum and natural gas are the main source of alkanes. However,
alkanes can be prepared by three methods.

The unsaturated hydrocarbons (alkenes and alkynes) are
converted into alkanes by catalytic hydrogenation. In this
process dihydrogen is passed through alkenes or alkynes in
the presence of finely divided catalysts such as Raney Ni, Pt
or Pd. These metals absorb dihydrogen gas on their surfaces
and activate the hydrogen-hydrogen bond. Platinum and
palladium catalyse the reaction at room tempreture.
However,higher tempreture (523-573k) and pressure are
required with nickle catalysts.
The hydrogenation reaction of unsaturated
hydrocarbon using nickle at a tempreture of 523-573K is
commonly known as Sabatier and Sender’s reaction or

i) Alkyl halides (except fluorides) on reduction with zinc and
dilute hydrochloric acid give alkanes.
For eg:

ii) Alkyl halides on treatment with sodium metal in dry
ethereal (free from moisture)solution give higher alkanes.
This reaction is known as Wurtz reaction and is used for
the preparation of higher alkanes containing even number
of carbon atom.

Decarboxylation Kolbe’s electrolytic
reaction method

i) Decarboxylation reaction :
Sodium salts of carboxylic acids on heating with soda lime
(mixture of sodium hydroxide and calcium oxide)gives
alkanes containing one carbon atom less than the
carboxylic acid. This process of elimination of carbon
dioxide from a carboxylic acid is known as
decarboxylation.

ii) Kolbe’s electrolytic method:
An aqueous solution of sodium or potassium salt of a
carboxylic acid on electrolysis gives alkane containing even
number of carbon atoms at the anode.

The reaction is supposed to follow the following path:
 .

i)

ii)At anode:

1.6 Properties of
Alkanes

Alkanes are almost non-polar molecules and therefore
the molecules are hold only by weak Van der Waals forces.
The weak intermolecular forces depend only upon the size

Alkanes have generally low boiling points because these are
non-polar and the molecules are held together only by weak
Van der Waals’ forces. With the increase in the number of
carbon atoms, the molecular size increases and therefore, the
magnitude of Van der Waals forces also increases.
Consequently, the boiling points increase with increase in
number of carbon atoms.
It has been observed that each carbon added
to the chain increases the boiling point by 20-30 k. the
boiling point of n-alkanes with increase in number of carbon
per molecule of the homologous series.

The melting points of alkanes do not shows regular variation
with increase in molecular size. It has been observed that, in
general, the alkanes with even number of carbon atoms have
Alkane melting points as H
higher C H C H
3 8 4 10 C compared toCthe immediately next
5 12 CH
6 14 H
7 16 CH
8 18

m.p.(K) alkanes with odd number 178.5
lower 85.9 138 143.3 of carbon atoms.
182.5 216.2

This is because the alkanes with even number of carbon atoms

Alkanes being non-polar in nature, are expected to be insoluble
in water(polar solvent). They dissolve in non-polar solvents
such as ether, benzene, carbon tetrachloride etc. The
solubility generally decreases with increase in molecular
mass. As we know, petrol is a mixture of hydrocarbon and is
used as a fuel for automobiles.

Alkanes are lighter than water. The density increase with the
increase in the number of the carbon atoms.

The reaction in which an atom or a group of atoms in a
molecule is replaced by some other atom or group of atom.
Alkanes undergo substitution reaction in which one or more
hydrogen atoms are replaced or substituted by different
atoms or groups such as halogen atom (Cl, Br or I), nitro
group(-NO2) or sulphonic acid (-SO3H) group.

This involves the replacement of one or more atoms of alkanes by the
corresponding number of halogens atoms. It is found that the rate of
reaction of alkanes with halogen is F2>Cl2>Br2>I2. Rate of
replacement of hydrogen of alkanes is:3˚>2˚>1˚.

For eg:

The reaction is initiated by homolysis of chlorine molecule in
the presence of light or heat, the Cl-Cl bond is weaker than
the C-C and C-H bond and hence, is easiest to break.

Chlorine free radicals attacks the methane molecule and
takes the reaction in the forward direction by breaking the
C-H bond to generate methyl free radical with the
formation of H-Cl.

The reaction stops after some time due to consumption of
reactants and/or due to following side reaction:
The possible chain terminating steps are:
a)
b)
c)

Though in (c) CH3-Cl, the one of the product is formed bur
free radicals are consumed and the chain is terminated.

Alkanes on heating in the presence of air or dioxygen are
completely oxidized to carbon dioxide and water with the
evolution of large amount of heat.

