View source for Chemistry:Lithium carbide

{{Short description|Chemical compound of lithium and carbon, an acetylide}}
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 450703945
|  ImageFile = lithium carbide.png
|   ImageSize = 120px
|   ImageName = Wireframe model of lithium carbide
|  PIN = Lithium acetylide
|  SystematicName = Lithium ethynediide
|  OtherNames = Dilithium acetylide<br />
Lithium dicarbon<br />
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 = {{chem|Li|2|C|2}}
|   MolarMass = 37.9034 g/mol
|   Density = 1.3 g/cm<sup>3</sup><ref>{{cite journal | title = Zur Kenntnis des Lithiumacetylids |author1=R. Juza |author2=V. Wehle |author3=H.-U. Schuster | journal = [[Chemistry:Zeitschrift für anorganische und allgemeine Chemie|Zeitschrift für anorganische und allgemeine Chemie]] | year = 1967 | volume = 352 | pages = 252 | doi = 10.1002/zaac.19673520506 | issue = 5–6}}</ref> 
|   Solubility = Reacts
|   SolubleOther = insoluble in organic solvents 
|   MeltingPt = 452°C<ref>{{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}}</ref>
|   BoilingPt = }}
}}
'''Lithium carbide''', {{chem|Li|2|C|2}}, often known as '''dilithium [[Chemistry:Acetylide|acetylide]]''', is a chemical compound of [[Chemistry:Lithium|lithium]] and [[Chemistry:Carbon|carbon]], an [[Chemistry:Acetylide|acetylide]]. It is an intermediate compound produced during [[Physics:Radiocarbon dating|radiocarbon dating]] procedures. {{chem|Li|2|C|2}} is one of an extensive range of lithium-carbon compounds which include the lithium-rich {{chem|Li|4|C}}, {{chem|Li|6|C|2}}, {{chem|Li|8|C|3}}, {{chem|Li|6|C|3}}, {{chem|Li|4|C|3}}, {{chem|Li|4|C|5}}, and the [[Chemistry:Graphite intercalation compound|graphite intercalation compound]]s {{chem|LiC|6}}, {{chem|LiC|12}}, and {{chem|LiC|18}}.<br/>{{chem|Li|2|C|2}} is the most thermodynamically-stable lithium-rich carbide<ref name=rev1>{{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}}</ref> and the only one that can be obtained directly from the elements. It was first produced by [[Biography:Henri Moissan|Moissan]], in 1896<ref>H. Moissan Comptes Rendus hebd. Seances Acad. Sci. 122, 362 (1896)</ref> who reacted coal with [[Chemistry:Lithium carbonate|lithium carbonate]]. 

:<chem>Li2CO3 + 4C->Li2C2 + 3CO   </chem>

The other lithium-rich compounds are produced by reacting lithium vapor with chlorinated hydrocarbons, e.g. [[Chemistry:Carbon tetrachloride|CCl<sub>4</sub>]]. Lithium carbide is sometimes confused with the drug [[Chemistry:Lithium carbonate|lithium carbonate]], {{chem|Li|2|CO|3}}, because of the similarity of its name.

==Preparation and chemistry==
In the laboratory samples may be prepared by treating [[Chemistry:Acetylene|acetylene]] with a solution of lithium in ammonia, on −40°C, with creation of addition compound of Li<sub>2</sub>C<sub>2</sub> • C<sub>2</sub>H<sub>2</sub> • 2NH<sub>3</sub> that decomposes in stream of hydrogen at room temperature giving white powder of Li<sub>2</sub>C<sub>2.</sub>

:<chem>C2H2 + 2Li -> Li2C2 + H2</chem>

Samples prepared in this manner generally are poorly crystalline. Crystalline samples may be prepared by a reaction between molten lithium and [[Chemistry:Graphite|graphite]] at over 1000&nbsp;°C.<ref name=rev1 /> Li<sub>2</sub>C<sub>2</sub> can also be prepared by reacting CO<sub>2</sub> with molten lithium.

