# Copper(I) acetylide

Copper(I) acetylide, copper carbide or cuprous acetylide, is a chemical compound with the formula Cu$_2$C$_2$. It is a copper(I) salt of acetylene. It consists of Cu$^+$ cations and acetylide anions $^−$C≡C$^−$, with the triple bond between the two carbon atoms. Although never characterized by X-ray crystallography, the material has been claimed at least since 1856. One form is claimed to be a monohydrate with formula Cu$_2$C$_2$·H$_2$O. Copper(I) acetylide is a reddish-brown explosive powder.

## Synthesis

Materials purported to be copper acetylide can be prepared by treating acetylene with a solution of copper(I) chloride and ammonia:
    C$_2$H$_2$(g) + 2 CuCl(s) → Cu$_2$C$_2$(s) + 2 HCl(g)
This reaction produces a reddish solid precipitate.

## Properties

When dry, copper acetylide is a heat and shock sensitive primary explosive, more sensitive than silver acetylide.
In acetylene manufacturing plants, copper acetylide is thought to form inside pipes made of copper or an alloy with high copper content, which may result in violent explosion. This led to abandonment of copper as a construction material in such facilities. Copper catalysts used in the chemical industry can also possess a degree of risk under certain conditions.

## Reactions

Copper(I) acetylide is the substrate of Glaser coupling for the formation of polyynes. In a typical reaction, a suspension of Cu$_2$C$_2$·H$_2$O in an amoniacal solution is treated with air. The copper is oxidized to Cu$^{2+}$ and forms a blue soluble complex with the ammonia, leaving behind a black solid residue. The latter has been claimed to consist of carbyne, an elusive allotrope of carbon:
    Cu$^+$($^−$C(≡C−C≡)$_n$C$^−$)Cu$^+$
This interpretation has been disputed.
Freshly prepared copper(I) acetylide reacts with hydrochloric acid to form acetylene and copper(I) chloride. Samples that have been aged with exposure to air or to copper(II) ions liberate also higher polyynes H(−C≡C−)$_n$H, with n from 2 to 6, when decomposed by hydrochloric acid. A "carbonaceous" residue of this decomposition also has the spectral signature of (−C≡C−)$_n$ chains. It has been conjectured that oxidation causes polymerization of the acetylide anions C2−2 in the solid into carbyne-type anions. $^−$C(≡C−C≡)$_n$C$^−$ or cumulene-type anions $^{2−}$C(=C=C=)$_n$C$^{2−}$.
Thermal decomposition of copper(I) acetylide in vacuum is not explosive and leaves copper as a fine powder at the bottom of the flask, while depositing a fluffy very fine carbon powder on the walls. On the basis of spectral data, this powder was claimed to be carbyne (−C≡C−)$_n$ rather than graphite as expected.

## Applications

Though not practically useful as an explosive due to high sensitivity, it is interesting as a curiosity because it is one of the very few explosives that do not liberate any gaseous products upon detonation.
The formation of copper(I) acetylide when a gas is passed through a solution of copper(I) chloride is used as a test for the presence of acetylene.
Reactions between Cu$^+$ and alkynes occur only if a terminal hydrogen is present (as it is slightly acidic in nature). Thus, this reaction is used for identification of terminal alkynes.