FSc Notes Part 2 Chemistry Important Chemicals Ethene C2H4

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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 not fulfilled by hydrogen atoms and double bonds are present between carbon atoms are called Unsaturated Hydrocarbon or alkenes.

The first member of alkene family composed of two carbon and four hydrogen atoms and is known as Ethene.

Structure of Ethene

Ethene is an unsaturated hydrocarbon, which is composed of two carbon and four hydrogen atoms. Its molecular formula is C2H4. The structural formula of ethene shows that each carbon contains two hydrogen and double bond is present two carbon atoms.

H2C = CH2

Each carbon atom of ethene is Sp2 hyberidized which contains three equivalent partially filled Sp2 hybrid orbitals and unhyberidized Pz orbital. The hybrid orbitals are arranged at the three corners of coplaner triangle with an angle of 120º.

Two Sp2 hybrid orbitals of carbon atoms overlapp with s atomic orbitals of hydrogen atoms to form a sigma bond between C – H due to the overlapping of Sp2 – S orbitals. The remaining Sp2 hybrid of each carbon atom overlapp with the Sp2 hybrid orbital of other carbon atom to form sigma bond between C – C due to the overlapping of Sp2 – Sp2.

The unhyberidized Pz orbital of each carbon atom are situated perpendicular to the Sp2 plane and parallel to the unhyberidized Pz orbitals of two different carbon atoms overlap side by side to form a pi bond between carbon atoms.

Preparation of Ethene

Ethene can be prepared by the following methods.

1. From Dehydrohalogenation of Ethyl Halide

Removal of hydrogen halide from a compound is called Dehydrogenation. When mono halo ethane or ethyl halide is treated with alcoholic potash (KOH), then dehydrohalogenation takes place as a result ethene is formed.

C2H5X + KOH —-> H2C = CH2 + H2O + KX

C2H5Cl + KOH —-> H2C = CH2 + H2O + KCl

2. From Dehalogenation Of Vicinal Dihalide

Removal of halogen from a compound is called Dehalogenation. Those dihalides in which hydrogen atoms are attached with two adjacent carbon atoms are called Vicinal Dihalide or 1, 2 – dihalo ethane reacts with powdered zinc metal then dehalogenation takes place as a result Ethene is formed.

C2H4X2 + Zn —-> C2H4 + ZnX2

C2H4Cl2 + Zn —-> C2H4 + ZnCl2

3. From Dehydration of Ethyl Alcohol (Ethanol)

Removal of water molecule from a compound is called Dehydration. In presence of catalyst concentrated H2SO4, when ethyl alcohol or ethanol is heated at about 170ºC then Dehydration takes place as a result ethene is formed.

C2H5OH —-> C2H4 + H2O

Dehydration of ethanol can also be called out in following two ways.

1. In presence of catalyst Al2O3 when ethanol is heated at (350 – 360ºC), then ethene is formed

C2H5OH —-> C2H4 + H2O

2. In presence of mixture of Al2O3 and H3PO4 at 250ºC, when ethanol is heated then ethene is formed.

C2H5OH —-> C2H4 + H2O