Power Point On Hydrocarbon

MADE BY :- SAMIRAN GHOSH
CLASS :- XI- ‘A’
SUBMITTED TO :- SMT. ARCHANA BHATNAGAR

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.
 Alkenes : These are hydrocarbon which contain at least one carbon-carbon
bond. For example: Ethene
 Alkynes: These are hydrocarbons which contain at least one carbon-carbon
triple bond. For example: Ethylene

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:
Alkanes
No double or Triple Bond CH3CH2CH3
Propane
Unsaturated Hydrocarbon:
1. Alkenes
2. Alkynes
Double Bond
Triple Bond
CH3–CH═CH2
Propene
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 .
Where 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˚.

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
CH3 CH2−CH3
In the example ,the longest chain has nine carbons and it is considered as
parent root chain and carbon atoms which are not included in parent
chain are called substituents.

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
CH−CH−CH−CH−CH−CH−CH−CH−CH
CH 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
CH3 CH3
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 substituents. So correct name of
compound is :3-Ethyl-5-methylheptane
CH3−CH2−CH−CH2−CH2−CH− CH2 −CH3
(3-Ethyl-6-methyloctane)
CH2CH3 CH3

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
CH3
5-(1,2- Dimenthylpropyl) nonane

Methods for
preparation of
alkanes
From
unsaturated
hydrocarbon
From alkyl
halides
From carboxylic
acids
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 reduction.
Methane cannot be prepared by this method because starting alkene or
alkyne must contain at least two carbon atom.
For eg:

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.
For eg:

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.
For eg:
Decarboxylation
reaction
Kolbe’s electrolytic
method

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:
iii)
iv) At cathode:

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 and the structure of the molecule. Due to weak forces, the C1 to C4
are gases, the next thirteen alkanes from C5 to C17 are liquid and the higher
member with more than 18 carbon atoms are solid at 298 K.
Properties
Physical properties Chemical properties

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.
Variations of boiling point of alkane with increase in number of C atoms.

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 higher melting points as compared to
the immediately next lower alkanes with odd number of carbon atoms.
This is because the alkanes with even number of carbon atoms have more
symmetrical structures and result in closer packing in the crystal structure
as compared to alkanes with odd number of carbon atoms. Therefore, the
attractive forces in the former are more and the melting points are higher
as compared to the alkanes with odd number of carbon atoms.
Alkane C3H8 C4H10 C5H12 C6H14 C7H16 C8H18
m.p.(K) 85.9 138 143.3 178.5 182.5 216.2

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:

ii.i) Initiation
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.
ii.ii) Propagation
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 methyl radical thus obtained attacks the second molecule of chlorine to form
CH3-Cl with the liberation of another chlorine free radical by homolysis of
chlorine molecule.

ii.iii) Termination
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:
Due to the evolution of large amount of heat during combustion, alkanes
are used as fuels

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
ii) When methane is mixed with oxygen and passed through heated
molybdenum oxide (Mo2O3), under pressure it is oxidised to methanal.
(CH3)3CH + O alk.KMnO4 HCHO + H2O

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 aromatic compounds. This process is called
aromatization.
CH3−(CH2)4−CH3
773K, 10-20 atm
Hexane Benzene

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
This reaction is called Fragmentation or Cracking or Pyrolysis. Pyrolysis of
hexane gives following product:

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.
Propylene

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
pi bond makes alkenes unstable
molecules in comparison to alkanes
and thus, alkenes can be changed
into single bondcompounds by
combining with the electrophilic
reagents.

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 4
For eg: CH3CH=CHCHCH3 CH2=CH−CH=CH2
CH3
4-Methylpent-2-ene Buta-1,3-diene

Isomerism in Alkanes
Structural Isomerism
Chain
Isomerism
Position
Isomerism
Geometrical 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:
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.
Cis-isomer is more polar than trans-isomers. These are distinguish on the
basis of their physical properties such as melting point, boiling point etc.

Preparation
Of Alkanes
From
Alkynes
From Alkyl
Halides
From Vicinal
Dihalides
From
Alcohols

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
CH≡CH+H2 Pd/C CH2=CH2
Ethyne Ethene

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.
Nature of halogen atom and the alkyl group determine rate of the reaction.
It is observed that for halogens, the rate is: Iodine>Bromine>Chlorine
while for alkyl group it is Tertiary> Secondary>Primary.

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.
For eg: CH3CH2OH H2SO4 or H3PO4 CH2=CH2+H2O
This reaction is also an example of β-elimination reaction because –OH group
takes out one hydrogen atom from the β- carbon atom.

