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:
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
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