Acetylide
In chemistry, an acetylide is a compound that can be viewed as the result of replacing one or both hydrogen atoms of acetylene by metallic or other cations. Calcium carbide is an important industrial compound, which has long been used to produce acetylene for welding and illumination. It is also a major precursor to vinyl chloride. Other acetylides are reagents in organic synthesis.
Nomenclature
The term acetylide is used loosely. It apply to an acetylene, where R = H or a side chain that is usually organic. The nomenclature can be ambiguous with regards to the distinction between compounds of the type and. When both hydrogens of acetylene are replaced by metals, the compound can also be called carbide, e.g. calcium carbide, which is calcium acetylide. When only one hydrogen atom is replaced, the anion may be called hydrogen acetylide or the prefix mono- may be attached to the metal, as in monosodium acetylide or sodium hydrogen acetylide,. Metal acetylide may be described as salts, but that description rarely comports with crystallographic analysis.Alkali and alkaline earth acetylides
In the absence of additional ligands, metal acetylides adopt polymeric structures wherein the acetylide groups are bridging ligands. Alkali metal acetylides have the general formula .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 solvated ions.Alkali metal and alkaline earth metal acetylides have the general formula . 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 metal oxides:
The ion has a closed shell ground state of 1Σ, making it isoelectronic to a neutral molecule, which may afford it some gas-phase stability. polymeric structures wherein the acetylide groups are bridging ligands.
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 phenylacetylide.
Preparation
Of the type
Acetylene and terminal alkynes are weak acids:Monopotassium and monosodium acetylide can be prepared by reacting acetylene with bases like sodium amide or with the elemental metals, often at room temperature and atmospheric pressure.
Copper acetylide can be prepared by passing acetylene through an aqueous solution of copper chloride because of a low solubility equilibrium. Similarly, silver acetylides can be obtained from silver nitrate.
In organic synthesis, acetylides are usually prepared by treating acetylene and alkynes with organometallic or inorganic Classically, liquid ammonia was used for deprotonations, but ethers are now more commonly used.
Lithium amide, LiHMDS, or organolithium reagents, such as butyllithium, are frequently used to form lithium acetylides:
Of the type and
is prepared industrially by heating carbon with lime at approximately 2,000 °C. A similar process can be used to produce lithium carbide.Dilithium acetylide,, competes with the preparation of the monolithium derivative.
Reactions
Ionic acetylides are typically decomposed by water with evolution of acetylene:Acetylides of the type are widely used in alkynylations in organic chemistry. They are nucleophiles 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.
Image:Acetylide carbonyl addition.png|600px|center|Reaction of ethyl propiolate with n-butyllithium to form the lithium acetylide.
The dimerization of acetylene to vinylacetylene proceeds by insertion of acetylene into a copper acetylide complex.
Coupling reactions
The copper-catalyzed Click reaction of terminal alkynes and azides proceeds via copper acetylide intermediates.Acetylides are sometimes used as intermediates in coupling reactions. Examples include Sonogashira coupling, Cadiot-Chodkiewicz coupling, Glaser coupling and Eglinton coupling.