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Alkynes: Reactions

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Alkynes: Reactions - Lesson Summary

Addition reactions of alkynes:
The addition reactions in alkynes are electrophillic in nature due to the availability of loosely held Pi electrons. Alkynes contain a triple bond during the reaction two molecules of dihydrogen, halogen or hydrogen halides added to alkynes to form a saturated molecule.
 


Vinylic cation is formed as an intermediate during the addition reaction of an alkyne whereas alkyl carbocation is formed as an intermediate during the addition reaction of an alkene. As vinylic cation is less stable than alkyl carbocation, alkynes are less reactive towards the addition reactions than alkenes.
 

 
Hence the addition reactions of alkynes take place under drastic conditions when compared to the addition reactions of alkenes.

Alkyne reacts with two molecules of dihydrogen in the presence of suitable catalysts like finely divided nickel, platinum or palladium to form the corresponding alkane.

Ex: Propyne reacts with two molecules of dihydrogen in the presence of finely divided nickel as catalyst to form propane.

CH 3-C≡CH+H 2  Pt/Pd/Ni →     [CH 3-CH=CH 2] + H 2 → CH 3-CH 2-CH 3 
Propyne                                           Propene                        Propane
 
Alkynes form dihaloalkene when treated with one molecule of halogen and form tetrahaloalkane when treated with two molecules of halogen.



Ex: Ethyne reacts with one molecule of  bromine forms  1,2-dibromo ethene and when it reacts two molecules of bromine it forms 1,1,2,2-tetrabromoethane.
 


Reddish brown colour of bromine decolorizes due to the formation of saturated 1, 1, 2, 2-tetrabromoethane. Thus this reaction is used to test the unsaturation in the molecule. 

Alkyne when treated with two molecules of hydrogen halide forms gem dihalide. When treated with one molecule of hydrogen halide forms alkenyl halide.

Addition in unsymmetrical alkynes takes place in accordance with Markownikov”s rule. According to this rule in an electrophillic addition reaction the negative part of an addendum attaches itself to the unsaturated carbon atom of the molecule having least number of hydrogen atoms.



Ex: Propyne reacts with two molecules of hydrogen bromide, and forms 2,2-dibromo propane in accordance with Markovnikoff’s rule. 
 


Similar to alkanes and alkenes, alkynes are also immiscible and do not react with water. But in the presence of dilute sulphuric acid and mercuric sulphate at 60 °C alkynes reacts with water molecule to form Aldehydes or Ketones.

 

Ex: Ethyne reacts with a water molecule in the presence of dilute sulphuric acid and mercuric sulphate at 60°C to give ethanal.


 
Polymerization:
Polymerization is of two types: linear polymerization and cyclic polymerization. When heated in the presence of a catalyst, alkynes undergo polymerization. In linear polymerization, the alkyne molecules form a long chain polymer.
 

 
Ex:  Linear polymerization of ethyne produces polyacetylene or polyethyne.
 

In cyclic polymerization, alkyne molecules add to each other, resulting in the formation of a cyclic compound.
Ex:  When ethyne is passed through a red hot iron tube at 873K it undergoes cyclic polymerization.



In cyclic polymerization, three molecules of ethyne polymerize to form benzene. Because of their polymerization abilities, alkynes combine to form many different compounds.

Ex: Benzene obtained by the polymerization of acetylene is used for the preparation of dyes, drugs and a large number of other organic compounds.

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