XII Chemistry BIEK Topic - Ethene C2H4

Ethene C2H4
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 hydracarbon, 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.

Diagram Coming Soon



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

4. From Hydrogenation of Ethyne

Introduction of hydrogen in a compound is called hydrogenation. In presence of catalyst Ni, Pt, Pd, when ethyne is heated with limited amount of Hydrogen at 250ºC then hydrogenation takes place as a result ethene is formed

H-C ≡ C - H + H2 ----> H2C = CH2




Physical Properties of Ethene
1. At ordinary temperature and pressure ethene exist as colourless gas with sweet smell.

2. It is slightly soluble in water but easily soluble in organic solvent.

3. Ethene gas is less denser than air.

4. Ethene gas produces Anaesthesia on inhilation.




Chemical Properties of Ethene
Ethene is unsaturated hydrocarbon which is composed of two carbon and four hydrogen atoms. Its molecular formula is C2H4. The structural formula shows that double bond is present between two carbon atoms in which one is sigma and other is pi bond.

Sigma bond is formed by the direct overlapping of orbitals and large amount of heat is required to break it. Where as pi bond is formed by the side by side overlapping of orbital and less amount of heat is required to break it. Hence sigma bond is stronger than pi bond.

When any attacking molecule reacts with ethene then double bond is broken into sigma bond (sigma bond) as a result two valencies are set free (one at each carbon). Now the attacking molecule occupies free valencies to form addition product.

H-C ≡ C-H + AB ----> H2CA - BCH2

Those atoms, molecules and ions, which are electron rich or contain negative charge are called Nucleophile. Those atoms, molecules or ion, which are electron defficient or contain positive charge are called Electrophile. Since pi electrons are present in ethene therefore it acts as Nucleophile. It reacts with any electrophillic reagent to form addition product. Hence the addition reactions of ethene are also called Electrophilic Addition Reaction.

Some important reactions of ethene are given below.


1. Hydrogenation

Introduction of Hydrogen in a compound is called Hydrogenation. In presence of catalyst Ni/Pt/Pd, when ethene is heated at about 250ºC then hydrogenation takes place as a result ethane is formed

H-C ≡ C-H \ H2 ----> H3C-CH3


2. Halogenation

Introduction of halogen in a compound is called Halogenation. When ethene reacts with halogen then halogenation takes place as a result di halo ethane is formed.

C2H4 + X2 ----> C2H4X2

The order of reactivity of halogen is Cl2 > Br2 > I2. It means that Cl2 and Br2 reacts at room temperature where as I2 reacts only in presence of catalyst C2H5OH.

C2H4 + Cl2 ----> C2H4Cl2

C2H4 + Br2 ----> C2H4Br2


3. Hydrohalogenation

Introduction of hydrogen halide (HX) in a compound is called Hydro Halogenation. When Halogen acid reacts with ethene the hydro halogenation takes place. As a result addition product mono halo ethane or ethyl halide is formed.

C2H4 + HX + C2H5X

The order of reactivity of halogen acid is HI > HBR ? HCl

C2H4 + HBr ----> C2H5Br


4. Addition of Water

In presence of catalyst concentrated H2SO4, when ethene reacts with water then ethyl alcohol or ethanol is formed.

C2H4 + H2O ----> C2H5OH


5. Addition of H2SO4

When ethene reacts with cold concentrated H2SO4 then ethyl hydrogen sulphate is formed.


6. Addition of Hypohalous Acid (HOX)

When halogen dissolves in water then halogen acid and hypohalous acid are formed.

X2 + H2O ----> HX + HOX

Cl2 + H20 ----> HCl + HOCl

When ethene reacts with hypohalous acid then ethane halohydrin is formed.

C2H4 + HOX ----> C2H4 - OH-X


7. Oxidation

When ethene is oxidized by dilute and alkaline solution of KMnO4 then ethylene glycol (1, 2 - ethandiol) is formed. As a result of this reaction purple colour of KmnO4 is decolourized. Therefore this test is used to distinguish between ethene in laboratory.

C2H4 + HOH ----> C2H4(OH)2


8. Combustion

When ethene is burnt in presence of air or oxygen, then carbon dioxide and water are formed with the evolution of large amount of heat energy.

C2H4 + 3O2 ----> 2CO2 + 2H2O -ΔH


9. Formation of Mustard Gas

When ethene reacts with sulphur mono chloride then mustard gas is formed which is highly poisonous gas.

C2H4 + S2Cl2 ----> S(CH2-CH2-Cl)2 + S


10. Polymerization

The phenomenon in which simpler and smaller molecules are converted into large and complex molecules is called Polymerization. The simple and smaller is called Monomer, where as large and complex molecule is called Polymer.

In presence of traces of oxygen, when ethene is heated at 200ºC under 1000 atm pressure then polymerization takes place as a result poly ethene is formed.

Diagram Coming Soon



Uses of Ethene
1. Ethene gas is used as fuel

2. Ethene gas is used as Anaesthetic.

3. Ethene gas is used in manufacture of plastics mustard gas etc.

4. Ethene gas is used for artificial ripening of raw fruit.