Lithium carbide,, 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. is one of an extensive range of lithium-carbon compounds which include the lithium-rich ,,,,,, and the graphite intercalation compounds,, and .
is the most thermodynamically-stable lithium-rich carbide[1] and the only one 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 vapor with chlorinated hydrocarbons, e.g. . Lithium carbide is sometimes confused with the drug lithium carbonate,, because of the similarity of its name.
In the laboratory samples may be prepared by treating acetylene with a solution of lithium in ammonia, on −40°C, with creation of adduct of that decomposes in stream of hydrogen at room temperature giving white powder of .
Samples prepared in this manner generally are poorly crystalline. Crystalline samples may be prepared by a reaction between molten lithium and graphite at over 1000 °C. can also be prepared by reacting with molten lithium.
Other method for production of is heating of metallic lithium in atmosphere of ethylene.
Lithium carbide hydrolyzes readily to form acetylene:
Lithium hydride reacts with graphite at 400°C forming lithium carbide.
Also can be formed when organometallic compound n-butyllithium reacts with acetylene in THF or used as a solvent, reaction is rapid and highly exothermic.
Lithium carbide reacts with acetylene in liquid ammonia rapidly to give a clear solution of lithium hydrogen acetylide.
Preparation of the reagent in this way sometimes improves the yield in an ethynylation over that obtained with reagent prepared from lithium and acetylene.
is a Zintl phase compound and exists as a salt, with the formula . Its 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 and caesium peroxide . 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.[3] The observed relatively short C-C distance of 120 pm indicates the presence of a C≡C triple bond. At high temperatures transforms reversibly to a cubic anti-fluorite structure.[4]
See main article: Radiocarbon dating.
There are a number of procedures employed, some that burn the sample producing that is then reacted with lithium, and others where the carbon containing sample is reacted directly with lithium metal.[5] The outcome is the same: is produced, which can then be used to create species easy to use in mass spectroscopy, like acetylene and benzene.[6] Note that lithium nitride may be formed and this produces ammonia when hydrolyzed, which contaminates the acetylene gas.