Question

Give a chemical test to distinguish between pent-1-yne and pent-2-yne.

Answer 1

Pent-1-yne is a terminal alkyne which gives grey colour precipitate of silver alkynide with ammoniacal silver nitrate solution and Pent-2-yne will not answer this test.
CH3CH2CH2C≡CH +AgNO3+NH4OH→CH3CH2CH2C≡CAg↓+NH4NO3+H2O
 

Answer 2

Answer:

Pent-1-yne reacts with Tollens reagent to form a white precipitate of silver salt whereas pent-2-yne do not give this reaction.

Explanation:

Terminal alkynes on reaction with Tollen's reagent form a white precipitate of silver salt.

Ammoniacal solution of silver nitrate is Tollen's reagent. It is prepared by adding a dilute solution of ammonium hydroxide to silver nitrate solution till the dirty white precipitate of AgOH first formed gets dissolved.

Pent-1-yne, being a terminal alkyne reacts with Tollens reagent to form a white precipitate of silver salt.

The reaction is shown below.

CH₃-CH₂-CH₂-C≡CH + [Ag(NH₃)₂]⁺ → CH₃-CH₂-CH₂-C≡C-Ag + NH₄⁺ + NH₃

Pent-2-yne do not give this reaction as it is not a terminal alkyne.