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Saturday, January 7, 2017

Properties & Acidic Character of Acetylene, Addition Reactions- of Hydrogen, Halogen, Hydrogen Halides and Water

Physical properties:

1. Physical properties are smaller to alkenes and alkanes.
2. First three are gases, next eight are liquids and higher ones are solids.
3. These are insoluble in water and soluble in organic solvents.

Chemical properties:

a) Acidic character of acetylene: Acetylene reacts with sodamide to form sodium acetylide with liberation of hydrogen gas.

HC ≡ CH + Na → HC ≡ C⁻ Na⁺ + ½ H₂
                            Monao sodium ethynide

The acidic character of ethyne compared to ethene and ethane is due to the greater percentage of S-Character.

In acetylene the percentage of s-character is 50% when as in case of ethylene and ethane are 33.3% and 25% respectively.

Only hydrogen atoms attached to triply bonded carbon atoms are acidic in nature.

CH₃ - C ≡ CH + Na⁺NH₂⁻ → CH₃ - C ≡ C⁻ Na+ + NH₃

b) Addition of reagent: Due to presence of triple bond alkynes undergo addition reactions with hydrogen and halogens, hydrogen halides.

c) Addition of dihydrogen:
d) Addition of Halogens:
e) Addition of Hydrogen Halides: Two molecules of hydrogen halides added to alkynes to form gem dihalides
f) Addition of water: Alkynes add to water molecule in presence of mercuric sulphate and dilute sulphuric acid at 333K to form carbonyl compounds.

g) Polymerization:
  • Linear Polymerization: Under suitable conditions linear polymerization of etheyne gives polyethylene.[CH = CH – CH = CH] is repeating until [-CH = CH – CH = CH -)n
  • Cyclic polymerization: Acetylene passing through a red hot iron tube at  undergoes cyclic polymerization.Three molecules polymerise to form benzene.