:<chem>10Li + 2CO2 -> Li2C2 + 4Li2O   </chem> 

Other method for production of Li<sub>2</sub>C<sub>2</sub> is heating of metallic lithium in atmosphere of [[Chemistry:Ethylene|ethylene]]. 

:<chem>6Li + C2H4 -> Li2C2 + 4LiH</chem>

Lithium carbide hydrolyzes readily to form acetylene:

:<chem>Li2C2 + 2H2O -> 2LiOH + C2H2</chem>

[[Chemistry:Lithium hydride|Lithium hydride]] reacts with graphite at 400°C forming lithium carbide.

:<chem>2LiH + 4C  ->  Li2C2 + C2H2</chem>

Also Li<sub>2</sub>C<sub>2</sub> can be formed when [[Chemistry:Organometallic chemistry|organometallic]] compound [[Chemistry:N-Butyllithium|n-Butyllithium]] reacts with ethyne in [[Chemistry:Tetrahydrofuran|THF]] or [[Chemistry:Diethyl ether|Et<sub>2</sub>O]] used as a solvent, reaction is rapid and highly exothermic.

:<chem>{C2H2} + 2BuLi ->[\ce{Et2O\ or\ THF}] {Li2C2} + 2C4H10</chem>

Lithium carbide reacts with acetylene in liquid ammonia rapidly to give a clear solution of lithium acetylide.

LiC≡CLi + HC≡CH → 2 LiC≡CH

Preparation of the reagent in this way sometimes improves the yield in an ethynylation over that obtained with reagent prepared from lithium and acetylene.

==Structure==
{{chem|Li|2|C|2}} is a [[Chemistry:Zintl phase|Zintl phase]] compound and exists as a [[Chemistry:Salt|salt]], {{chem|2Li|+|C|2||2−}}. Its reactivity, combined with the difficulty in growing suitable [[Earth:Single crystal|single crystal]]s, has made the determination of its crystal structure difficult. It adopts a distorted ''anti''-fluorite crystal structure, similar to that of rubidium peroxide ({{chem|Rb|2|O|2}}) and caesium peroxide ({{chem|Cs|2|O|2}}). Each Li atom is surrounded by six carbon atoms from 4 different acetylides, with two acetylides co-ordinating side -on and the other two end-on.<ref name=rev1 /><ref>{{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}}</ref> The observed C-C distance of 120 pm indicates the presence of a C≡C triple bond.
At high temperatures {{chem|Li|2|C|2|}} transforms reversibly to a cubic anti-fluorite structure.<ref>{{cite journal |author1=U. Ruschewitz |author2=R. Pöttgen | title = Structural Phase Transition in {{chem|Li|2|C|2}} | journal = [[Chemistry:Zeitschrift für anorganische und allgemeine Chemie|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}}</ref>

==Use in radiocarbon dating==
{{main|Physics:Radiocarbon dating}}
There are a number of procedures employed, some that burn the sample producing [[Chemistry:Carbon dioxide|CO<sub>2</sub>]] that is then reacted  with lithium, and others where the carbon containing sample is reacted directly with lithium metal.<ref>{{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 = [[Biology:Cellular and Molecular Life Sciences|Cellular and Molecular Life Sciences]] | doi = 10.1007/BF02146038 | year = 1964 | volume = 20 | pages = 47–48| s2cid = 31319813 }}</ref> The outcome is the same: Li<sub>2</sub>C<sub>2</sub> is produced, which can then be used to create species easy to use in mass spectroscopy, like acetylene and benzene.<ref>[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 }}</ref>  Note that [[Chemistry:Lithium nitride|lithium nitride]] may be formed and this produces [[Chemistry:Ammonia|ammonia]] when hydrolyzed, which contaminates the acetylene gas.

==References==
{{reflist}}

{{Lithium compounds}}

[[Category:Lithium compounds]]
[[Category:Carbides]]

{{Sourceattribution|Lithium carbide}}