Test to distinguish between triple bond and double bond
Answers
Question:
=> Give a test to distinguish between triple bond and double bond.
Alkenes and Alkynes:
=> As we know, aliphatic hydrocarbons containing double bonds are called alkenes and aliphatic hydrocarbons containing triple bonds are called alkynes.
=> We can use Ammoniacal Silver Nitrate (NH4OH + AgNO3) also known as Tollen's Reagent or Ammoniacal Cuprous Chloride (NH4OH + CuCl) solution to distinguish between alkenes and alkynes.
The Test Used:
=> If a white precipitate is obtained with Ammoniacal Silver Nitrate or a red precipitate is obtained with Ammoniacal Cuprous Chloride, the compound is an alkyne and contains at least one triple bond.
=> If no precipitate is obtained with Ammoniacal AgNO3 or Ammoniacal CuCl , then the compound is an alkene and has at least one double bond.
Mechanism of the Test:
=> Alkenes do not react with Tollen's reagent or Ammoniacal Cuprous Chloride as the double bond is more stable, and is not easily oxidized by these reagents.
=> Alkynes however, are much less stable and more reactive than alkenes, they react easily with Tollen's reagent or Ammoniacal Cuprous Chloride to give precipitates of Acetylides.
Reactions taking place:
=> When Acetylene (Alkyne) is bubbled through Tollens reagent:
(Alkyne) (Silver Acetylide - White ppt.)
H-C≡C-H + 2AgNO3 + 2NH4OH ---------> Ag-C≡C-Ag↓ + 2NH4NO3 + 2H2O
=> When Acetylene (Alkyne) is bubbled through Ammoniacal CuCl:
( Alkyne) (Copper [I] Acetylide - Red ppt.)
H-C≡C-H + 2CuCl + 2NH4OH ---------> Cu-C≡C-Cu↓ + 2NH4Cl + 2H2O