Hydrocarbons which will react with Tollen's reagent conform to the general formula

Analyze the question: Tollens' reagent is ammoniacal silver nitrate, [Ag(NH₃)₂]⁺. We need the general formula of hydrocarbons that react with it.

Concept: Tollens' reagent reacts with hydrocarbons only if they are terminal alkynes, meaning they have a hydrogen on a triple-bonded carbon (R-C≡C-H). That hydrogen is weakly acidic, so it is replaced by silver, giving a white precipitate of silver acetylide (or a silver alkynide).

Equation (for ethyne): HC≡CH + 2[Ag(NH₃)₂]⁺ → AgC≡CAg ↓ + 2NH₃ + 2NH₄⁺

For propyne: CH₃-C≡CH gives CH₃-C≡C-Ag.

General formulas of the options:

• A. CₙH₂ₙ: alkenes (and cycloalkanes). They do not react with Tollens' reagent.
• B. CₙH₂ₙ₊₂: alkanes. These are unreactive.
• C. CₙH₂ₙ₋₂: alkynes (also dienes). Terminal alkynes react.
• D. CₙH₂ₙ₊₁OH: alcohols. These are not hydrocarbons.

Answer: The hydrocarbons that react are the terminal alkynes, whose general formula is CₙH₂ₙ₋₂, so the answer is C.

Note: Not every CₙH₂ₙ₋₂ compound reacts, since non-terminal alkynes such as but-2-yne do not. Among the options, though, C is the only one that fits.

Exam tip: Remember "Tollens = terminal alkyne" (a triple bond at the end of the chain). Alkenes and alkanes never give the silver precipitate. Also, don't confuse this with the aldehyde test: aldehydes give a silver mirror, but they are not hydrocarbons.