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Difficulty: MediumAlkenes and Alkynes: Preparation, Reactions, and Unsaturation Tests

An organic compound XX with the molecular formula C4H6C_4H_6 rapidly decolorizes bromine water in tetrachloromethane, but produces no precipitate when treated with ammoniacal silver nitrate solution. What is the IUPAC name of compound XX?

  1. But-2-yneAnswer
  2. B
    But-1-yne
  3. C
    But-1-ene
  4. D
    But-2-ene

Answer

But-2-yne
The compound But-2-yne (CH3−C≡C−CH3CH_3-C\equiv C-CH_3) has the molecular formula C4H6C_4H_6 and contains an internal carbon-carbon triple bond. As an unsaturated hydrocarbon, it readily decolorizes bromine water. However, because its triple bond is located between carbon-2 and carbon-3, it lacks a terminal acetylenic hydrogen atom (−C≡C−H-C\equiv C-H). Therefore, it cannot react with ammoniacal silver nitrate to form a precipitate, matching all given experimental observations.

Step-by-Step Solution

1
Determine the structural class using the given molecular formula C4H6C_4H_6.
The formula C4H6C_4H_6 fits the general formula CnH2n−2C_nH_{2n-2} (alkynes or alkadienes), indicating two degrees of unsaturation.
Alkynes with 4 carbon atoms have the formula C4H6C_4H_6.
2
Analyze the response to bromine water in tetrachloromethane.
Decolorization confirms the presence of carbon-carbon multiple bonds (unsaturation).
Electrophilic addition of bromine occurs across the triple bond.
3
Analyze the reaction with ammoniacal silver nitrate solution.
The absence of a precipitate indicates that the alkyne is non-terminal (internal).
Only terminal alkynes containing an acidic acetylenic hydrogen atom (−C≡C−H-C\equiv C-H) react with Tollens' reagent ([Ag(NH3)2]+[Ag(NH_3)_2]^+) to yield a insoluble silver alkynide precipitate.
4
Identify the correct IUPAC name among the C4H6C_4H_6 isomers.
But-2-yne (CH3−C≡C−CH3CH_3-C\equiv C-CH_3) is an internal alkyne, whereas But-1-yne (CH3−CH2−C≡CHCH_3-CH_2-C\equiv CH) is a terminal alkyne.
Since compound XX does not form a precipitate, it must be the internal alkyne, But-2-yne.

Key Concept

Distinction between terminal and internal alkynes using Tollens' reagent (ammoniacal silver nitrate)
Estimated Time:1m 30s
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Topics

Alkenes and Alkynes: Preparation, Reactions, and Unsaturation Tests

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