Alkynes can be prepared through several methods: 1. From calcium carbide by reacting calcium carbide with water to produce acetylene. 2. From vicinal dihalides by treating them with alcoholic potassium hydroxide to undergo dehydrohalogenation and form alkyne. 3. Alkynes readily react with hydrogen in the presence of catalysts like nickel, platinum or palladium through a reaction called hydrogenation.

1. Alkynes and their
preparation

2. Preparation Of Alkynes
From calcium carbide From vicinal dihalides

3. From calcium carbide
Acetylene is prepared in the laboratory as well as
an industrial scale by the action of water on
calcium carbide.
CaC2 + 2H2O HC≡CH +
Ca(OH)2
Calcium carbide required for this purpose is
obtained by heating calcium oxide (from
limestone) and coke in an electric furnace at

4. From vicinal dihalides
Vicinal dihalides on treatment with alcoholic potassium
hydroxide undergo dehydrohalogenation. One
molecule of hydrogen halides is eliminated to form
alkenyl halide which on treatment with sodamide gives
alkyne.

5. Alkynes react readily with hydrogen in the
presence of finely divided Ni, Pt or Pd as a
catalyst. The reaction is called hydrogenation.
HC≡CH+H2
Pt/Pd/Ni [H2C=CH2] H2 CH3−CH3

6. Addition of halogens
Reddish orange colour of the solution of
bromine in carbon tetrachloride is decolourised.
This is used as a test for unsaturation. This test
is known as bromine water test.

7. Addition of hydrogen halides
Two molecule of hydrogen halides(HCl, HBr and
HI) add to alkynes to form gem dihalides (in
which two halogens are attached to the same
carbon atom). For example:

8. Addition of water
• Alkenes react with water in the presence of mercuric
sulphate (HgSO4) and sulphuric acid at 337K. The
product are carbonyl compounds (aldehydes and
ketones). For eg:

9. polymerisation
a)Linear polymerisation
Linear polymerisation of ethyne takes place to
produce polyacetylene of polythyne which is a
high molecular weight polyene containing
repeating units of (CH=CH−CH=CH).

10. Cyclic polymerisation
Alkynes have larger tendency to polymerize then alkenes
and, therefore these give low molecular mass polymers
alkynes when passed through a red hot iron tube at
873k polymerize to give aromatic hydrocarbons. For eg:
This is the best route for entering from aliphatic to aromatic
compounds.
b