Copper(I) acetylide

Copper(I) acetylide, or cuprous acetylide, is a chemical compound with the formula Cu2C2. 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 .). It is a reddish solid, that easily explodes when dry.
Synthesis.
Materials purported to be copper acetylide can be prepared by treating acetylene with a solution of copper(I) chloride and ammonia:
This reaction produces a reddish solid precipitate.
Properties.
When dry, copper acetylide is a heat and shock sensitive high explosive, more thermally sensitive than silver acetylide.
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 was found to be the cause of explosions in acetylene plants, and led to abandonment of copper as a construction material in such plants. Copper catalysts used in petrochemistry can also possess a degree of risk under certain conditions.
Reactions.
Copper acetylide is the substrate of Glaser coupling for the formation of polyynes. In a typical reaction, a suspension of . in an amoniacal solution is treated with air. The copper is oxidized to 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:
This interpretation has been disputed.
Freshly prepared copper 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−)nH, 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 in the solid into carbyne-type anions .C(≡C−C≡)nC2− or polycumulene-type anions C(=C=C=)mC4−.
Thermal decomposition of copper 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≡C−)"n"C rather than graphite as expected.
Applications.
Though not practically useful as an explosive due to high sensitivity and reactivity towards water, 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 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.