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physical and chemical properties of alkynes

Alkynes:

Physical Properties

            Low-molecular alkynes have a slightly higher boiling point than the corresponding alkenes. At roomtemperature, ethyne, propyne and but-1-yne are gases, but starting with but-2-yne, alkynes with medium molecular weight are liquids. Since alkynes show a great tendency to polymerize, sometimes violently, they have to be handled carefully. Though ethyne explodes under pressure, it is available as a solution in acetone in gas cylinders filled with porous material, such as kieselguhr or pumice.
           Physical properties and reactivity of alkynes are determined by the sp hybridization of the C atoms. Though a C?C triple bond is shorter and stronger than a C=C double or even a C-C single bond, the bonding energy clearly is less than the threefold energy of a single bond. The ?i bond, too, is weaker than the one in alkenes. These facts explain the high reactivity of alkynes.

Chemical Properties:


Combustion of Alkynes :
  Ethyne burn in air with a luminous, smoky flame, forming carbon dioxide and water.

Oxidation of Alkynes:
    Ethyne is oxidised by a dilute aqueous solution of potassium permanganate to form oxalic acid. Thus, if ethyne is bubbled through a solution of potassium permanganate the solution is decolourised. This is Baeyer's test for unsaturated organic compounds.

Addition Reactions of Alkynes:
    Because of the unsaturated nature of ethyne addition reactions can occur across the triple bond.
     Addition of Hydrogen
          When acetylene and hydrogen are passed over a nickel catalyst at 150 degC, (or over platinum black catalyst at room temperature) ethene is first formed and then this is further reduced to ethane.

Polymerisation of Alkynes due to Triple Bond:
        Two molecules of ethyne can be combined to produce vinyl ethyne, HC(CCH=CH2, by passing the ethyne into a saturated solution of cuprous chloride in to ammonium chloride continuously in such a way that low conversions of starting material occur.
     This linear polymerisation can be extended by altering the conditions of reaction. For example,
    HCCCH=CH2 + HCCH ------->CH2=CHC(CCH=CH2 )n 

    Vinyl Ethyne  + Ethyne ------>DiVinyl Ethyne 

Substitution Reaction of Alkynes:
       When ethyne is passed through a solution of sodium in liquid ammonia then sodium acetylide is formed and hydrogen is liberated.
                          liq.NH3
HCCH   +   2Na    ==>       2HCCNa    + H2
                                 Sodium Acetylide      






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