An organic functional group which can likely decolorize ammoniacal silver nitrate is?

Analyze the question: Ammoniacal silver nitrate is Tollens' reagent, and the question asks which group reacts with it. Strictly, "decolorize" is loose wording, since the reagent is colourless to begin with. What the question really tests is which of these groups gives a positive reaction.

Concept: Ammoniacal silver nitrate, [Ag(NH₃)₂]⁺, reacts with two kinds of compounds:

1. Aldehydes, which are oxidised and deposit a silver mirror (Tollens' test).
2. Terminal alkynes (R-C≡C-H), which have an acidic hydrogen on the triple-bonded carbon. They react to form a white precipitate of silver alkynide (silver acetylide).

Solution:

• Alkene (A): No acidic hydrogen, so it does not react with the reagent. It decolorizes bromine water and acidified KMnO₄ instead.
• Alkane (B): Saturated and unreactive towards this reagent.
• Alkyne (C): Terminal alkynes react: HC≡CH + 2[Ag(NH₃)₂]⁺ → AgC≡CAg (white precipitate) + 2NH₄⁺ + 2NH₃ This is the characteristic test that distinguishes terminal alkynes from alkenes and alkanes.
• Alkanol (D): Alcohols do not react with Tollens' reagent. Only aldehydes do, and alkanols are not aldehydes.

Answer: The only option that reacts with ammoniacal silver nitrate is the alkyne, so the answer is C.

Tip: Remember the pairs of tests.

• Alkenes and alkynes both decolorize bromine water.
• Only terminal alkynes react with ammoniacal silver nitrate (white precipitate) or ammoniacal copper(I) chloride (red precipitate).

So if the question mentions "ammoniacal", think "alkyne" (or aldehyde, if it's offered as an option).