Concept:Ammoniacal silver nitrate is Tollens’ reagent, which detects reducing organic groups such as terminal alkynes.
Explanation:Tollens’ reagent contains the diamminesilver(I) ion,
[Ag(NH3)2]+.
Most organic compounds do not react with it, but terminal alkynes, containing the group
−C≡CH, react readily.
In this reaction, the terminal hydrogen is replaced by silver, forming a white precipitate of silver alkynide,
R−C≡CAg.
The silver ion is removed from solution, so the reagent loses its colour and decolorizes.
Alkenes, alkanes, and alkanols do not give this characteristic reaction with ammoniacal silver nitrate.
Therefore, the organic functional group likely to decolorize ammoniacal silver nitrate is the alkyne group, especially when it is terminal.
Answer:C. alkyne