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

Physical properties:

  • Physical properties are smaller to alkenes and alkanes.
  • First three are gases, next eight are liquids and higher ones are solids.
  • 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

Mono 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:

\(HC\equiv CH+{{H}_{2}}\xrightarrow{Pt/Pd/Ni}\left[ {{H}_{2}}C=C{{H}_{2}} \right]\xrightarrow{{{H}_{2}}}C{{H}_{3}}-C{{H}_{3}}\)

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.Addition of Water

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.
    Cyclic Polymerization