ALKYNE
The alkynes are acyclic unsaturated
organic compounds made up of carbon and hydrogen only. These compounds are
unsaturated, have a triple bond along with single bonds. They have less hydrogen
than alkanes and alkenes.
They contain at least one triple
bond in their molecules Some alkyne have only one triple bond these are known
as mone-yne and some compounds have two double bonds are known as diyne.
Properties of alkenes
1. Alkynes are more
reactive as compared to alkanes due to their unsaturated nature.
2. The name of
alkynes ends with –yne.
3. The general formula
of alkenes is CnH2n-2, here n shows the number of carbon atoms
present in a molecule.
4. The lower member
of the alkyne class is gases but the higher member exits as liquid and solids.
5. They have at
least one triple bond.
6. These are
non-polar compounds, and insoluble in water but readily dissolve in benzene and carbon tetrachloride.
7. Among all compounds of alkyne, only ethyne/acetylene
has a garlic-like smell and other members of alkynes are colorless and odorless.
8. When burning in the
air, carbon dioxide and water is formed.
9. When a molecular
mass of alkynes increases with the increase of chain length, their melting and
boiling point also increases.
10. Alkynes have acidic nature.
Methods for the preparation of alkynes
Alkynes are
prepared by different methods which are as follows.
v Deydrohalogenation of vicinal dihalide
When vicinal dihalide react with a strong base
two molecules of hydrogen halide were removed from two adjacent carbon atoms, then
converted into alkyne.
dehydrohalogenation
v Industrial preparation
We can prepare
alkyne on an industrial scale by using calcium carbide. In the method, calcium
carbide reacts with water and is converted into alkyne.
on industrial scale
v From haloform
Among all the
alkyne, ethyne can be prepared by haloform reaction.
from haloform
v By electrolysis
In this method, the metal salt of the unsaturated dicarboxylic acid is hydrolyzed in water and converted
into alkyne.
electrolysis
v Dehalogenation of tetrahalide
Alkyne can also
be prepared by the reaction of tetrahaloalkane with zinc .
tetrahalide
Reactions of alkynes
Alkynes are more reactive as
compared to alkane due to their unsaturated nature. Alkynes have two pi bond
that’s why they react quickly with other compounds. It undergoes different types of reactions.
v Addition
reaction
v Oxidation
reaction
v Polymerization
v Nature
of alkynes
1.
Addition
reactions
In all addition
reactions two molecules of alkyne are used.
v
Addition of halogen acid
Alkyne react with halogen
acid and di haloalkanes are formed. It is a 2 step reaction, first alkene is
formed and then alkyne is formed.
hydrogen halide
v
Addition of hydrogen
This reaction occurs in the
presence of catalysts Ni, Pt, Pd. Alkane is formed after hydrogenation.
hydrogenation
v
Addition of halogen
In this reaction, tetrahalide
is formed after the addition of halogen in alkyne. The first dihalide is formed.
tetrahalide
v
Addition of cyanide
When alkyne reacts with hydrogen
cyanide acrylonitrile is formed.
 |
| acrylonitrile |
v
Addition of ammonia
When alkyne reacts with ammonia
methyl cyanide/ acetonitrile is formed.
methyl cyanide
v
Addition of water
When alkyne reacts with water
in the presence of sulphuric acid and mercuric sulfate at high temperatures.
with water
2.
Acidic nature of
alkyne
alkyne
Alkyne has acidic nature because
alkyne carbon is sp hybridized. When alkyne reacts with sodamide in the presence
of liquid ammonia, metal acetylides are formed. Different acetylides has
different colours.
metal acetylides
3.
Oxidation
reaction
o
Reaction with potassium per magnate.
When alkyne reacts with strong alkaline KMnO4, glyoxal,
and oxalic acid is formed
oxidation
o Reaction with ozone
When ozone reacts with alkyne, the carboxylic acid is formed.
With ozone
o
Combustion
When alkyne is treated with oxygen, it
will burn with luminous flame and
produce carbon dioxide and water and heat is also evolved in this reaction.
4.
Polymerization
It is a process in
which monomers combine with each other and resulting product will be polymer. Polymerization
of alkyne give both type of products linear and cyclic depending upon
conditions. The polymers formed in this reaction are low molecular weight
polymers.
Ø
Formation of vinyl acetylene and divinyl acetylene
When ethyne reacts with a solution
of cuprous chloride and ammonium chloride then vinyl acetylene is formed. It is further
converted into divinyl acetylene when reacts with ethyne again.
vinyl acetylene
Ø
Formation of benzene
Ethyne polymerize in Cu tube
at 300 ºC and converted into benzene.
benzene formation
Alkynes use
Alkynes are used as artificial ripening of fruits.
Alkynes are used
in the preparation of acetic acid, alcohol, alkanes, alkenes, and carbonyl
compounds.
They are also
used for the manufacturing of many polymers products like polyvinyl ethers,
polyvinyl acetate, vinyl acetylene, chloroprene, and neoprene rubber.
neoprene and chloroprene
We can also use
alkynes as a local anesthetic.
They are also
used for the manufacture of acetylene tetrachloride.
They are used in oxyacetylene torch.
They are also used for the formation of metal
acetylides.
A large amount of heat is also obtained from the
combustion of alkynes.
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