Chemistry - What happens when an alkyne reacts with ammoniacal Cu2Cl2?
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:
- Carl Glaser, "Beiträge zur Kenntniss des Acetenylbenzols," Berichte der deutschen chemischen Gesellschaft 1869, 2(1), 422–424 (https://doi.org/10.1002/cber.186900201183).
- 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).