Dilithium acetylide
Template:Short description Script error: No such module "about". Template:Chembox 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 Template:Chem2 subunits attached to lithium, resulting in a polymeric structure.[1] Template:Chem2 is one of an extensive range of lithium-carbon compounds, which include the lithium-rich [[tetralithiomethane|Template:Chem2]], Template:Chem2, Template:Chem2, Template:Chem2, Template:Chem2, Template:Chem2, and the graphite intercalation compounds Template:Chem2, Template:Chem2, and Template:Chem2. It is an intermediate compound produced during radiocarbon dating procedures.
Template:Chem2 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. [[carbon tetrachloride|Template:Chem2]]. Lithium carbide is sometimes confused with the drug lithium carbonate, Template:Chem2, because of the similarity of its name.
Preparation and reactions
In the laboratory samples may be prepared by treating acetylene with butyl lithium:[3]
Instead of butyl lithium, a solution of lithium in ammonia can be used to prepare Template:Chem2. In this case, a transient adduct Template:Chem2 if 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.[1] Template:Chem2 can also be prepared by reacting [[Carbon dioxide|Template:Chem2]] with molten lithium.Script error: No such module "Unsubst".
Other method for production of Template:Chem2 is heating of metallic lithium in atmosphere of ethylene. Lithium hydride is a coproduction:
Lithium carbide hydrolyzes readily to form acetylene as well as Lithium hydroxide:
Lithium hydride reacts with graphite at 400°C forming lithium carbide.
Lithium carbide reacts with acetylene in liquid ammonia rapidly to give a 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.Script error: No such module "Unsubst".
Structure
Template:Chem2 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 (Template:Chem2) and caesium peroxide (Template:Chem2). 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.[1][4] The relatively short C-C distance of 120 pm indicates the presence of a C≡C triple bond. At high temperatures Template:Chem2 transforms reversibly to a cubic anti-fluorite structure.[5]
Use in radiocarbon dating
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There are a number of procedures employed, some that burn the sample producing [[carbon dioxide|Template:Chem2]] 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: Template:Chem2 is produced, which can then be used to create species easy to use in mass spectroscopy, like acetylene and benzene.[7] Note that lithium nitride may be formed and this produces ammonia when hydrolyzed, which contaminates the acetylene gas.
References
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- ↑ a b c d Script error: No such module "Citation/CS1".
- ↑ H. Moissan Comptes Rendus hebd. Seances Acad. Sci. 122, 362 (1896)
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- ↑ University of Zurich Radiocarbon Laboratory webpage Template:Webarchive
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