Lithium carbide

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Lithium carbide
Wireframe model of lithium carbide
Names
Preferred IUPAC name
Lithium carbide
Systematic IUPAC name
Dilithium(1+) ethyne
Other names
Dilithium acetylide

Lithium dicarbon

Lithium percarbide
Identifiers
1070-75-3 YesY
ChemSpider 59503 YesY
EC number 213-980-1
Jmol-3D images Image
PubChem 66115
Properties
Li
2
C
2
Molar mass 37.9034 g/mol
Density 1.3 g/cm³[1]
Melting point > 550°C
Solubility insoluble in organic solvents
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
 N verify (what isYesY/N?)
Infobox references

Lithium carbide, Li
2
C
2
, often known as dilithium acetylide, is a chemical compound of lithium and carbon, an acetylide. It is an intermediate compound produced during radiocarbon dating procedures. Li
2
C
2
is one of an extensive range of lithium-carbon compounds which include the lithium-rich Li
4
C
, Li
6
C
2
, Li
8
C
3
, Li
6
C
3
, Li
4
C
3
, Li
4
C
5
, and the graphite intercalation compounds LiC
6
, LiC
12
, and LiC
18
.
Li
2
C
2
is the most thermodynamically-stable lithium-rich compound[2] and the only one that can be obtained directly from the elements. It was first produced by Moissan, in 1896[3] who reacted coal with lithium carbonate. 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, Li
2
CO
3
, because of the similarity of its name.

Preparation and chemistry[edit]

In the laboratory samples may be prepared by bubbling acetylene through a solution of lithium dissolved in liquid ammonia.

C
2
H
2
+ 2 Li → Li
2
C
2
+ H2

Lithium carbide prepared in this manner generally has very poor crystallinity, being essentially amorphous. Purer samples may be prepared by a reaction between molten lithium and graphite at over 1000°C.[2] Li2C2 can also be prepared by reacting CO2 with molten lithium.

Lithium carbide is reactive and hydrolyses very readily to form acetylene gas, C2H2, and LiOH.

Structure[edit]

Li
2
C
2
is a Zintl phase compound and exists as a salt, 2Li+
C
2
2−
. It's reactivity, combined with the difficulty in growing suitable single crystals, has made the determination of its crystal structure difficult. It adopts a distorted anti-fluorite crystal structure, similar to that of rubidium peroxide (Rb
2
O
2
) and caesium peroxide (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.[4][2] The observed C-C distance of 120 pm indicates the presence of a C≡C triple bond. At high temperatures Li
2
C
2
transforms reversibly to a cubic anti-fluorite structure.[5]

Use in radiocarbon dating[edit]

Main article: 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.[6] The outcome is the same: Li2C2 is produced, which can then be used to create species easy to mass, like acetylene and benzene.[7] Note that lithium nitride may be formed and this produces ammonia when hydrolyzed, which contaminates the acetylene gas.

References[edit]

  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. ^ a b c 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. 
  3. ^ H. Moissan Comptes Rendus hebd. Seances Acad. Sci. 122, 362 (1896)
  4. ^ Juza, Robert; Opp, Karl (November 1951). "Metallamide und Metallnitride, 24. Mitteilung. Die Kristallstruktur des Lithiumamides". Zeitschrift fur anorganische und allgemeine Chemie (in German) 266 (6): 313–324. doi:10.1002/zaac.19512660606. 
  5. ^ U. Ruschewitz, R. Pöttgen (1999). "Structural Phase Transition in Li
    2
    C
    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.
     
  6. ^ 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. doi:10.1007/BF02146038. 
  7. ^ University of Zurich Radiocarbon Laboratory webpage