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Alkynes: Preparation

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Alkynes: Preparation - Lesson Summary

Industrial preparation of ethyne from calcium carbide:
Limestone is subjected to thermal decomposition results in the formation of quick lime. Quick lime obtained is heated to high temperatures with coke to get calcium carbide.

          CaCO 3  ∆ →        CaO              + CO 2
    Lime Stone       Quick Lime

            CaO +         3C     ∆ →    CaC 2                    + CO
       Quick Lime     Coke            Calcium Carbide
 
When this calcium carbide is subjected to hydrolysis, ethyne or acetylene is formed.
 

 
Preparation of ethyne from vicinal dihalides:
A vicinal dihalide molecule has two halogen atoms, each linked to two adjacent carbon atoms.
1,2-dihalo alkanes are called vicinal dihalides.

Alkynes can also be prepared from vicinal dihalides. When treated with alcoholic potash, the molecule undergoes dehydrohalogenation. In dehydrohalogenation, first, an atom of halogen on one carbon and an atom of hydrogen present on adjacent carbon in the alkyl dihalide are eliminated as one molecule of hydrogen halide results in the formation of alkenyl halide.


 
Alkenyl halide when treated with a strong base such as sodamide, another molecule of hydrogen halide is eliminated, thus resulting in the formation of an alkyne.



Ex:  Ethylidene chloride (1, 2-dichloro ethane) when treated with alcoholic potassium hydroxide forms vinyl chloride and this when treated with sodamide, forms ethyne.


 
As the chlorine atom is directly attached to double bonded carbon atom in vinyl chloride, it is made highly unreactive.  In other words, as the non bonding electrons of chlorine and the pi electrons of carbon –carbon double bond are in conjugation, the halogen atom acquires a partial double bond character and the removal of it becomes difficult.

   

Hence, a strong base such as sodamide is required for the removal of hydrogen chloride from vinyl chloride to form Ethyne.

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