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ALKYNES

Methods of Preparation of Alkynes:

1. From Calcium carbide:
CaCO3 heat CaO + CO2
CaO + 3C CaC2 + CO
CaC2 + 2 H2O Ca (OH)2 + C2H2
2. From Vicinal dihalides:

CH2(Br)-CH2(Br) + alc. KOH CH2=CH-Br NaNH2 C2H2
Chemical Properties:
1. Acidic nature of Alkynes:

(a)Acetylene is acidic in nature. The s-character of C-H bond of Acetylene is higher in comparison to
C-H bond of ethene and ethane due to sp hybridization in Acetylene. So, the release of H from
+
ethene and ethane is difficult and they are less acidic. The acid strength order is:
(i)Ethyne > Ethene > Ethane

(ii)Ethyne > Propyne > But-2-yne
(b)Formation of Sodium acetylides: Acetylene reacts with sodium in liquid ammonia or sodamide to
form sodium acetylide and liberate H2 or NH3 gas.

(i) CH CH + Na Liq. NH3 CH C-Na + ½ H2
(ii) CH C-Na + Na Liq. NH3 Na-C C-Na + ½ H2
(iii) CH CH + 2NaNH2 Na-C C-Na + 2NH3
2. Addition reactions:
(a) Addition of Hydrogen:

CH CH H2/Ni CH2=CH2 H2/Ni CH3-CH3
(b) Addition of Halogen: Test for unsaturation
CH CH Br2/CCl4 CH-Br=CH-Br Br2/CCl4 CHBr2-CHBr2
(c) Addition of Hydrogen halides:
CH CH HBr CH2=CH-Br HBr CHBr2-CH3
(d) Addition of water:
+
2+
CH CH H2O, Hg /H CH2=CH-OH Isomerization CH3-CHO
3. Polymerization:
(a) Linear polymerization:
n CH CH polymerisation -(CH=CH-CH=CH)-n
(b) Cyclic polymerization: