Which of the following alkynes will yield a red precipitate of cuprous acetylide when treated with ammoniacal cuprous chloride ()?
Answer and explanation
Correct answer: C. But-1-yne
- Step 1: Ammoniacal cuprous chloride is a diagnostic reagent specific for terminal alkynes ().
- Step 2: Terminal alkynes possess weakly acidic acetylenic protons that are replaced by copper(I) ions to form insoluble metal acetylide precipitates.
- Step 3: But-1-yne is a terminal alkyne and reacts according to: .
Answer
But-1-yne.
Why not the others: But-2-yne is an internal alkyne lacking an acidic acetylenic hydrogen; Hex-3-yne is a symmetrical internal alkyne; Pent-2-yne is an internal alkyne and gives no precipitate.
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