Dilithium acetylide
{{Short description|Chemical compound of lithium and carbon, an acetylide}}
{{About||the chemical Li4C|Tetralithium carbide}}
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 450703945
| ImageFile =
| ImageSize = 120px
| ImageName = Wireframe model of lithium carbide
| PIN = Lithium acetylide
| SystematicName = Lithium ethynediide
| OtherNames = {{ubl|Dilithium acetylide|Lithium dicarbon|Lithium percarbide}}
| Section1 = {{Chembox Identifiers
| InChI1 = 1/C2.2Li/c1-2;;/q-2;2*+1
| InChIKey1 = ARNWQMJQALNBBV-UHFFFAOYAB
| CASNo_Ref = {{cascite|correct|??}}
| CASNo = 1070-75-3
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = GZ7TQ3WG5P
| ChemSpiderID = 59503
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| EINECS = 213-980-1
| PubChem = 66115
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C2.2Li/c1-2;;/q-2;2*+1
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = ARNWQMJQALNBBV-UHFFFAOYSA-N
| SMILES = [Li+].[Li+].[C-]#[C-]
| InChI = 1S/C2.2Li/c1-2;;/q-2;2*+1
| InChIKey = ARNWQMJQALNBBV-UHFFFAOYSA-N}}
| Section2 = {{Chembox Properties
| Formula = {{chem2|Li2C2}}
| MolarMass = 37.9034 g/mol
| Appearance = Powder
| Density = 1.3 g/cm3{{cite journal | title = Zur Kenntnis des Lithiumacetylids |author1=R. Juza |author2=V. Wehle |author3=H.-U. Schuster | journal = Zeitschrift für anorganische und allgemeine Chemie | year = 1967 | volume = 352 | pages = 252 | doi = 10.1002/zaac.19673520506 | issue = 5–6}}
| Solubility = Reacts
| SolubleOther = insoluble in organic solvents
| MeltingPt = 452°C{{cite journal | title = Thermal properties of lithium carbide and lithium intercalation compounds of graphite |author1=Savchenko, A.P. |author2= Kshnyakina, S.A. |author3=H.-Majorova, A.F. | journal = Neorganicheskie Materialy | year = 1997 | volume = 33 | pages = 1305–1307 | issue = 11}}
| BoilingPt = }}
| Section9 = {{Chembox Related
| OtherCompounds = {{ubl|Tetralithiomethane|Monolithium acetylide|Monosodium acetylide|Copper(I) acetylide|Silver acetylide|Calcium carbide}}
}}
}}
Dilithium acetylide is an organometallic compound with the formula Li2C2. It is typically derived by double deprotonation of acetylene. X-ray crystallography confirms the presence of {{chem2|C\tC}} subunits attached to lithium, resulting in a polymeric structure. {{chem2|Li2C2}} is one of an extensive range of lithium-carbon compounds, which include the lithium-rich tetralithiomethane, {{chem2|Li6C2}}, {{chem2|Li8C3}}, {{chem2|Li6C3}}, {{chem2|Li4C3}}, {{chem2|Li4C5}}, and the graphite intercalation compounds {{chem2|LiC6}}, {{chem2|LiC12}}, and {{chem2|LiC18}}. It is an intermediate compound produced during radiocarbon dating procedures.
{{chem2|Li2C2}} is the most thermodynamically-stable lithium-rich carbide{{cite journal|last1=Ruschewitz|first1=Uwe|title=Binary and ternary carbides of alkali and alkaline-earth metals|journal=Coordination Chemistry Reviews|date=September 2003|volume=244|issue=1–2|pages=115–136|doi=10.1016/S0010-8545(03)00102-4}} and the only one that can be obtained directly from the elements. It was first produced by Moissan, in 1896H. Moissan Comptes Rendus hebd. Seances Acad. Sci. 122, 362 (1896) who reacted coal with lithium carbonate.
:{{chem2|Li2CO3 + 4 C → Li2C2 + 3 CO}}
The other lithium-rich compounds are produced by reacting lithium vapor with chlorinated hydrocarbons, e.g. carbon tetrachloride. Lithium carbide is sometimes confused with the drug lithium carbonate, {{chem2|Li2CO3}}, because of the similarity of its name.
