FSc Notes Part 2 Chemistry Important Chemicals Methane CH4

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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