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My text book provided me with 2 reactions concerning alkynes (Specifically Terminal Alkynes).

The first one was the dimerisation of an alkyne on passing it through ammoniacal $\ce{Cu2Cl2}$.

All was fine until I encountered the next reaction.

It was a test to distinguish between terminal alkynes and other alkene and alkynes.

It involved the fact that treating a terminal alkyne with ammoniacal $\ce{Cu2Cl2}$ would result in the alkyne forming a red precipitate as copper replaces hydrogen of one of the terminal hydrogen.

However now I was confused. Which of these reactions actually happens? Or are there specific conditions for each of these reactions that my text missed out?

Please help me in this aspect.

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Terminal alkynes (including acetylene) form a $\color{red}{\text{red precipitate}}$ of copper acetylides when it is passed through ammoniacal cuprous chloride solution:

$$\ce{R-C#C-H + CuCl + NH3 -> R-C#C-Cu + NH4Cl}$$

However, in the presence of oxygen, this reaction proceeds further to dimerize the corresponding alkyne. This reaction, the oldest homo-coupling of terminal alkyne reaction, was first discovered by Carl Andreas Glaser in 1869 (Ref.1, 2), hence the reaction is named Glaser-coupling after him:

$$\ce{2R-C#C-Cu ->[O2][NH3/EtOH] R-C#C-C#C-R }$$

The actual Glaser's reaction was (ref.2):

$$\ce{2Ph-C#C-H ->[CuCl/O2][NH3/EtOH] Ph-C#C-C#C-Ph}$$

References:

  1. Carl Glaser, "Beiträge zur Kenntniss des Acetenylbenzols," Berichte der deutschen chemischen Gesellschaft 1869, 2(1), 422–424 (https://doi.org/10.1002/cber.186900201183).
  2. Chao Liu, "Chapter 1: Introduction," In Lecture Notes in Chemistry 102: Transition Metal Catalyzed Oxidative Cross-Coupling Reactions; Aiwen Lei, Editor; Springer-Verlag GmbH DE (Part of Springer Nature): Berlin, Germany, 2019, pp. 1-10 (ISBN: 978-3-662-58102-5).
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$$\ce{H-C#C-H ->[Cu2Cl2][NH4Cl] H-C#C- Cu+ }$$

This then attacks $\ce{H-C#C-H}$ to give $\ce{H-C#C-CH=CH2}$

Note: This occurs only for terminal alkynes and only in acidic medium(NH4Cl here).

Reference:M S Chouhan Advanced Problems In Organic Chemistry For JEE

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