Preparation and reactions
In the laboratory samples may be prepared by treating acetylene with butyl lithium:{{cite journal|doi=10.1016/S0022-328X(00)89260-8|title=Friedel–Crafts reactions of bis(trimethylsilyl)polyynes with acyl chlorides; a useful route to terminal-alkynyl ketones|year=1972|last1=Walton|first1=D.R.M.|last2=Waugh|first2=F.|journal=Journal of Organometallic Chemistry|volume=37|pages=45–56}}
:{{chem2|C2H2 + 2 BuLi → Li2C2 + BuH}}
Instead of butyl lithium, a solution of lithium in ammonia can be used to prepare {{chem2|Li2C2}}. In this case, a transient adduct {{chem2|Li2C2*C2H2*2NH3}} if formed. It decomposes with release of ammonia at room temperature.
Samples prepared from acetylene generally are poorly crystalline. Crystalline samples may be prepared by a reaction between molten lithium and graphite at over 1000 °C. {{chem2|Li2C2}} can also be prepared by reacting Carbon dioxide with molten lithium.{{cn|date=January 2025}}
:{{chem2|10 Li + 2 CO2 → Li2C2 + 4 Li2O}}
Other method for production of {{chem2|Li2C2}} is heating of metallic lithium in atmosphere of ethylene. Lithium hydride is a coproduction:
:{{chem2|6 Li + C2H4 → Li2C2 + 4 LiH}}
Lithium carbide hydrolyzes readily to form acetylene as well as Lithium hydroxide:
:{{chem2|Li2C2 + 2 H2O → 2 LiOH + C2H2}}
Lithium hydride reacts with graphite at 400°C forming lithium carbide.
:{{chem2|2 LiH + 4 C → Li2C2 + C2H2}}
Lithium carbide reacts with acetylene in liquid ammonia rapidly to give a lithium hydrogen acetylide.
:{{chem2|LiC\tCLi + HC\tCH → 2 LiC\tCH}}
Preparation of the reagent in this way sometimes improves the yield in an ethynylation over that obtained with reagent prepared from lithium and acetylene.{{cn|date=January 2025}}
Structure
{{chem2|Li2C2}} could be viewed as a Zintl phase. It is not a salt. It adopts a distorted anti-fluorite crystal structure, similar to that of rubidium peroxide ({{chem2|Rb2O2}}) and caesium peroxide ({{chem2|Cs2O2}}). Each lithium atom is surrounded by six carbon atoms from 4 different acetylide anions, with two acetylides co-ordinating side -on and the other two end-on.{{cite journal|last1=Juza|first1=Robert|last2=Opp|first2=Karl|title=Metallamide und Metallnitride, 24. Mitteilung. Die Kristallstruktur des Lithiumamides|journal=Zeitschrift für anorganische und allgemeine Chemie|date=November 1951|volume=266|issue=6|pages=313–324|doi=10.1002/zaac.19512660606|language=German}} The relatively short C-C distance of 120 pm indicates the presence of a C≡C triple bond. At high temperatures {{chem2|Li2C2}} transforms reversibly to a cubic anti-fluorite structure.{{cite journal |author1=U. Ruschewitz |author2=R. Pöttgen | title = Structural Phase Transition in {{chem|Li|2|C|2}} | journal = Zeitschrift für anorganische und allgemeine Chemie | volume = 625 | issue = 10 | pages = 1599–1603 | doi = 10.1002/(SICI)1521-3749(199910)625:10<1599::AID-ZAAC1599>3.0.CO;2-J | year = 1999}}
Use in radiocarbon dating
{{main|Radiocarbon dating}}
There are a number of procedures employed, some that burn the sample producing carbon dioxide that is then reacted with lithium, and others where the carbon containing sample is reacted directly with lithium metal.{{cite journal | author = Swart E.R. | title = The direct conversion of wood charcoal to lithium carbide in the production of acetylene for radiocarbon dating | journal = Cellular and Molecular Life Sciences | doi = 10.1007/BF02146038 | year = 1964 | volume = 20 | pages = 47–48| s2cid = 31319813}} The outcome is the same: {{chem2|Li2C2}} is produced, which can then be used to create species easy to use in mass spectroscopy, like acetylene and benzene.[http://www.geo.unizh.ch/c14/ University of Zurich Radiocarbon Laboratory webpage] {{webarchive|url=https://web.archive.org/web/20090801100716/http://www.geo.unizh.ch/c14/ |date=2009-08-01}} Note that lithium nitride may be formed and this produces ammonia when hydrolyzed, which contaminates the acetylene gas.
References
{{reflist}}
{{Lithium compounds}}
{{Carbides}}