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

Which of the following reagents forms a characteristic precipitate when reacted with propyne, but produces no precipitate when reacted with propene?

  1. Ammoniacal silver nitrate solutionAnswer
  2. B
    Bromine water
  3. C
    Acidified potassium tetraoxomanganate(VII) solution
  4. D
    Concentrated tetraoxosulfate(VI) acid

Answer

Ammoniacal silver nitrate solution
Ammoniacal silver nitrate solution contains complexed silver ions [Ag(NH3)2]+[Ag(NH_3)_2]^+ which react selectively with the acidic hydrogen attached to the triply bonded carbon in terminal alkynes like propyne (CH3C≡CHCH_3C\equiv CH), forming a insoluble white precipitate of silver propynide. Propene lacks this acidic acetylenic hydrogen and gives no precipitate.

Step-by-Step Solution

1
Identify the structural difference between propyne and propene
Propyne (CH3C≡CHCH_3C\equiv CH) is a terminal alkyne containing a weakly acidic hydrogen attached to a triply-bonded carbon atom, whereas propene (CH3CH=CH2CH_3CH=CH_2) is an alkene.
Terminal alkynes possess acidic acetylenic hydrogens (R−C≡C−HR-C\equiv C-H), unlike alkenes.
2
Evaluate the chemical reagents for specific reactivity with terminal acetylenic hydrogen
Ammoniacal silver nitrate solution ([Ag(NH3)2]+[Ag(NH_3)_2]^+) reacts with terminal alkynes to precipitate silver dicarbide/alkynide (a white precipitate). Alkenes do not undergo this substitution reaction.
Reagents like bromine water and acidified KMnO4KMnO_4 test for general double/triple bond unsaturation and react with both compounds.

Key Concept

Distinction between terminal alkynes and alkenes using ammoniacal silver nitrate solution
Estimated Time:45s
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Topics

Alkenes and Alkynes: Preparation, Reactions, and Unsaturation Tests

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