The general combustion equation for any alkane is:

Alkanes on heating with a regulated supply of dioxygen or air
at high pressure and in the presence of suitable catalyst give
a variety of oxidation product:

i)When a mixture of methane and oxygen in the molar ratio of
9:1 is compressed to about1100 atmospheres and passed
through copper tubes at 575 K, methane is oxidised to
methanol.

2CH4 + O2 Cu/575K/1100 atm. 2CH3OH

Alkane isomerise to branched chain alkanes when heated
with anhydrous aluminium chloride (AlCl3) and hydrogen
chloride at 573 K under a pressure of about 30-35
atmosphere. CH3

CH3CH2CH2CH3 anhy.AlCl3,HCl CH3−CH−CH3
n-butane isopropane

The alkanes containing six or more carbon atoms when
heated at about 773K under high pressure of 10-20 atm in
the presence of catalyst on alumina gel get converted to

On passing a mixture of steam and methane over heated nickle
(supported over alumina, Al2O3) catalyst at 1273 K,
methane is oxidised to carbon monoxide and hydrogen is
evolved.
CH4 +H2O CO + 3H2

When higher alkanes are heated to high tempreture in the
presence of alumina or silica catalysts, the alkanes break
down to lower alkanes and alkenes. For eg:
C3H8 C2H4 + CH4 or C3H6 + H2

Chemist represent conformations in two simple ways:
a)Sawhorse representation b)Newman projection

In this projection, the molecule is viewed along the axis of
the model from an oblique angle. The central carbon-
carbon bond (C-C) s drawn as a straight line slightly
tilted to right for the sake of clarity. The front carbon is
shown as the lower left hand carbon and there are carbon
is shown as the upper right hand carbon.

In this method, the molecule is viewed from the front along
the carbon-carbon bond axis. The two carbon atoms
forming the σ bond are represented by two circle; one
behind the other so that only the front carbon is seen. The
front carbon atom is shown by a point whereas the carbon
further from the eye is represented by the circle.
Therefore, the C-H bonds of the front carbon are depicted
from the centre of the circle while C-H bonds of the back
carbon are drawn from the circumference of the circle at an
angle of 120˚ at each other.

Alkenes are unsaturated hydrocarbons containing carbon-
carbon double bond (C═C)in their molecules. They have the
general formula CnH2n. The simplest member of alkene
family is ethene, C2H4. The alkenes are also called olefins
(Greek olefiant meaning oil forming) because the larger
member of the series (such as ethylene, propylene, etc react
with chlorine to form oily products.

Carbon-Carbon double bond in alkenes consists of one strong
sigma(σ) bond (bond enthalpy about 397kJ mol-1 due to head
on overlapping of sp2 hybridised orbitals and one weak pi()
bond(bond enthalpy about 284 kJ mol-1)obtained by lateral
or sideways overlapping of the two 2p orbitals of the two
carbon atom. The double bond is shorter in bond length
(134pm) than the single bond (154pm). Alkenes are easily
attacked by reagents or compounds which are in search of
electron(electrophilic reagents)because they behave as source
of loosely held mobile electron. The presence of weaker  bond
makes alkenes unstable molecules in comparison to alkanes
and thus, alkenes can be changed into single bond

According to IUPAC system alkenes are named similar to
alkanes with the following modification:
i)The longest continues chain should include both the carbon
atoms of the double bond.
ii)The suffix used for alkene is –ene
iii)The chain is numbered from the end that gives the lower
number to the first carbon atom of the double bond.
iv)If there are two or more double bonds the ending ane of the
alkane is replaced by adiene or atiene.
1 2 3 4 5 1 2 3

Isomerism in
Alkanes

Structural Geometrical
Isomerism Isomerism

Chain Position
Isomerism Isomerism

Alkenes show following types of structural isomerisms:

The isomers differ with respect to the chain of carbon atoms.
as in alkanes, ethene (C2H4) and propene(C3H6) can have
only one structure but alkenes higher than propene have
different structures.
For eg: 4 3 2 1 But-1-ene
CH3−CH2−CH=CH2

The isomers differ in the position of the double bonds. For eg:

The compounds which have the same structural formula but
differ in the spatial arrangement of atoms or groups of
atoms about the double bond are called geometrical isomers
and the phenomena is known as geometrical isomerism.
The isomers in which similar atoms or groups lie on the
same side of the double bond is called cis-isomers while the
other in which they are displaced on opposite sides, is
called trans-isomerism.

