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Concept
Acetylide
Summary
In organometallic chemistry, acetylide refers to chemical compounds with the chemical formulas and , where M is a metal. The term is used loosely and can refer to substituted acetylides having the general structure (where R is an organic side chain). Acetylides are reagents in organic synthesis. The calcium acetylide commonly called calcium carbide is a major compound of commerce.
Alkali metal and alkaline earth metal acetylides of the general formula MC≡CM are salt-like Zintl phase compounds, containing C22− ions. Evidence for this ionic character can be seen in the ready hydrolysis of these compounds to form acetylene and metal oxides, there is also some evidence for the solubility of C22− ions in liquid ammonia. The C22− ion has a closed shell ground state of 1Σ, making it isoelectronic to a neutral molecule N2, which may afford it some stability.
Analogous acetylides prepared from other metals, particularly transition metals, show covalent character and are invariably associated with their metal centers. This can be seen in their general stability to water (such as silver acetylide, copper acetylide) and radically different chemical applications.
Acetylides of the general formula RC≡CM (where R = H or alkyl) generally show similar properties to their doubly substituted analogues. In the absence of additional ligands, metal acetylides adopt polymeric structures wherein the acetylide groups are bridging ligands.
Terminal alkynes are weak acids:
RC≡CH + R′′M R′′H + RC≡CM
To generate acetylides from acetylene and alkynes relies on the use of organometallic or inorganic superbases in solvents which are less acidic than the terminal alkyne. In early studies liquid ammonia was employed, but ethereal solvents are more common.
Lithium amide, LiHMDS, or organolithium reagents, such as butyllithium, are frequently used to form lithium acetylides:
{H-C{\equiv}C-H} + \overset{butyllithium}{BuLi} ->[\ce{THF}][-78^\circ\ce C] {Li-!{\equiv}!-H} + BuH
Sodium or potassium acetylides can be prepared from various inorganic reagents (such as sodium amide) or from their elemental metals, often at room temperature and atmospheric pressure.
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