Chemical Test for Terminal Alkynes | Chemistry for JAMB PDF Download

Introduction

Terminal alkynes are hydrocarbons that possess a carbon-carbon triple bond at the terminal or end position of the carbon chain. These compounds play a crucial role in various chemical reactions and organic synthesis. To identify the presence of terminal alkynes, several chemical tests can be conducted. 

Chemical Tests for Terminal Alkynes

Bromine Test:

Terminal alkynes react readily with bromine water (a solution of bromine in water) to form a reddish-brown color precipitate of 1,2-dibromoalkene. This reaction occurs due to the addition of bromine across the carbon-carbon triple bond.
Procedure:

• Take a test tube and add a small amount of the organic compound suspected to be a terminal alkyne.
• Add a few drops of bromine water to the test tube.
• Observe the color change. If a reddish-brown precipitate is formed, it confirms the presence of a terminal alkyne.
  R-C≡CH + Br₂ → R-CBr=CHBr

Sodium Ammonia Test (Lindlar's Catalyst Test):

Terminal alkynes can be distinguished from internal alkynes by their response to sodium ammonia (NaNH₂) in the presence of Lindlar's catalyst. This test selectively reduces terminal alkynes to corresponding cis-alkenes.
Procedure:

• Prepare a solution of sodium in liquid ammonia by dissolving a small piece of sodium metal in liquid ammonia.
• Add a small amount of the organic compound suspected to be a terminal alkyne to the sodium ammonia solution.
• Add a small amount of Lindlar's catalyst (a mixture of palladium and calcium carbonate) to the solution.
• Stir the mixture and allow it to react for some time.
• If the compound is a terminal alkyne, it will be reduced to a cis-alkene.
• Perform additional tests to confirm the presence of the alkene.
  R-C≡CH + 2Na + 2NH₃ + Lindlar's catalyst → R-CH=CH₂ (cis-alkene) + NaNH₂

Note: Internal alkynes do not react with sodium ammonia and Lindlar's catalyst.

Conclusion

Chemical tests play a vital role in identifying the presence of terminal alkynes. The bromine test and the sodium ammonia test (Lindlar's catalyst test) are commonly used to differentiate terminal alkynes from other organic compounds. It is essential to understand these tests thoroughly to perform accurate identification and characterization of terminal alkynes in organic chemistry experiments.