Alkynes can be reduced to alkenes using palladium charcoal
(palladised charcoal) catalyst partially deactivated with
poison like sulphur compounds or quioline. Partially
deactivated palladised charcoal is known as Lindlar’s
catalyst. Alkynes can also be reduced to alkenes with
sodium in liquid ammonia (called Birch reduction).

For eg: CH3−C≡C−CH3 Pd- C, H2 CH3CH═CHCH3
But-2-yne But-2-ene
CH3–C≡CH+H2 CH3–CH=CH2
Propyne Propene
Pd/C

Alkene can be prepared from alkyl halides(usually bromides or
iodides) by treating with alcoholic potash(potassium
hydroxide dissolved in ethanol). This reaction removes a
molecule of HX and therefore, the reaction is called
dehydrohalogenation. In this reaction, the hydrogen atom is
eliminated from β carbon atom (carbon atom next to the
carbon to which halogen is attached). Therefore, the reaction
is also called β–elimination reaction.

Dihalogen derivatives of alkanes in which two halogens atoms
are attached to adjacent carbon atoms (called vicinal
dihalogen derivatives) are converted to alkenes by heating
with zinc dust in ethyl alcohol. For eg:
CH3CHBr−CH2Br+Zn CH3CH=CH2+ZnBr

Alkenes are prepared from alcohols by heating with protonic
acids such as sulphuric acid at about 443K. This reaction is
called dehydration of alcohols
CH3CH2OH H2SO4 or H3PO4 CH2=CH2+H2O

In general, alkenes have higher melting point than the
corresponding alkanes. This is due to the reason that p-
electrons of a double bond are more polarizable than s-electron
of single bonds. As a result, the intermolecular force of
attraction are stronger in alkenes than alkanes. The melting
and boiling point of alkenes in general, increase with increase
in molecular mass.

Alkenes are weakly polar. The p-electron of the double bond
can be easily polarized. Therefore, their dipole moments are
higher than those of alkanes. The dipole moment of alkene
depends upon the position of the groups bonded to the two
double bonded carbon atoms. The symmetrical trans alkenes
are non-polar and hence have zero dipole moment. However,
unsymmetrical trans-alkenes have small dipole moment
because the two dipoles opposes each other but they do not
cancel out each other exactly since they are unequal. On the
other hand, both symmetrical and asymmetrical cis-alkenes
are polar and hence have finite dipole moments. This is
because the two dipoles of individual bonds are on the same
side and hence have a resultant dipole moment.

Alkenes add up on molecule of dihydrogen gas in the presence of
finally divided nickle, palladium or platinum to form alkanes.

Halogens like bromine or chlorine add up to alkene to form
vicinal dihalides. The reddish orange colour of bromine
solution in carbon tetrachloride is discharged when bromine
adds up to an unsaturation site. This reaction is used as a test
for unsaturation. Addition of halogen to alkene is an example
of electrophilic addition reaction.

Markovnikov, a Russian chemist made a generalisation in 1869. these
generalisation led Markovnikov to frame a rule call Markovnikov
rule. The rule stated that:

“During the addition across unsymmetrical multiple
bond, the negative part of the addendum (attacking
molecule)joins with the carbon atom which carries
smaller number of hydrogen atoms while the positive
part goes to the carbon atom with more hydrogen
atom.”

Cold concentrated sulphuric acid adds to alkenes in
accordance with Markovnikov rule to form alkyl hydrogen
sulphate by the electrophilic addition reaction.

In the presence of a few drops of concentrated sulphuric acid
alkenes react with water to form alcohols, in accordance

Alkenes react with cold dilute aqueous or alkaline potassium
permanganate solution to form 1,2-diols called glycols.
The glycols contain two –OH groups on adjacent carbon
atoms. This reaction of addition of two hydroxyl groups
to each end of double bond is called hydroxylation of the
double bond.
2KMnO4+H2O 2KOH+2MnO2+3[O]

When alkene is treated with hot acidic potassium
permanganate or potassium dichromate solution the
alkene gets split up at the double bond forming carboxylic

Alkenes are oxidised with ozone to form ozonides which are
unstable compounds. These are reduced with zinc and
water forming aldehydes and ketones. The reaction is
called ozonolysis.

Polymerisation is a process in which a large number of simple
(same or different) molecules combine to form a bigger

Alkynes are unsaturated hydrocarbon having carbon-carbon
triple bonds in their molecules. There general formula is
CnH2n-2. The simplest member of this class is ethyne (C2H2)
which is properly known on acetylene.

