In chemistry, an acetylide is a compound that can be viewed as the result of replacing one or both hydrogen atoms of acetylene (ethyne) by metal or other . Calcium carbide is an important industrial compound, which has long been used to produce acetylene for welding and carbide lamp. It is also a major precursor to vinyl chloride.[ Other acetylides are in organic synthesis.
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Nomenclature
The term acetylide is used loosely. It applies to an acetylene , where R = H or a side chain that is usually organic.
Alkali and alkaline earth acetylides
In the absence of additional ligands, metal acetylides adopt polymeric structures wherein the acetylide groups are . Alkali metal acetylides have the general formula (M = Li, Na, K, etc).They are sometimes represented as but the C---M bonding might also be described as polar covalent. They dissolve without decomposition in ammonia. Such solutions are proposed to contain Solvation ions.
Alkali metal and alkaline earth metal acetylides have the general formula (M' = Mg, Ca, etc). Rather than salt-like, they can be considered Zintl phase compounds, containing ions, with a triple bond between the two carbon atoms. They undergo ready hydrolysis to form acetylene and :
The ion has a closed Electron shell ground state of 1Σ, making it Isoelectronicity to a neutral molecule , which may afford it some gas-phase stability.
File:Na2C2structure.jpg| Structure of sodium acetylide .[S. Hemmersbach, B. Zibrowius, U. Ruschewitz (1999): "Na2C2 und K2C2: Synthese, Kristallstruktur und spektroskopische Eigenschaften". Zeitschrift für anorganische und allgemeine Chemie, volume 625, issue 9, pages 1440-1446. ]
File:CAMNEJ.svg| Structure of the cluster formed from complexed to N,N,N′,N′-tetramethyl-1,6-diaminohexane (methylene groups omitted for clarity). Color key: turquoise = Li, blue = N.
Transition metal acetylides
Acetylides of the transition metals, show evidence of covalent character, e. g. for being neither dissolved nor decomposed by water and by radically different chemical reactions. The inventory of such complexes numbers in the hundreds. Even the stoichiometrically simple silver acetylide and copper acetylide appear highly covalent..
File:XAPGIF2.png | Portion of the structure of the polymer copper(I) phenylacetylide .
Preparation
Of the type
Acetylene and terminal are weak acids:
Monopotassium and monosodium acetylide can be prepared by reacting acetylene with bases like sodium amide
In organic synthesis, acetylides are usually prepared by treating acetylene and alkynes with organometallic
Lithium amide,
Of the type and
Calcium carbide is prepared industrially by heating carbon with lime (calcium oxide) at approximately 2,000 °C.
Dilithium acetylide, , competes with the preparation of the monolithium derivative .[
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Reactions
Ionic acetylides are typically decomposed by water with evolution of acetylene:
Acetylides of the type are widely used in in organic chemistry. They are that add to a variety of electrophilic and unsaturated substrates.
A classic application is the Favorskii reaction, such as in the sequence shown below. Here ethyl propiolate is deprotonated by N-Butyllithium to give the corresponding lithium acetylide. This acetylide adds to the carbonyl center of cyclopentanone. Hydrolysis liberates the alkynyl alcohol.
The dimerization of acetylene to vinylacetylene proceeds by insertion of acetylene into a copper(I) acetylide complex.
Coupling reactions
The copper-catalyzed Click reaction of terminal alkynes and azides proceeds via copper(I) acetylide intermediates.
Acetylides are sometimes used as intermediates in coupling reactions. Examples include Sonogashira coupling, Cadiot-Chodkiewicz coupling, Glaser coupling and Eglinton coupling.
Hazards
Some acetylides are notoriously explosive.
See also
