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 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 between 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 120°

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 parallal to the unhyberidized Pz orbitals of other carbon atom. The unhyberidized Pz orbitals of two different carbon atoms overlap side by side to form a pi bond between carbon atoms.

Therefore in ethene one sigma and one pi bond is present 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 + H₂0 + KX

C₂H₅Cl + KOH ® H₂C=CH₂ + H₂0 + 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 ehtane 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 170°C 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 - 360°C), then ethene is formed.

C₂H₅OH ® C₂H₄ + H₂O

2.      In presence of mixture of Al₂O₃ and H₃PO₄ at 250°C, 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 250°C then hydrogenation takes place as a result ethene is formed.

H-CºC-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 single 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-CºC-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 250°C then hydrogenation takes place as a result ethane is formed.

H-CºC-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.

C₂H₄ + HHSO₄