• C2H2 H:C:::C:H H—C C—H
Acetylene
(ethyne)

Ethyne is the simplest molecule of alkyne series. In the triple
bond formation, one sp hybridised orbital of one carbon
atom overlaps axially (head on) with the similar sp hybrid
orbital of the other carbon atom to form σ bond. Each of
the two unhybridised orbitals of one carbon overlaps
sidewise with the similar orbital of the other carbon atom
to form two weak  bonds. The remaining sp hybrid of each
carbon atom overlaps with 1s orbital of hydrogen to form
C-H bond. Thus, carbon to carbon triple bond is made up
of one σ bond and two  bonds.

In IUPAC system they are named as derivatives of
corresponding alkanes replacing ‘ane’ by the suffix ‘yne’.
The following rules should be followed:
i)The longest continues chain should include both the carbon
atoms of the triple bond.
ii) The suffix used for alkyne is – yne.
iii) The chain is numbered from the end which gives the lower
number to the first carbon atom of the triple bond.
iv) The positions of the substituents are indicated.

Alkynes exhibit the following structural isomerisms:

The isomers differ in the chain of carbon atoms. For example,
the molecule having molecular formula C5H8 shows chain
isomers as:
5 4 3 2 1
CH3−CH2−CH2−C≡CH

Pent-1-yen

Alkynes having more than four carbon atoms show position
isomerism. For example: 4 3 2 1

Preparation Of
Alkynes

From calcium carbide From Vicinal Dihalides

Acetylene is prepared in the laboratory as well as an
industrial scale by the action of water on calcium carbide.
CaC2 + 2H2O HC≡CH + Ca(OH)2

Calcium carbide required for this purpose is obtained by
heating calcium oxide (from limestone) and coke in an

Vicinal dihalides on treatment with alcoholic potassium hydroxide
undergo dehydrohalogenation. One molecule of hydrogen halides is
eliminated to form alkenyl halide which on treatment with sodamide
gives alkyne.

The first three members (ethyne, propyne, butyne) of the family
are gases at room tempreture, the next eight are liquid while
the higher ones are . All alkynes are colourless.
However, ethyne has characteristic odour of garlic smell.

Alkynes are weakly polar in nature. They are lighter than

The melting and boiling point of the members of the family are slightly
higher as compared to those of the corresponding members of alkane
and alkene families. This is due to the fact that the alkynes have
linear structure and therefore, their molecules are more closely packed
in space as compared to alkanes and alkenes. The magnitude of
attractive forces among them are higher and therefore, the melting
and boiling point are also higher. The melting and boiling point
increase with increase in molecular mass of the alkynes.
Hydrocarbon Ethane Ethene Ethyne
m.p. (K) 101 104 191

b.p. (K) 184.5 171 198

Alkynes react readily with hydrogen in the presence of finely
divided Ni, Pt or Pd as a catalyst. The reaction is called
hydrogenation.
HC≡CH+H2 Pt/Pd/Ni [H2C=CH2] H CH3−CH3
2

Reddish orange colour of the solution of bromine in carbon
tetrachloride is decolourised. This is used as a test for
unsaturation.

Two molecule of hydrogen halides(HCl, HBr and HI) add to
alkynes to form gem dihalides (in which two halogens are
attached to the same carbon atom). For example:

Alkenes react with water in the presence of mercuric sulphate
(HgSO4) and sulphuric acid at 337K. The product are
carbonyl compounds (aldehydes and ketones). For eg:

Linear polymerisation of ethyne takes place to produce
polyacetylene of polythyne which is a high molecular
weight polyene containing repeating units of (CH=CH−CH=CH).

Alkynes have larger tendency to polymerize then alkenes and,
therefore these give low molecular mass polymers alkynes
when passed through a red hot iron tube at 873k
polymerize to give aromatic hydrocarbons. For eg:

These hydrocarbons are also known as ‘arenes’. Since most of
them possess pleasant odour (Greek; aroma means pleasant
smelling), the class of compounds was named as ‘aromatic
compounds’. The parent member of the family is benzene
having the molecular formula C6H6. it has hexagonal ring
of six carbon atoms with three double bond in alternate
position.

Aromatic compounds containing benzene ring are known as

The stability of benzene can be explained on the basis of
concept of resonance. Kekule in1865 gave a ring structure
for benzene in which the positions of the three double bonds
are not fixed. He suggested that the double bond keep on
changing their positions an this is called Resonance. The
resonance structure of benzene is supported by the following
facts:
i)The carbon-carbon bond length in benzene is 139 pm which is
intermediate between bond lengths for C-C bond (154
pm)and C=C bond (134 pm) and the value is the same for all
the bonds.
ii)Due to resonance the -electron charge in benzene gets

According to the orbital concept, each carbon atom in benzene
is sp2- hybridised and one orbital remains unhybridised. Out
of the three hybrid orbitals, two overlap axially with the
orbitals of the neighbouring carbon atoms on both side to
form σ-bond. The third hybridised orbital of the carbon atom
overlaps with the half-filled orbital of the hydrogen atom
resulting in C-H bonds. Thus, benzene has a planar
structure –with bond angle of120˚ each.

