Question

Question #86848

Answer 1

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"`.

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