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Difficulty: Very hardAlkenes and Alkynes: Preparation, Reactions, and Unsaturation Tests

An acyclic hydrocarbon XX with the molecular formula C5H8C_5H_8 rapidly decolourises bromine water. However, when XX is treated with ammoniacal silver nitrate solution, no precipitate is observed. Upon complete catalytic hydrogenation in the presence of a nickel catalyst, XX is converted into pentane. What is the IUPAC name of hydrocarbon XX?

Answer: pent-2-yne / 2-pentyne / Pent-2-yne / 2-Pentyne

Answer

Pent-2-yne (or 2-pentyne)
Hydrocarbon XX has the molecular formula C5H8C_5H_8, corresponding to two degrees of unsaturation. Complete hydrogenation to pentane confirms an unbranched five-carbon chain. Decolourisation of bromine water verifies unsaturation. Because XX yields no precipitate with ammoniacal silver nitrate solution, it lacks acidic terminal acetylenic hydrogens (R−C≡C−HR-C \equiv C-H). Therefore, the triple bond must be located internally between carbon-2 and carbon-3, making the compound pent-2-yne.

Step-by-Step Solution

1
Determine the degree of unsaturation and carbon skeleton of hydrocarbon XX.
Degree of unsaturation is 2, and the carbon skeleton is a straight 5-carbon chain.
The molecular formula C5H8C_5H_8 corresponds to CnH2n−2C_nH_{2n-2}, indicating two degrees of unsaturation (an alkyne or alkadiene). Complete catalytic hydrogenation yields pentane (C5H12C_5H_{12}), proving an unbranched five-carbon chain.
2
Analyze the reaction with bromine water.
Hydrocarbon XX contains carbon-carbon multiple bonds.
Decolourisation of bromine water confirms the presence of unsaturation.
3
Evaluate the test with ammoniacal silver nitrate solution.
Hydrocarbon XX is an internal (non-terminal) alkyne.
Terminal alkynes possess acidic acetylenic hydrogen atoms (R−C≡C−HR-C \equiv C-H) that react with ammoniacal silver nitrate to form a characteristic white silver acetylide precipitate. The absence of a precipitate rules out pent-1-yne and confirms that the triple bond is located internally at C-2.
4
Deduce the final IUPAC name of hydrocarbon XX.
pent-2-yne
Combining a straight 5-carbon chain with an internal triple bond between carbon-2 and carbon-3 gives pent-2-yne.

Key Concept

Distinction between terminal and non-terminal alkynes using ammoniacal silver nitrate test and carbon skeleton determination via hydrogenation
Estimated Time:2m 0s
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Topics

Alkenes and Alkynes: Preparation, Reactions, and Unsaturation Tests

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