Examkin

Question

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

An unknown gaseous hydrocarbon XX decolorizes acidified potassium tetraoxomanganate(VII) solution and produces a reddish-brown precipitate when bubbled into an ammoniacal solution of copper(I) chloride. Which of the following IUPAC structural formulas represents hydrocarbon XX?

  1. CH3CH2C≡CH\text{CH}_3\text{CH}_2\text{C}\equiv\text{CH}Answer
  2. B
    CH3C≡CCH3\text{CH}_3\text{C}\equiv\text{CCH}_3
  3. C
    CH3CH=CHCH3\text{CH}_3\text{CH}=\text{CHCH}_3
  4. D
    CH3CH2CH2CH3\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3

Answer

The correct structural formula is CH3CH2C≡CH\text{CH}_3\text{CH}_2\text{C}\equiv\text{CH} (but-1-yne).
The compound CH3CH2C≡CH\text{CH}_3\text{CH}_2\text{C}\equiv\text{CH} (but-1-yne) contains a carbon-carbon triple bond which decolorizes acidified KMnO4\text{KMnO}_4 via oxidation. Furthermore, because it is a terminal alkyne with a hydrogen atom directly bonded to an spsp-hybridized carbon, it reacts with ammoniacal copper(I) chloride solution to yield a reddish-brown precipitate of copper(I) acetylide.

Step-by-Step Solution

1
Analyze the reaction with acidified potassium tetraoxomanganate(VII) (\text{KMnO}_4) solution.
The decolorization of acidified KMnO4\text{KMnO}_4 proves that hydrocarbon XX is unsaturated (contains carbon-carbon double or triple bonds). This eliminates saturated alkanes like butane.
Unsaturated hydrocarbons undergo oxidation addition across carbon-carbon multiple bonds.
2
Analyze the reaction with ammoniacal copper(I) chloride (\text{Cu}_2\text{Cl}_2) solution.
Formation of a reddish-brown precipitate (copper(I) diallylide/acetylide) confirms the presence of a terminal alkyne carrying an acidic acetylenic hydrogen attached to an spsp-hybridized carbon atom (−C≡C−H-\text{C}\equiv\text{C}-\text{H}).
Only terminal alkynes have sufficiently acidic hydrogen atoms to be substituted by copper(I) or silver ions in ammoniacal solutions.
3
Differentiate between terminal alkynes, internal alkynes, and alkenes based on structural formulas.
CH3CH2C≡CH\text{CH}_3\text{CH}_2\text{C}\equiv\text{CH} is but-1-yne (a terminal alkyne) which fulfills both conditions. CH3C≡CCH3\text{CH}_3\text{C}\equiv\text{CCH}_3 is an internal alkyne lacking a terminal acidic hydrogen.
Internal alkynes and alkenes fail the ammoniacal copper(I) chloride test despite being unsaturated.

Key Concept

Distinction between general unsaturation tests and terminal alkyne confirmation tests.
Estimated Time:1m 30s
Rate this question

Topics

Alkenes and Alkynes: Preparation, Reactions, and Unsaturation Tests

More questions from this topic