Formula Li2C2 Density 1.3 g/cm³ | Molar mass 37.9034 g/mol | |
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 compound 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. The other lithium-rich compounds are produced by reacting lithium vapor with chlorinated hydrocarbons, e.g. CCl4.
Contents
Lithium carbide is sometimes confused with the drug lithium carbonate, Li
2CO
3, because of the similarity of its name.
Lithium carbide top 7 facts
Preparation and chemistry
In the laboratory samples may be prepared by bubbling acetylene through a solution of lithium dissolved in liquid ammonia.
C2H
2 + 2 Li → Li
2C
2 + H2
Lithium carbide prepared in this manner generally has very poor crystallinity, being essentially amorphous. Purer 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.
Lithium carbide is reactive and hydrolyses very readily to form acetylene gas, C2H2, and LiOH.
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. 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.
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 mass, like acetylene and benzene. Note that lithium nitride may be formed and this produces ammonia when hydrolyzed, which contaminates the acetylene gas.