Question #86848

1 Answer
Ernest Z.
Feb 18, 2017

Alkanes do not react with copper(I) chloride, but ethyne and terminal alkynes do react.

Explanation:

When ethyne is passed through an ammoniacal solution of copper(I) chloride, a red-brown precipitate of copper(I) acetylide is produced.

#"H-C≡C-H" + "2CuCl" + "2NH"_3 → underbrace("Cu-C≡C-Cu")_color(red)("red-brown ppt") + "2NH"_4"Cl"#

Terminal alkynes also react to form a red-brown precipitate. Thus.

#underbrace("CH"_3"-C≡C-H")_color(red)("a terminal alkyne") + "CuCl" + "NH"_3 → underbrace("CH"_3"-C≡C-Cu")_color(red)("red-brown ppt") + "2NH"_4"Cl"#

Non-terminal alkynes like but-2-yne, #"CH"_3"C≡CCH"_3#, lack an acidic hydrogen, so they do not react with ammoniacal #"CuCl"#.

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You can, therefore, use this reaction to distinguish between terminal and non-terminal alkynes.

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