There is still one unhybridised 2p-orbital left on each carbon

The resultant -orbital cloud is spread over all the six carbon atoms
(shown in fig c.). As a result, there are two continuous rings of -
electron clouds, one above and the other below the plane of the
carbon atoms(shown in fig d.).

c)

d) electron cloud

Aromatic compounds are those which resembles benzene in chemical
behaviour. These compounds contain alternate double and single
bonds in a cyclic structure. They undergo substitution reaction rather
than addition reaction. This characteristic be behaviour is called
aromaticity. The aromaticity depends upon the electronic structure of
the molecule.

Cyclopentadienyl anion

The main essential for aromaticity are:
 Delocalisation: the molecule should contain a cyclic cloud of
delocalized  electron above and below the plane of the
molecule
 Planarity: for the delocalisation of -electron the ring must
be planar to allow cyclic overlap of p-orbitals. Therefore,
for a molecule to be aromatic, the ring must be planar.
 (4n+2) electron: for aromaticity, the -electron could must
contain a total of (4n+2) electrons where n is an integer
equal to 0,1,2,3……..n . This is known as Huckel Rule.

 Decarboxylation of aromatic acid
benzene is prepared in the laboratory by heating sodium
benzoate with soda lime.

 Reduction of phenol
Benzene can be prepared from phenol by distillation with
zinc.

Benzene and its containing up to eight carbon
atoms are colourless liquids with characteristic smell.
Aromatic hydrocarbons are immiscible with water but are
soluble in organic solvents.
They are inflammable and burn with sooty flame.
They are toxic and carcinogenic in nature.
The melting and boiling point of aromatic hydrocarbon
increase with increasing molecular mass. This is due to
increase in magnitude of van der Waals’ forces of

Chemical
Properties

Mechanism of
Electrophilic
electrophilic Addition reaction
substitution reaction
substitution reaction

The replacement of a hydrogen atom in the ring by a nitro (-
NO2) group is called nitration. It is carried out by heating
benzene with the nitrating mixture consisting of
concentrated nitric acid and sulphuric acid to about 330K.

The replacement of a hydrogen atom in the ring by a halogen

The replacement of a hydrogen atom in the ring by a
sulphonic acid (-SO3H) group is called sulphonation. It is
carried out by heating benzene with fuming sulphuric acid
and oleum.

When benzene is treated with an alkyl halide in the presence
of anhydrous aluminium chloride, alkylbenene is formed.

According to experimental evidences, SE (S= substitution;
E= electrophilic) reaction are supposed to proceed via the
following three steps:
a)Generation of the electrophile.
b)Formation of carbocation intermediate.
c)Removal of proton from the carbonation intermediate.

The attacking reagent may not be strong electrophile.
Therefore, first of all an electrophile is generated by some

The electrophile E+ approaches the -electron cloud of the
aromatic ring and forms a bond with carbon, creating a
positive charge on the ring. This results in the formation of
a sigma complex (called arenium ion).

The arenium ion gets stabilized by resonance

The carbocation formed loses a proton to the nucleophile
(Nuˉ) present in the reaction mixture to form a
substitution product. During this step, the aromatic
character of the benzene ring is restored and this step is
fast.
The loss of proton allows the two electrons
from the carbon-hydrogen bond to move to regenerate the
aromatic ring and thus restoring the aromatic character.

Benzene reacts with hydrogen in the presence of a catalyst
such as nickel, or platinum at 473 to 573 K under pressure
to form cyclohexane.

Benzene reacts with chlorine or bromine in the presence of
sunlight and absence of halogen carrier to form benzene
hexachloride.

When monosubstituted benzene is subjected to further
substitution, three possible disubstituted products are not formed
in equal amounts. Two types of behaviour are observed. Either
ortho and para products or meta product is predominantly formed.
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.
a)Ortho and para directing groups
b)Meta directing group

The groups which direct the incoming group to ortho and
para position are called ortho and para directing groups.
As an example, let us discuss the directive influence of –
OH (phenolic) 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 an example of Nitro group.

Benzene and polynuclear hydrocarbon 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 reaction and finally damage DNA and
cause cancer.

Presented By: Arjav Patel,
11-C
Anandalaya,Anand.

Hydrocarbon arjav patel

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