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.
The boiling points of alkene show a regular gradation with the increase in
number of carbon atoms like alkanes. In general, for each added –CH2 group
the boiling point rises by 20˚-30˚.

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 are lighter than water. These are insoluble in water because
they are non-polar. However, they readily dissolve in organic
solvents like alcohol, benzene, ether, carbon tetrachloride, etc.

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.
Hydrogen halides (HCl, HBr, HI) add up to alkenes to form alkyl
halides. The order of reactivity of the hydrogen halides is
HI>HBr>HCl. Like addition of halogens to alkenes, addition of
hydrogen halides is also 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 with the Markovnikov rule.

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.
For eg: 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 acids or ketones. This is also called oxidative
cleavage of alkanes.
For eg: CH3−CH=CH−CH3
KMnO4/H+
2CH3COOH
But-2-ene Ethanoic acid

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 molecule of higher molecular mass. The small
molecule are called monomers while the bigger molecule are called macromolecules
or polymers.

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.

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 pi 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 pi
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.
For eg: 4 3 2 1 1 2 3 4 5 6
CH3CH2C≡CH CH3CH2C≡C−CH2CH3
But-1-yne Hex-3-yne

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-yne
Alkynes having more than four carbon atoms show position isomerism.
For example: 4 3 2 1 4 3 2 1
CH3−CH2−C≡CH CH3−C≡C−CH3
But-1-yne But-2-yne

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 electric furnace at 2275 K.
CaCO3
Heat
CaO + CO2
Preparation Of
Alkynes
From calcium carbide From Vicinal Dihalides

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 solid.
All alkynes are colourless. However, ethyne has characteristic odour of
garlic smell.
Alkynes are weakly polar in nature. They are lighter than water and
immiscible with water but are soluble in organic solvents such as
petroleum ether, carbon tetrachloride, benzene, etc.

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] H2 CH3−CH3
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: This
is the best route for
entering from aliphatic
to aromatic compounds.

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 benzenoids and those
not containing a benzene ring are known as non-benzenoids. For eg:

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 pi-electron charge in benzene gets distributed over
greater area i.e., it gets delocalised. As a result of delocalisation the energy
of the resonance hybrid decreases as compared to contributing structure by
about 50kJ mol-1. the decrease in energy is called resonance energy.
Therefore, it is stabilised and behaves as a saturated hydrocarbon.
iii)If the positions of double bonds are fixed. We expect two isomers of 1,2-
dichlorobenzene as shown below (one having Cl atoms attached to C-C
bond and the other having Cl atoms attached to C=C bond).

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 atom. Each one of these
orbitals can overlap sidewise with similar orbital of the carbon atoms on either sides to
form two sets of pi-bonds. (Shown in fig a. and b. respectively)
a) b)

The resultant pi-orbital cloud is spread over all the six carbon atoms
(shown in fig c.). As a result, there are two continuous rings of pi-
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
pi-electron above and below the plane of the molecule
Planarity: for the delocalisation of pi-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) pi-electron: for aromaticity, the pi-electron could must contain a
total of (4n+2)pi electrons where n is an integer equal to 0,1,2,3……..n
. This is known as Huckel Rule.
Benzene, 6 pi e- Naphthalene, 10 pi e- Anthracene, 14 pi e-
(n=1) (n=2) (n=3)

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 attraction with increase in molecular size. Amongst
isomeric arenes, (i.e., o-,m- and p- xylenes), the p- isomer has the highest
melting point because it is most symmetrical.

Chemical
Properties
Electrophilic
substitution reaction
Mechanism of
electrophilic
substitution reaction
Addition 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 atom
(F, Cl, Br or I) is called halogenation.
Arenes react with halogen in the
presence of a Lewis acid like
anhydrous FeCl3, FeBr3 or AlCl3
to yield haloarenes.

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.
The reaction of benzene with acyl halide
or acid anhydride in the presence of lewis
acid (AlCl3) Yields acyl benzene

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
preliminary reaction. For example , during chlorination of
benzene, an electrophile (Cl +) is generated by reacting
with anhydrous AlCl3 used as catalyst.

The electrophile E+ approaches the pi-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 resulting carbocation has three important contributing structures
which spread the positive charge over the remaining carbon atom.

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.

On completely burning with oxygen, benzene gives carbon
dioxide and water with the evolution of a large amount of
energy.

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 resonance structures of
phenol show that the overall
electron density on the benzene
ring increases in comparison to
benzene. Therefore, it is an
activating 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.
Nitro group reduces the electron
density in the benzene ring due to
its strong-I effect. Nitrobenzene
is the resonance hybrid of the
following structures.

 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.

Power Point On Hydrocarbon

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