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.


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):

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


  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).

$$\ce{H-C#C-H ->[Cu2Cl2][NH4Cl] H-C#C- Cu+ }$$

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


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.