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Dilithium acetylide

LiC≡CLi
Names
Preferred IUPAC name Lithium acetylide
Systematic IUPAC name Lithium ethynediide
Other names Dilithium acetylide Lithium dicarbon Lithium percarbide
Identifiers
CAS Number	1070-75-3
3D model (JSmol)	Interactive image
ChemSpider	59503
ECHA InfoCard	100.012.710
EC Number	213-980-1
PubChem CID	66115
UNII	GZ7TQ3WG5P
InChI InChI=1S/C2.2Li/c1-2;;/q-2;2*+1Key: ARNWQMJQALNBBV-UHFFFAOYSA-N InChI=1S/C2.2Li/c1-2;;/q-2;2*+1Key: ARNWQMJQALNBBV-UHFFFAOYSA-N InChI=1/C2.2Li/c1-2;;/q-2;2*+1Key: ARNWQMJQALNBBV-UHFFFAOYAB
SMILES ..#
Properties
Chemical formula	Li2C2
Molar mass	37.9034 g/mol
Appearance	Powder
Density	1.3 g/cm
Melting point	452°C
Solubility in water	Reacts
Solubility	insoluble in organic solvents
Related compounds
Related compounds	Tetralithiomethane Monolithium acetylide Monosodium acetylide Copper(I) acetylide Silver acetylide Calcium carbide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). N verify (what is  ?) Infobox references

Dilithium acetylide is an organometallic compound with the formula L_i2C₂. It is typically derived by double deprotonation of acetylene. X-ray crystallography confirms the presence of C≡C subunits attached to lithium, resulting in a polymeric structure. Li₂C₂ is one of an extensive range of lithium-carbon compounds, which include the lithium-rich Li₄C, Li₆C₂, Li₈C₃, Li₆C₃, Li₄C₃, Li₄C₅, and the graphite intercalation compounds LiC₆, LiC₁₂, and LiC₁₈. It is an intermediate compound produced during radiocarbon dating procedures.

Li₂C₂ is the most thermodynamically-stable lithium-rich carbide and the only one that can be obtained directly from the elements. It was first produced by Moissan, in 1896 who reacted coal with lithium carbonate.

Li₂CO₃ + 4 C → Li₂C₂ + 3 CO

The other lithium-rich compounds are produced by reacting lithium vapor with chlorinated hydrocarbons, e.g. CCl₄. Lithium carbide is sometimes confused with the drug lithium carbonate, Li₂CO₃, because of the similarity of its name.

Preparation and reactions

In the laboratory samples may be prepared by treating acetylene with butyl lithium:

C₂H₂ + 2 BuLi → Li₂C₂ + BuH

Instead of butyl lithium, a solution of lithium in ammonia can be used to prepare Li₂C₂. In this case, a transient adduct 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. Li₂C₂ can also be prepared by reacting CO₂ with molten lithium.

10 Li + 2 CO₂ → Li₂C₂ + 4 Li₂O

Other method for production of Li₂C₂ is heating of metallic lithium in atmosphere of ethylene. Lithium hydride is a coproduction:

6 Li + C₂H₄ → Li₂C₂ + 4 LiH

Lithium carbide hydrolyzes readily to form acetylene as well as Lithium hydroxide:

Li₂C₂ + 2 H₂O → 2 LiOH + C₂H₂

Lithium hydride reacts with graphite at 400°C forming lithium carbide.

2 LiH + 4 C → Li₂C₂ + C₂H₂

Lithium carbide reacts with acetylene in liquid ammonia rapidly to give a lithium hydrogen 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

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 (Rb₂O₂) and caesium peroxide (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. The relatively short C-C distance of 120 pm indicates the presence of a C≡C triple bond. At high temperatures Li₂C₂ transforms reversibly to a cubic anti-fluorite structure.

Use in radiocarbon dating

There are a number of procedures employed, some that burn the sample producing CO₂ that is then reacted with lithium, and others where the carbon containing sample is reacted directly with lithium metal. The outcome is the same: Li₂C₂ is produced, which can then be used to create species easy to use in mass spectroscopy, like acetylene and benzene. Note that lithium nitride may be formed and this produces ammonia when hydrolyzed, which contaminates the acetylene gas.

References

1. R. Juza; V. Wehle; H.-U. Schuster (1967). "Zur Kenntnis des Lithiumacetylids". Zeitschrift für anorganische und allgemeine Chemie. 352 (5–6): 252. doi:10.1002/zaac.19673520506.
2. Savchenko, A.P.; Kshnyakina, S.A.; H.-Majorova, A.F. (1997). "Thermal properties of lithium carbide and lithium intercalation compounds of graphite". Neorganicheskie Materialy. 33 (11): 1305–1307.
3. ^ Ruschewitz, Uwe (September 2003). "Binary and ternary carbides of alkali and alkaline-earth metals". Coordination Chemistry Reviews. 244 (1–2): 115–136. doi:10.1016/S0010-8545(03)00102-4.
4. H. Moissan Comptes Rendus hebd. Seances Acad. Sci. 122, 362 (1896)
5. Walton, D.R.M.; Waugh, F. (1972). "Friedel–Crafts reactions of bis(trimethylsilyl)polyynes with acyl chlorides; a useful route to terminal-alkynyl ketones". Journal of Organometallic Chemistry. 37: 45–56. doi:10.1016/S0022-328X(00)89260-8.
6. Juza, Robert; Opp, Karl (November 1951). "Metallamide und Metallnitride, 24. Mitteilung. Die Kristallstruktur des Lithiumamides". Zeitschrift für anorganische und allgemeine Chemie (in German). 266 (6): 313–324. doi:10.1002/zaac.19512660606.
7. U. Ruschewitz; R. Pöttgen (1999). "Structural Phase Transition in Li
   ₂C
   ₂". Zeitschrift für anorganische und allgemeine Chemie. 625 (10): 1599–1603. doi:10.1002/(SICI)1521-3749(199910)625:10<1599::AID-ZAAC1599>3.0.CO;2-J.
8. Swart E.R. (1964). "The direct conversion of wood charcoal to lithium carbide in the production of acetylene for radiocarbon dating". Cellular and Molecular Life Sciences. 20: 47–48. doi:10.1007/BF02146038. S2CID 31319813.
9. University of Zurich Radiocarbon Laboratory webpage Archived 2009-08-01 at the Wayback Machine

• Lithium compounds
• Acetylides
• Carbides