{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 440410848
| ImageFile=Copper acetylide.png
| ImageSize=120px
| IUPACName=Dicuprous acetylide
| OtherNames=
|Section1={{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|??}}
| CASNo=1117-94-8
| PubChem=19021056
| SMILES=[C-]#[C-].[Cu+].[Cu+]
| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}
| ChemSpiderID = 14318114
| InChI = 1/C2.2Cu/c1-2;;/q-2;2*+1
| InChIKey = SQDLRJMJSRRYGA-UHFFFAOYAK
| StdInChI_Ref = {{stdinchicite|changed|chemspider}}
| StdInChI = 1S/C2.2Cu/c1-2;;/q-2;2*+1
| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
| StdInChIKey = SQDLRJMJSRRYGA-UHFFFAOYSA-N
 }}
|Section2={{Chembox Properties
| C=2 | Cu=2
| Appearance= red-brown powder
| Density=
| MeltingPt=
| BoilingPt=
| Solubility=
 }}
|Section3={{Chembox Hazards
| MainHazards= explosive
| FlashPt=
| AutoignitionPt =
| PEL = TWA 1 mg/m<sup>3</sup> (as Cu)<ref name=PGCH>{{PGCH|0150}}</ref>
| REL = TWA 1 mg/m<sup>3</sup> (as Cu)<ref name=PGCH/>
| IDLH = TWA 100 mg/m<sup>3</sup> (as Cu)<ref name=PGCH/>
 }}
}}

'''Copper(I) acetylide''', '''Kupfercarbid''' or '''cuprous acetylide''', is a [[chemical compound]] with the formula [[copper|Cu]]<sub>2</sub>[[carbon|C]]<sub>2</sub>. Although never characterized by [[X-ray crystallography]], the material has been claimed at least since 1856.<ref name=cataldo/> One form is claimed to be a mono[[hydrate]] with formula {{chem|Cu|2|C|2}}.{{chem|H|2|O}} 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<sub>2</sub>H<sub>2</sub> (g) + 2 CuCl (s) → Cu<sub>2</sub>C<sub>2</sub> (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]].<ref name="CataldoCasari2007">{{cite journal|last1=Cataldo|first1=Franco|last2=Casari|first2=Carlo S.|title=Synthesis, Structure and Thermal Properties of Copper and Silver Polyynides and Acetylides|journal=Journal of Inorganic and Organometallic Polymers and Materials|volume=17|issue=4|year=2007|pages=641–651|issn=1574-1443|doi=10.1007/s10904-007-9150-3|s2cid=96278932}}</ref>

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.<ref>{{cite web |url = http://www.msha.gov/Accident_Prevention/Tips/acetylenegas.htm |title = Mine Safety and Health Administration (MSHA) - Accident Prevention Program - Miner's Tips - Hazards of Acetylene Gas |accessdate = 2008-06-08 |archive-url = https://web.archive.org/web/20080706064005/http://www.msha.gov/Accident_Prevention/Tips/acetylenegas.htm |archive-date = 2008-07-06 |url-status = dead}}</ref> This led to abandonment of copper as a construction material in such facilities.<ref>{{cite web|url=http://www.ilo.org/encyclopedia/?print&nd=857200158 |accessdate=February 8, 2013 |url-status=dead |archiveurl=https://web.archive.org/web/20071001010729/http://www.ilo.org/encyclopedia/?print&nd=857200158 |archivedate=October 1, 2007 |title=Copper}}</ref> Copper [[catalyst]]s used in the [[chemical industry]] can also possess a degree of risk under certain conditions.<ref>{{cite web|url = http://aiche.confex.com/aiche/s06/preliminaryprogram/abstract_43768.htm |title = The Safe Use of Copper -Containing Catalysts in Ethylene Plants |accessdate = 2008-06-08}}</ref>

== Reactions ==
Copper acetylide is the substrate of [[Glaser coupling]] for the formation of [[polyyne]]s. In a typical reaction, a suspension of {{chem|Cu|2|C|2}}.{{chem|H|2|O}} in an [[ammonium|amoniacal]] solution is treated with air. The copper is [[oxidized]] to {{chem|Cu|2+}} and forms a blue soluble [[coordination compound|complex]] with the [[ammonia]], leaving behind a black solid residue. The latter has been claimed to consist of [[linear acetylenic carbon|carbyne]], an elusive [[allotrope]] of carbon:<ref name=cataldo2>Franco Cataldo (1999), ' 'A study on the structure and electrical properties of the fourth carbon allotrope: carbyne''. Polymer International, volume 44, issue 2, pages 191–200. {{doi|10.1002/(SICI)1097-0126(199710)44:2}}
</ref>
:{{chem|Cu|+}} <sup>−</sup>C(≡C−C≡)<sub>n</sub>C<sup>−</sup> {{chem|Cu|+}}
This interpretation has been disputed.<ref name=kroto> H. Kroto (2010), [http://www.rsc.org/chemistryworld/Issues/2010/November/CarbyneOtherMythsAboutCarbon.asp ''Carbyne and other myths about carbon'']. RSC Chemistry World, November 2010.
</ref>

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 [[polyyne]]s H(−C≡C−)<sub>n</sub>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−)<sub>''n''</sub> chains. It has been conjectured that oxidation causes polymerization of the [[acetylide]] anions {{chem|C|2|2-}} in the solid into [[linear acetylenic carbon|carbyne]]-type anions .C(≡C−C≡)<sub>n</sub>C<sup>2−</sup> or [[cumulene|polycumulene]]-type anions C(=C=C=)<sub>m</sub>C<sup>4−</sup>.<ref name=cataldo> Franco Cataldo (1999), ''From dicopper acetylide to carbyne''.Polymer International, volume 48, issue 1, pages 15-22. {{doi|10.1002/(SICI)1097-0126(199901)48:1}}
</ref>

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 [[linear acetylenic carbon|carbyne]] C(−C≡C−)<sub>''n''</sub>C rather than graphite as expected.<ref name=cataldo/>

==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 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<sup>+</sup> 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.

== See also ==
* [[Carbide]]
* [[Walter Reppe]]
* [[Alkynation|Ethynylation]]

== References ==
{{reflist}}

{{Copper compounds}}
{{Carbides}}

[[Category:Copper(I) compounds]]
[[Category:Acetylides]]
[[Category:Explosive chemicals]]