Lithium carbide

Lithium carbide

Names
Preferred IUPAC name Lithium carbide
Systematic IUPAC name Dilithium(1+) ethyne
Other names Dilithium acetylide Lithium dicarbon Lithium percarbide
Identifiers
CAS Number	1070-75-3 Y
3D model (Jmol)	Interactive image
ChemSpider	59503 Y
ECHA InfoCard	100.012.710
EC Number	213-980-1
PubChem	66115
InChI InChI=1S/C2.2Li/c1-2;;/q-2;2*+1 Y Key: ARNWQMJQALNBBV-UHFFFAOYSA-N Y InChI=1S/C2.2Li/c1-2;;/q-2;2*+1 Key: ARNWQMJQALNBBV-UHFFFAOYSA-N InChI=1/C2.2Li/c1-2;;/q-2;2*+1 Key: ARNWQMJQALNBBV-UHFFFAOYAB
SMILES [Li+].[Li+].[C-]#[C-]
Properties
Chemical formula	Li 2C 2
Molar mass	37.9034 g/mol
Density	1.3 g/cm³[1]
Melting point	> 550°C
Solubility	insoluble in organic solvents
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

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^[2] and the only one that can be obtained directly from the elements. It was first produced by Moissan, in 1896^[3] 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 bubbling acetylene through a solution of lithium dissolved in liquid ammonia.

C
2H
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.^[2] 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.^[2][4] 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.^[5]

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.^[6] The outcome is the same: Li₂C₂ is produced, which can then be used to create species easy to mass, like acetylene and benzene.^[7] Note that lithium nitride may be formed and this produces ammonia when hydrolyzed, which contaminates the acetylene gas.

References