Dilithium acetylide is an organometallic compound with the formula Li2C2. 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.
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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 is 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
Dilithium acetylide 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.