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Preparation of alkynes

  1. Hydrolysis of carbides
     
    CaC2 + 2H2O Description: 45727.png + Ca(OH)2
     
    Mg2C3 + 4H2O CH3 – C ≡ CH + 2Mg(OH)2
  2. From acetylene
     
  3. Dehydrohalogenation of vicinal dihalides
     
    Description: 45761.png
  4. Dehalogenation of vicinal tetrahalides
     
    Description: 45775.png

Chemical properties of the alkynes

Acetylene is less reactive than ethylene towards most of the electrophilic reagents.
 
Description: 45805.png
 
Towards hydrogenation (which do not involve electrophilic attack), alkynes are more reactive than alkenes.
  1. Hydrogenation:
     
    Description: 45820.png
     
    Description: 45829.png
  2. Addition of halogen acids:
     
    Description: 45836.png
     
    RO + H – Br  RO – H + Br
     
    Description: 45847.png 
  3. Addition of halogens:
     
    Description: 45856.png
     
    Description: 45866.png
  4. Addition of water:
     
    Description: 45873.png
  5. Addition of boron hydrides
     
    2R – C ≡ CH + B2H6 2R – CH = CH – BH2
     
    R – C ≡ CH + R – CH = CH – BH2 (R – CH = CH)2BH
     
    R – C ≡ CH + (R – CH = CH)2BH (R – CH = CH)3B
     
    Description: 45882.png
     
    Description: 45913.pngDescription: 45905.png
     
    Description: 45920.png
  6. Ozonolysis
     
    Description: 45930.png
  7. Acidity of alkynes
     
    RC ≡ CH + NH2 RC ≡ C + NH3
     
    Description: 45941.png
     
    Identification of terminal alkynes
     
    Description: 45948.png




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