Methane CH4
Introduction
Organic compounds, which are composed
of only carbon and hydrogen atoms are known as hydrocarbons. Those hydrocarbons
in which all the valencies of carbon atom are fulfilled by hydrogen atoms are
called Saturated Hydrocarbon or alkanes. The first member of alkane family
composed of one carbon and four hydrogen atoms and is known as Methane.
Structure of Methane
Methane is a saturated hydrocarbon,
which is composed of one carbon and four hydrogen atoms. Its molecular formula
is CH4. The structural formula of methane shows that all the valencies of
carbon atoms are fulfilled by hydrogen atoms.
The carbon atoms of methane is Sp3
hyberidized which contains four equivalent partially filled Sp3 hybrid
orbitals, these bybrid orbitals are arranged at the four corners of regular
tetrahedron (tetra hedral structure) with an angle of 109.5º.
The Sp3 hybrid orbitals 1 of carbon
atoms overlapp with s atomic orbitals of hydrogen atoms to form a sigma bond
between C – H due to the overlapping of Sp3 – S orbitals
Preparation of Methane
Methane can be prepared by the
following methods.
1. From Sodium Acetate
When concentrated solution of NaOH
reacts with acetic acid then sodium acetate is formed.
CH3COOH + NaOH —-> CH3COONa + H2O
When anhydrous sodium acetate reacts
with NaOH then methane is obtained.
CH3OONa + NaOH —-> CH4 + Na2CO3
2. From Methyl Magnesium Iodide
In presence of dry ether when alkyl
halide reacts with magnesium metal the Alkyl Magnesium halide is formed. This
compound was first synthesized by a German Chemist Grignar, therefore, it is
also known as Grignard’s Reagent.
3. From Reduction of Methyl Iodide
Methyl iodide can be reduced to
methane by the following methods.
a) By Nascent
Hydrogen
When concentrated HCl reacts with
powdered zinc metal then nascent Hydrogen is obtained.
2HCl + Zn —-> ZnCl2 + 2[H]
Nascent Hydrogen acts as strong
reducing agent. When methyl iodide is reduced with nascent hydrogen then
methane is formed.
b) By
Catalytic Reduction of Methyl Iodide
In presence of catalyst Palladium
(Pd), when methyl iodide reacts with hydrogen then methane is formed
Physical Properties of Methane
1. At ordinary temperature and
pressure methane exist as colourless, odourless, non-poisonous gas.
2. Methane gas is less soluble in H2O
but easily in organic solvents.
3. Methane molecule is non-polar and
symmetric.
4. Methane gas is lighter than air.
Chemical Properties of Methane
Reactivity
Methane is a saturated hydrocarbon
which is composed of one carbon and four hydrogen atoms. Its molecular formula
is CH4. The structural formula shows that all the valencies of carbon atom are
fulfilled by Hydrogen atoms. Therefore, at ordinary temperature and pressure,
methane is chemically unreactive. It does not react with any acid, base or
oxidizing agent such as KMnO4 or K2Cr2 O7. Under special circumstances methane
shows substitution reaction.
Those reactions in which any atoms or
molecule is replaced by other atom or molecule is called substitution reaction.
Some important chemical reaction of
methane are given below.
1. Halogenation
Introduction of halogen in a compound
is called halogenation. In presence of sunlight, when chlorine (halogen) reacts
with methane then chlorination (halogenation) takes place as a result hydrogen
atom of methane is replaced by chlorine atom to form substituted product
monochloro methane or methyl chloride is formed.
CH4 + Cl2 —-> CH3Cl + HCl
In presence of sunlight and excess
chlorine further substitution takes place till hydrogen atoms are replaced by
chlorine atoms.
CH3Cl + Cl2 —-> CH2Cl2 + HCl
CH2Cl2 + Cl2 —-> CHCl3 + HCl
CHCl3 + Cl2 —-> CCl4 + HCl
Mechanism
The mechanism of chlorination
(halogenation) in methane proceeds through the following steps.
First Step
In presence of sunlight chlorine
molecule dissociate into two chlorine free radicals.
Second Step
Chlorine fee radical combines with
methane molecule to form HCl and methyl free radical.
Third Step
Methyl free radical combines with
other chlorine molecule to form a substituted product methyl chloride or
monochloro methane and chlorine free radical.
2. Combustion
When methane is heated in presence of
air and oxygen then carbondioxide and water are formed with the evolution of
large amount of heat energy.
CH4 + 2O2 —-> CO2 + 2H2O -ΔH
3. Cracking Or Pyrolysis
The phenomenon in which large
molecules are broken into smaller and simple molecule is called Pyrolysis. When
Methane is heated in absence of air or oxygen at about 600ºC then Pyrolysis
takes place. As a result carbon black and Hydrogen are formed.
Uses of Methane
1. Methane gas is used as domestic
fuel.
2. Methane gas is used in manufacture
of methanol, carbon black and polishes etc.
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