FSc Notes Part 2 Chemistry Important Chemicals Ethyne C2H2

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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 triple bond is present between two carbon atoms are known as Unsaturated Hydrocarbon or alkynes.

The first member of alkyne family composed of two carbon and two hydrogen atoms and is known as Ethyne or Acetylene.

Structure of Ethyne

Ethyne is a unsaturated hydrocarbon, which is composed of two carbon and two hydrogen atoms. Its molecular formula is C2H2. The structural formula of ethyne shows that each carbon contains one hydrogen atom and triple bond is present between two carbon atoms.

HC ≡ CH

Each carbon atom of ethyne is Sp hyberidized which contains two equivalent partially fill Sp hybrid orbitals and two unhyberidized P orbital. The hybrid orbitals are arranged at the two corners of linear structure with an angle of 180º.

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

The unhyberidized P orbital of each carbon atom are situated perpendicular to the Sp plane and parallel to the unhyberidized Pz orbitals of other carbon atom. The unhyberidized P orbitals of two different carbon atoms overlap side by side to form pi bonds between carbon atoms.

Therefore in ethyne one sigma and two pi bonds are present between carbon atoms.

Preparation of Ethyne

Ethyne can be prepared by the following methods.

1. From Dehalogenation of Tetra Halo Ethane

Removal of halogen from a compound is called Dehalogenation. When tetra halo ethane reacts with powdered zinc metal, then dehalogenation takes place as a result ethyne is formed.

C2H2X4 + 2Zn —-> HC ≡ CH + 2ZnX2

C2H2Cl4 + 2Zn —-> HC ≡ CH + 2ZnCl2

2. From Dehydrohalogenation of Vicinal Dihalide

Removal of hydrogen halide from a compound is called Dehydrogenation. Those dihalides in which halogen atoms are present at two adjacent carbon atoms are called vicinal di halides. When vicinal dihalide (1, 2 – dihalo ethane) reacts with alcoholic potash (KOH) then dehydrohalogenation takes place as a result ethyne is formed.

C2H4X2 + 2KOH —-> HC ≡ CH + 2H2O + 2KX

C2H4Cl2 + 2KOH —-> HC ≡ CH + 2H2O + 2KCl

3. From Calcium Carbide

When Calcium Carbide (CaC2) reacts with cold water then ethyne is formed.

Physical Properties

1. At ordinary temperature and pressure, ethyne exist as colourless gas with sweet smell.
2. Ethyne gas is sparingly soluble in water but easily soluble in organic solvents.
3. It is less denser than air.
4. Liquid ethyne is unstable in nature.