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 &minus; 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 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.

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 &minus;. 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. . 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