Chemical compound of lithium and carbon, an acetylide
Chemical compound
Lithium carbide, Li
2C
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
2C
2 is one of an extensive range of lithium-carbon compounds which include the lithium-rich Li
4C, Li
6C
2, Li
8C
3, Li
6C
3, Li
4C
3, Li
4C
5, and the graphite intercalation compounds LiC
6, LiC
12, and LiC
18.
Li
2C
2 is the most thermodynamically-stable lithium-rich carbide[3] and the only one that can be obtained directly from the elements. It was first produced by Moissan, in 1896[4] 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
2CO
3, because of the similarity of its name.
Preparation and chemistry
In the laboratory samples may be prepared by treating acetylene with a solution of lithium in ammonia, on −40°C, with creation of addition compound of Li2C2 • C2H2 • 2NH3 that decomposes in stream of hydrogen at room temperature giving white powder of Li2C2.
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.[3] Li2C2 can also be prepared by reacting CO2 with molten lithium.
Other method for production of Li2C2 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 Li2C2 can be formed when organometallic compound n-Butyllithium reacts with ethyne in THF or Et2O used as a solvent, reaction is rapid and highly exothermic.
![{\displaystyle {\ce ((C2H2}+2BuLi->[{\ce {Et2O\ or\ THF))]{Li2C2}+2C4H10))}](https://wikimedia.org/api/rest_v1/media/math/render/svg/69e46f1798c0434e9898b0aabf9c75db128bb6fc)
Lithium carbide reacts with acetylene in liquid ammonia rapidly to give a clear solution of lithium 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
Li
2C
2 is a Zintl phase compound and exists as a salt, 2Li+
C
22−
. 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 (Rb
2O
2) and caesium peroxide (Cs
2O
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.[3][5] The observed C-C distance of 120 pm indicates the presence of a C≡C triple bond.
At high temperatures Li
2C
2 transforms reversibly to a cubic anti-fluorite structure.[6]
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.[7] 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.[8] Note that lithium nitride may be formed and this produces ammonia when hydrolyzed, which contaminates the acetylene gas.
