Lithium carbide

Lithium carbide, Li₂C₂, often known as dilithium acetylide, is a chemical compound of lithium and carbon. It is an intermediate compound produced during radiocarbon dating procedures. 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.
Li₂C₂ is the thermodynamically stable lithium rich compound and is the only one of them that can be obtained directly from the elements. It was first produced by Moissan, in 1896^[2] who reacted coal with lithium carbonate. The other lithium rich compounds are produced by reacting lithium vapour with chlorinated hydrocarbons e.g. CCl₄. Lithium carbide is sometimes confused with the drug lithium carbonate, Li₂CO₃, because of the similarity of its name.

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Preparation and chemistry

To prepare pure samples in the laboratory molten lithium + graphite are reacted at high temperature. Li₂C₂ can also be prepared by reacting CO₂ with molten lithium. It is reactive and hydrolyses very readily to form acetylene gas , C₂H₂, and LiOH.

Structure

Li₂C₂ is an ionic salt and is formulated 2Li+ C₂²⁻. It has a similar structure to Rb₂O₂ and Cs₂O₂. 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 prior that is then reacted with lithium, and others where the carbon containing sample is reacted directly with lithium metal.^[3]. The outcome is the same, Li₂C₂ is produced. Note that lithium nitride may be formed and this produces ammonia when hydrolysed, which contaminates the acetylene gas.

General references

• Structural Phase Transition in Li₂C₂ U. Ruschewitz R Pottgen Zeitschrift für anorganische und allgemeine Chemie Volume 625, Issue 10 , Pages 1599 - 1603